The Evolution of Oilfield Batteries
Well logging, measurements-while-drilling and seismic acquisition
require custom power sources. A dedicated development effort that
began in 1984 as a small project to meet short-term needs has grown
into a specialized design and manufacturing team that produces
primary and rechargeable batteries, and fuel cells for E&P services.
Don Hensley
Marvin Milewits
Wenlin Zhang
Rosharon, Texas, USA
For help in preparation of this article, thanks to Bill Jones,
Bic Nguyen, Chris Spring, Henry Stevenson and Tony
Veneruso, Schlumberger Perforating & Testing Center,
Rosharon, Texas, USA; and Ting Lau, Schlumberger
Sugar Land Product Center, Sugar Land, Texas.
ARC5 (Array Resistivity Compensated), IRIS (Intelligent
Remote Implementation System), MSRT (MultiSensor
Recorder/Transmitter), RAB (Resistivity-at-the-Bit) and
UNIGAGE are marks of Schlumberger.
42
An autonomous power supply is not required
if an extension cord of sufficient length is
available. Likewise, a downhole tool does
not need batteries when run on an electricline cable, which provides, in addition to
mechanical conveyance, a direct link to the
surface for power transmission and signal
telemetry. Wireline operations, however,
require a relatively unobstructed vertical path
from surface to the measurement depth in
openhole or cased wellbore, but many wells
today are inclined, high-angle, extendedreach or horizontal. Other wireline limitations involve gauges that need to be placed
below valves in production tubulars or tools
that must be rotated. The first downhole batteries were developed for pressure buildup,
production logging, measurements-whiledrilling (MWD) and logging-while-drilling
(LWD) tools that have no way of receiving
power from the surface.
Until the early 1980s, most production logging tools recorded pressure, temperature
and flow rate solely with mechanically
driven components. These devices contained
a stylus to scratch marks on a carbon-coated
metallic cylinder that turned at a constant
slow rate. Specially trained field personnel
read the excursion marks with a microscope
and plotted digitized values versus elapsed
time on paper. Results were entered manually into proprietary computing software to
analyze formation pressures, drive mechanisms and reservoir boundaries.
This mechanical tool, the J200, was
replaced by a battery-powered version, the
J300, which was a predecessor of solid-state
downhole processor (SSDP) models. With
each new-generation device, data stability,
precision and accuracy as well as tool autonomy were improved by energy-efficient electronics and high-performance batteries.
Through better information and improved
analysis, formation evaluation and reservoir
characterization were also enhanced by the
evolution of battery technology. To appreciate the role of batteries in oilfield services,
the unique demands of subsurface environments need to be defined.
Oilfield Review
Basic Electrochemical Cells
A review of conventional electrochemical,
or galvanic, cells sets the stage for understanding downhole batteries (see “Battery
Terminology,” page 45). The anode and
cathode—the electrodes—of conventional
cells are composed of solid materials surrounded by liquid electrolyte. A porous
separator isolates the electrodes mechanically to prevent an internal short circuit, but
allows ion flow, or diffusion. When a conductive path or electronic device is connected to the battery, electrons released
from the anode in a continuous oxidation
process flow through this external load, performing work by electric potential.
Inside the cell, in a process called oxidation-reduction, ions are released from both
the anode and cathode as charge neutrality is maintained (below). The cell produces electric power as long as the anode
supplies electrons and the cathode accepts
electrons. Electron flow (current) stops or
is limited if the anode or cathode are consumed, ions cannot reach the cathode, the
external current path is interrupted, the
anode contacts the cathode or the ion diffusion limit is reached.
Electron flow
Load
Electrolyte
Autumn 1998
e-
Cathode
cells with liquid cathodes are not used by
Schlumberger in oilfield services (see
“Inside a Lithium Cell,” page 46).
Operating life—Battery life requirements
range from several days to a year. Longer
lasting power is achieved with larger cells
and energy efficient, or battery friendly, electronics. Chemistry, cell composition, tool
loads and temperature also affect battery life.
Shelf life—Global oil and gas operations
dictate that batteries often spend weeks in
transit to job locations. Once on site, batteries may be stored for long periods until
work begins, so shelf life must be long
enough to provide reliable power under
most conditions and standby situations.
Production logging and formation evaluation tools must be accurate and reliable and
batteries are essential in meeting these
requirements. This article reviews the history
of battery technology, and discusses current
state-of-the-art power source development at
the Schlumberger Perforating & Testing (SPT)
Center in Rosharon, Texas, USA.
Anode
Temperatures—Logging tools are tested
at the surface before a trip into the well,
so batteries must function from arctic to
extreme downhole conditions. Well temperatures routinely range from 70 to 200°F
[20 to 100° C], but can get as hot as 400°F
[200°C] or higher.
Limited space—Tool diameters have to be
smaller than the borehole and lengths need
to be minimized to facilitate handling.
Batteries must fit into the available shape
and size of a tool compartment.
Shock and vibration—Tools and batteries
are subjected to the same conditions and
must meet the same standards. However,
rugged cell designs also have to be costeffective and disposable.
Power and safety—A battery is designed
to provide only the power required to operate a tool. Excess power increases the danger and chance of a leak or failure. For this
reason, commercially available spiral-wrap
+
Separator
■Basic electrochemical cells. A conventional
battery is a can of chemicals that performs
work by virtue of electrical potential difference between the anode and cathode. Specific electrochemical reactions produce electron flow, or current, to the external load,
providing power for tools or devices.
43
An Early Downhole Power Source
■Early SSDP Battery Chemistries.
Chemistry
Temperature operating range
Alkaline
Lithium copper oxide
Lithium copper oxyphosphate
–22 to 176° F [–30 to 80° C]
–22 to 257° F [–30 to 125° C]
122 to 347° F [50 to 175° C]
■The early SSDP battery pack. In this one-piece, all-metal, welded housing, individual
cells were locked in place with epoxy resin. This design was costly and made the packs
a disposal problem.
3.6
[LiSOCI2]
3.4
3.2
3.0
[LiSO2]
Cell voltage, volts (V)
2.8
2.6
[LiMnO2]
2.0
[MgMnO2]
1.8
1.6
[LiFeS2]
A Specialized Downhole Power Source
[ZnAg2O]
1.4
[ZnHg0]
1.2
[Alkaline-MnO2]
1.0
[Zn-air]
[CdHgO]
0.8
[Zn-carbon]
0.6
10
20
30
40
50
60
70
80
90 100
Discharged capacity, %
■ Voltage discharge for primary battery chemistries. High-voltage lithium chemistries
including lithium thionyl chloride [LiSOCl2], lithium sulfur dioxide [LiSO2] and lithium
manganese dioxide [LiMnO2] were evaluated. Other electrochemistries include
magnesium manganese dioxide [MgMnO2], lithium iron disulfide [LiFeS2], zinc
mercury oxide [ZnAg2O], zinc silver oxide [ZnHgO], zinc air [Zn-air], alkaline manganese dioxide [alkaline-MnO2], heavy-duty zinc carbon [Zn-carbon] and mercad,
or cadmium silver oxide [CdHgO].
44
Optimal battery designs address electrochemistry, tool power needs, packaging and
diverse considerations such as cost, shelf
life, shipping regulations, remaining life
and disposal. These factors were not always
considered prior to the 1980s when batteries were a last-minute tool addendum, usually obtained from outside vendors.
In 1984, a Flopetrol-Johnston group in
Melun, France, requested that engineering
counterparts in the USA develop another
source for the SSDP pressure-recording tool
battery. Prior to that time, there were three
SSDP batteries, each with different
chemistries and temperature ranges (above
left). Because each cell chemistry had limited operating temperatures, field engineers
had to plan each job carefully. One unusual
prejob procedure required lithium copper
oxyphosphate packs to be short circuited
intentionally to warm them up above 122°F
[50°C]. Wireline operators then had to lower
the tool into the hole quickly before the batteries cooled down. Operations in cold
weather were problematic.
The original SSDP battery was a one-piece
pack, consisting of a welded metal tube that
contained the cells and had two solid end
pieces for connectors (left). Individual electrochemical cells, typically AA size, were held in
place by epoxy. These packs were difficult to
dispose of and expensive.
Batteries for reliable oilfield service
require cell components, processes and
packaging to be addressed in a systematic
fashion. Beginning with basic electrochemical technology, a second SSDP battery with one chemistry for the entire range
of operating temperatures was developed.
To ensure safe and cost-effective battery
operation, both packaging and cell electrochemistry were reevaluated.
In the search for a new battery supply, electrochemistry was scrutinized first. Downhole
tools are relatively small, but increasingly
need more operating power. High-voltage
chemistries require fewer cells to meet
these power requirements, fit the space available inside tools more easily and reduce cost
(left). Lithium thionyl chloride [LiSOCl2]
appeared to be best for oilfield service in part
because only one battery type was needed for
most temperatures, simplifying field inventory.
1. Gabano JP (ed): Lithium Batteries. London, England:
Academic Press, 1983.
Oilfield Review
Lithium thionyl chloride chemistry was discovered almost by accident. In 1969, JeanPaul Gabano, a French chemist, was
developing a rechargeable lithium battery
comprised of a lithium cell containing
thionyl chloride electrolyte with dissolved
chlorine acting as the cathode. The cell
demonstrated high-current rechargeable performance and, surprisingly, continued to
produce current even after the chlorine
depleted. An evaluation determined that
thionyl chloride could also serve as the cathode and the LiSOCl2 battery was born.1
Theoretically, lithium thionyl chloride batteries should not exist. Normally, when an
anode touches a cathode, oxidation and
reduction begin immediately and continue in
an abrupt short-circuit reaction. During cell
manufacture, however, when liquid thionyl
chloride is poured into the lithium metal cell,
there are no violent reactions because of the
instantaneous formation of a LiCl layer on the
freshly wetted lithium metal surface. This passivation layer seals the surface from further
direct contact with the liquid cathode and
prevents dangerous reactions.
This chemistry, with one of the highest
energy densities of practical cells, has some
limitations and intrinsic safety issues.
Lithium metal mixed with water is
flammable and explosive, the liquid electrolyte is corrosive and toxic, and high-rate
electrode structures are susceptible to
explosions when shorted. Low-rate electrodes have the same hazards at high temperatures. Explosions can also result from
forced over-discharge and cell charging.
This chemistry also exhibits a voltage delay,
or passivation, with load onset, which is a
function of storage time and temperature.
As a result, lithium batteries must be predischarged—depassivated—before use.
Lithium thionyl chloride chemistry was not
widely used at first and had been available
commercially for fewer than 10 years when
this chemical reevaluation began. There
were initial concerns and problems related
to premature market introduction, including
leaks and failures in airline emergency lighting, an early use of these batteries. In
response, strict shipping regulations were
put into effect. Batteries with liquid cathodes
and more than 0.5, but less than 12, grams
of lithium had to pass specific tests and be
placed in special containers before being
shipped on cargo aircraft. By 1984, five vendors were making this type of cell and most
of the associated problems were understood
and had been addressed.
Autumn 1998
Battery Terminology1
Ampere (A): A coulomb per second,
the basic international standard unit of
electric current.
Anode: The electrode where oxidation, or loss
of electrons, occurs in electrochemical cells.
Aqueous: Substances that are like water or
water based.
Battery pack: A single galvanic cell or group
of cells connected through series or parallel
circuits and housed in a modular enclosure
to provide power to electronic devices.
Cathode: The electrode where reduction, or
electron gain, occurs in electrochemical cells.
Cell charging: Electric current flowing into a
cell from an external source or other cells.
Conductivity: The material property of conducting or transmitting electric current.
Coulomb: The quantity of electricity transported in one second by a one-ampere
current, the basic international standard unit
of electric charge.
Current: The transfer, or flow, rate of electric
energy, or electricity, measured in amperes
(A), or one coulomb per second.
Deep discharging: Depletion of a battery
below the normal end-of-life voltage, but
still above zero volts (V).
Electrode: An electronic conductor that acts
as an electron source or sink, usually made
of metal and immersed in electrolyte solution.
Electrolyte: An ionic solution capable of
conducting electric current.
Energy density: Electrical energy of a
unit mass or volume expressed in W-hr/kg
or W-hr/L.
Galvanic cell: An arrangement consisting of
two electrodes and an electrolyte that produces electric current from a spontaneous
chemical reaction when the electrodes are
connected externally.
Gravimetric energy density: Electric energy
in a unit mass expressed in W-hr/kg.
Hermetic seal: A gas-tight and nonconductive
external barrier that allows an electrical connection with an internal cell electrode.
Ion: A positive or negative charged atom
through either gaining or losing electrons.
Ion diffusion rate: A measure of the movement
or travel of ions from the anode to the cathode.
Overdischarging: Depletion of a battery
below zero volts (V) and into voltage reversal.
Oxidation: The process of electrons being
removed at the anode in the electrochemical
reaction of a galvanic cell.
Parallel circuit: An electrical path formed by
connecting positive terminals through one
conductor and negative terminals through
another conductor, so current can flow from
each cell to the external load.
Passivation: The formation of solid products
on electrode surfaces.
Reduction: The process of electrons being
donated at the cathode in the electrochemical
reaction of a galvanic cell.
Series circuit: An electrical path formed by
connecting battery cell positive terminals to
negative terminals in a sequence, so current
can flow through each cell in succession to
the external load.
Short circuit: A very conductive path placed
across the terminals of a battery (external
short circuit) or contact between anode and
cathode (internal short circuit) that generates
large currents and damages subsequent
cell performance.
Volt (V): The difference in electric potential
that makes a 1-amp current flow through a
1-ohm resistance, the basic international
standard unit of electromotive force.
Voltage delay: The immediate drop in voltage
below normal operating values caused by passivation. Voltage recovery may occur slowly or
not at all, depending on passivation severity.
Volumetric energy density: Electric energy
in a unit volume expressed in W-hr/L.
Watt (W): A 1-ampere current under 1-volt of
electric potential; 1 joule per second or
1/746 horsepower; the basic international
unit of electric power.
1. Hibbert DB and James AM: Dictionary of
Electrochemistry, Second Edition. New York,
New York, USA: Wiley-Interscience, 1984.
45
The next concern in developing a new battery source was packaging. The previous
method of enclosing cells in a metallic tube
was costly and did not enhance safety.
Instead, a new design concept separated the
battery into two parts (right). Rather than dispose of the entire depleted battery pack, only
the section containing cells is removed. The
remaining housing, consisting of the metal
tube and end pieces, is reused. Because cells
can leak, the reusable battery package is gastight to protect tools against corrosive leakage from the encapsulated cells. Pressure
relief valves are built into the end-caps to
allow controlled venting.
The result of this new chemistry and packaging was a 50% reduction in SSDP battery
cost. This backup source soon became the
primary source and the first stage of newgeneration battery developments was complete. Currently, the hazards and limitations
of lithium-based batteries cannot be eliminated completely, but must be understood
and addressed by battery developers, tool
designers and end-users. Schlumberger considers battery safety issues to be of paramount
importance and addresses them in detail during the evaluation and design process.
■A new pack design. A novel approach replaced expensive one-piece, metal batteries by
building packs in sections. A removable cell holder is disposable, but the housing can be
reused. The end-caps have integral valves to relieve internal pressure in the event of an
accidental cell leak.
Inside a Lithium Cell
There are four major components of primary,
or nonrechargable, lithium thionyl chloride
cells—anode, cathode, electrolyte salt and
separator (right). The negative terminal is
lithium metal foil or lithium alloy, which
serves as the anode. In this chemistry, the
cathode and electrolyte are the same and are
called “catholyte.” The liquid thionyl chloride [SOCl2] cathode reaction takes place on
a high-surface-area carbon electrode, which
serves as the positive terminal. A common
electrolyte salt, lithium aluminum chlorate
[LiAlCl4], is dissolved in the thionyl chloride
to increase ion conductivity and improve
current rates. A layer of nonwoven fiberglass
material physically and electrically isolates
the lithium anode from the positive terminal.
■Idealized lithium cell. Major components of a nonrechargable, or primary, lithium thionyl chloride battery are the anode, cathode, electrolytic salt and a
separator. For this chemistry, the cathode and electrolyte are the same. Electrons from the lithium anode
flow toward the positive terminal by way of the external circuit, or load. Lithium ions diffuse through the
catholyte—combined cathode and electrolyte—
toward the positive terminal. Thionyl chloride at the
positive terminal meets returning electrons, which are
reduced to form chloride ions, sulfur and sulfur dioxide. Chloride ions combine with lithium ions to form
lithium chloride, a solid that is deposited on the carbon electrode reaction site.
46
Can
Separator
Carbon electrode
Anode
Liquid cathode
Electron flow
Load
+
Separator
Glass-to-metal
seal (GMS)
Carbon electrode
Anode
Liquid cathode
Anode reaction:
+
4Li
4Li +4e
Overall reaction:
4Li + 2SOCI2
(solid) (liquid)
Can
Cathode reaction:
S+SO2+4CI
2SOCI2+4e
4LiCl + S + SO2
(solid) (solid) (gas)
Oilfield Review
Cell chemistry—Lithium, with optimal
electrochemical potential (3-volt) and electrochemical equivalence (3.86-A-hr/g), is
the best choice for downhole battery
anodes. Combined with nonaqueous
thionyl chloride, it has one of the highest
voltages and energy densities of practical
battery systems. During discharge, lithium
oxidation occurs at the negative anode terminal and thionyl chloride reduction takes
place at the positive carbon electrode.
Thionyl chloride serves as both electrolyte
and cathode reactant. The carbon electrode
is not part of the cell reaction, but provides
active sites where reactions take place. There
are six modes of capacity loss and current
stoppage. If the anode is consumed, there is
no electron source and current ceases. If the
catholyte is consumed, there are no reactants to receive electrons. If a cell leaks or
has no active reaction sites, current stops. If
the load, or external circuit, is disconnected,
electrons cannot move to the positive terminal. If the anode and carbon electrode touch,
current will not flow. The ion diffusion limit
is reached when loads are too high for the
available electrode surface area and cells
cannot support external current due to
excessive ion flow that results in depressed
voltage or current.
Premature depletion of most primary
lithium batteries with liquid cathodes occurs
when carbon electrode sites become
blocked by lithium chloride or other solid
discharge products, even if lithium and
thionyl chloride remain. This is called cathode passivation. For safety, lithium cells have
excess thionyl chloride with respect to the
anode material. This stops dendritic growth,
microfingers of lithium metal, on wet-dry
electrode interfaces, which can cause an
internal short circuit. As a result, at the end
of battery life there is thionyl chloride
remaining and minimum wet-dry interfaces.
Cell structure—In a typical cylindrical cell
design, the solid components form concentric shells (above right). For an anode,
lithium foil is swaged against the inner wall
of a stainless steel tube, making the container a negative terminal. Nonwoven glass
paper is placed against the inside diameter
(ID) of the lithium foil as a separator and a
highly porous carbon plug is placed inside
the separator shell. In the center of this carbon plug, a nickel or stainless steel screen
serves as current collector with a connection
to the top-mounted, glass-to-metal seal (GMS)
for outside termination, which is the positive
cell terminal. Liquid thionyl chloride with
dissolved electrolyte salt fills most cell voids.
Hermetically
welded seam
Positive terminal
Cover
Glass-to-metal
seal (GMS)
Top insulator
Separator
Anode
Cathode
Can
Current
collector
Insulating sleeve
Spot-welded
negative terminal
Bottom insulator
■Limited anode-cathode surface. Bobbin-type cylindrical cells are designed for low power
and moderate to low current discharge rates. The solid internal components form concentric
shells. The anode is lithium foil and nonwoven glass paper is used as a separator. A porous
carbon plug forms the interior ring. This plug has a steel screen in the center to collect current and is connected to the positive terminal. Liquid thionyl chloride with dissolved electrolytic salt fills the void spaces. [Adapted from Linden, reference 2.]
The bobbin design with limited anode and
cathode surface area for a given cell size is
suitable for low discharge rates. The challenge is to design electrodes with sufficiently high surface area to satisfy tool load
requirements without decreasing safety.
This can be achieved with appropriate
safety features using the more popular spiral-wrap cell (below).2
Cells are designed to have intrinsic safety at
ambient conditions for battery assemblers
and end users in the field. Failures occur
only if there is a short circuit at hot downhole temperatures. In addition to designing
safe, high-performance cells, the issue of
preventing anodes and cathodes from touching during high-shock and vibration applications like MWD and LWD was addressed by
SPT. Proprietary designs for cylindrical and
annular cells were implemented.
■High anode-cathode surface area. Tightlywrapped, spiral cylindrical cells are commercially available for high power and high current discharge rates. [Adapted from Linden,
reference 2.]
2. Linden D (ed): Handbook of Batteries. New York,
New York, USA: McGraw-Hill, Inc., 1995.
Autumn 1998
47
Troubleshooting Batteries
Short circuits
Causes:
External—conductivity across terminals is too
high; can occur accidentally during pack fabrication or battery operation.
Internal—accidental anode and cathode contact;
can occur during high shock and vibration.
Mechanisms:
Heat and gas are produced in an accelerated
chemical reaction. Solid discharge products form
on the carbon electrode, block surface reaction
sites and gradually decrease current output. An
explosion may occur if cell temperature rises
above 80°C.
Cures:
Design low-rate cells with optimum electrode
surface area.
Confirm safety at ambient and higher temperatures with low resistance, short circuit and
mechanical tests.
Use correct safety fuse at pack level.
Temperature buildup
Causes:
Environmental—increase in external temperature.
Internal—discharge current is too high and poor
heat dissipation.
Mechanisms:
At temperatures above 356°F [180°C], a lithium
anode melts. A short circuit can develop depending
on cell orientation and mechanical shock environment. At temperatures below 180°C, hydrostatic
bursting and leaking welds occur due to thionyl
chloride’s [SOCl2] high thermal expansion coefficient. If external temperature is low, because it is an
exothermic process, high battery temperature can
result from a discharge current that is too high.
Cures:
Use lithium alloy anode to increase melting
temperature.
Leave sufficient void volume when filling cell
with SOCl2.
Assure that battery is right size for tool load.
Design pack to have good thermal conductivity.
Cell charging
Causes:
Battery with single string of cells has an external
charging current.
A charging current flows into a battery when the
positive and negative terminals of an external
power source are connected to the corresponding
battery terminals.
A battery with multiple strings has a charging
condition when one or more strings with a higher
voltage imparts a charging current into a string
with lower voltage.
Mechanisms:
Charging current forces lithium ions to flow backwards towards anode.
Ions plate back onto lithium anode to form fine
dendrites, or metal “fingers.”
High surface area lithium dendrites are unstable
in SOCl2.
48
Cures:
Use a series diode in each battery string.
If required, use series diode even in a singlebattery string to avoid inadvertent charging by
an external power source.
Forced overdischarge
Causes:
Occurs when using two or more cells in a pack.
Less likely to occur if cells have uniform capacity.
Environmental factors like shocks may cause cells
to deplete differentially.
Most likely to occur near the end of battery life.
May take a long time, since driving currents are
small.
Usually does not occur during short periods of
tool operation because cells are relatively fresh.
Mechanisms:
An active cell depletes available lithium and turns
from power source to a sink.
When an external current flows through the cell,
the terminals reverse polarity.
Lithium ions flow back toward anode and
form dendrites.
Dendrites tend to form more along wet-dry
electrolyte interfaces.
Surface dendrites form slowly with low
driving currents.
Cures:
Do not deplete batteries to 0 volts.
Design tool to stop high load at a 2-volt per cell
pack equivalent. For example, 4 volt end-of-life for
a two cell-pack.
Treat packs with an open circuit voltage (OCV)
of less than 3.6-volt per cell pack equivalence
with caution.
Cell leaks
Causes:
Each cell is a miniature pressure vessel with four
weld rings on cylindrical top and bottom lids,
which can become problem areas.
High internal pressure, even on good welds, produces leaks.
Nominal internal pressure on defective welds can
produce leaks.
Mechanisms:
Escaping SOCl2 liquid combines with moisture to
form hydrochloric acid and sulfur dioxide gas.
Hydrochloric acid corrodes container exterior,
forming pits and further damaging the tool housing and electronics, and the battery. Batteries may
also build up internal pressure during thermal
cycling.
Cures:
Fabricate welds with good penetration into the
can material.
Do not overfill cells with thionyl chloride.
Operate batteries within their temperature rating.
Do not subject batteries to repeated thermal cycles.
Lithium Battery Performance
Anode passivation, self-discharge, carbon
pore blocking (cathode passivation) and failure mechanisms influence battery performance and longevity. Designers must
address these factors during development of
downhole power sources.
Anode passivation—When a lithium anode
comes in contact with thionyl chloride electrolyte, while filling a cell for example, a solid
electrolyte interface (SEI), or passivation layer,
is generated immediately on the lithium surface. This thin layer protects the anode surface
from further chemical reaction. However, for
elevated temperature and long storage periods, continuous passivation layer growth
results in capacity loss. Lithium thionyl chloride batteries would not exist without a SEI
layer. The cell reaction would proceed uncontrollably upon initial catholyte filling.
Because of SEI microdefects, the chemical
reaction proceeds at a low rate, but never
stops, resulting in slow layer growth. Less
passivation occurs at low temperatures. The
reaction is faster at high temperatures. For
prolonged storage under no load, especially
at high temperature, the SEI layer gets quite
thick and causes observed voltage delays
when loads are placed on a battery. Voltage
drops below normal operating values initially, but recovers. For severe passivation,
voltage drops farther and takes longer to
recover, if at all. The voltage delay is worse
for batteries stored at high temperatures with
no load for long periods and then discharged
in cold conditions.
To reduce and prevent voltage delay,
lithium batteries should be stored in controlled environments at about 70°F [25°C].
Batteries need to be depassivated before use
by applying a temporary load, which is
higher than the required tool current, until
voltage recovers. If passivation is heavy, it is
best to discharge the pack with a low current
for an extended time before applying higher
currents to address the difference between
thin and thick passivation layers. At SPT, a
special depassivation box was designed to
provide a timed constant current, which turns
off when the proper voltage is met or the battery fails to depassivate in a given time.
Self-discharge—Anode passivation reduces
a self-discharge condition. If a lithium anode
is fully passivated, the rate of self-discharge is
decreased dramatically. Self-discharge also
takes place during normal battery depletion.
When a cell is activated, the SEI is disrupted
continuously by ion flow from the anode surface beneath, exposing more lithium to
thionyl chloride. This process occurs in parallel with the normal electrochemical reaction, resulting in two discharge reactions: an
Oilfield Review
external current and a parasitic internal current. Capacity losses can be appreciable. At
high temperatures for long periods, self-discharge has a more severe effect. The effect is
decreased without sacrificing power output
by minimizing the lithium surface area that is
open to electrolyte fluid.
Carbon pore blocking—During lithium battery discharge, solids form inside and outside
of the carbon electrode. Solids precipitate in
carbon electrode pores and block access to
unused reaction sites, resulting in less capacity and lower voltage. This carbon passivation
occurs late in battery life. Unlike anode passivation, when solid products form on the
anode surface, it is not reversible. Carbon
plugging is more severe for high discharge
rates at low to moderate temperatures.
During discharge, chloride ions, a cathode reaction product, form inside the carbon electrode. Lithium ions, an anode
reaction product, dissolve from the lithium
anode, diffuse toward the carbon cathode
and combine with chloride ions to form
solid lithium chlorine [LiCl]. At low discharge rates, LiCl is uniformly distributed in
the carbon electrode. At higher discharge
rates, Li ions cannot diffuse deep into carbon cathodes because of ion-diffusion rate
limits. Precipitation of LiCl occurs primarily
on the outer carbon-electrode surface,
which plugs the pores and results in inefficient use of interior reaction sites. Higher
temperatures and lower currents increase
ion diffusivity and promote recovery of
some carbon volume. Cathode blocking is
more persistent and much harder to remove
than anode passivation.
Failure mechanisms—Lithium thionyl chloride batteries are ideal for oilfield service;
however, caution must be exercised when
handling or using this chemistry due to high
energy content. Failure modes, which normally cause hot cells or annoying leaks in less
potent batteries may create venting or runaway reactions. The reasons for this difference
are inherent high energy density and hermetic construction of lithium cells. If a short
circuit occurs, it lasts longer, gets hotter and
builds up higher internal pressure, creating a
greater hazard than lower energy systems like
nonhermetic, alkaline cells. There are five
failure modes, which can cause cell or tool
damage from resulting leaks (see
“Troubleshooting Batteries,” previous page).
Custom Cell Development
After a second SSDP battery was successfully
introduced in 1985, the next pack to be
developed was for the MSRT MultiSensor
Recorder/Transmitter tool. The MSRT pack fits
in the tool annular battery compartment and
houses AA-size cells in a carousel configuration (right). The pack is an inexpensive combination of phenolic tubes and end-caps with
cells connected in series and parallel.
The next achievement in battery technology came in 1987 with development of the
first LWD tool. Until this time, Schlumberger
built the packaging, but used commercial
cell vendors. The problems with this
approach were confidentiality, limited control, delivery uncertainty and lack of differentiation from competitors. The key to
advancing to the next stage of battery development was control of cell manufacturing.
■The MSRT MultiSensor Recorder/Transmitter tool. Built using inexpensive phenolic
tubes and end-caps, the MSRT annular battery pack holds AA-size, series and parallel
connected cells in a carousel configuration.
Autumn 1998
49
Top
Fuse
Cell
Bore
Metallic tube
Each cell contains
proprietary structures
for extra ruggedness.
Stiffening
elements
A
Bore
Cells
A
Section A-A
Bottom
■Batteries for drilling. Because MWD and LWD tools are subjected to extended shocks
and vibration, power sources for these applications rely on rugged cell and battery
packaging to meet performance and reliability requirements.
Annular Pack Designs
Cylindrical cells
Packaging was important, but rugged cells
with custom sizes and shapes are essential to
battery success.
A vendor was secured to develop and produce standard and custom lithium cells
quickly, economically and with utmost confidentiality. This move allowed development
of a special lithium cell for LWD tools.
Because LWD tools are part of the drillstring,
the tool and battery pack must endure hundreds of hours of drilling without failures. In
addition, since batteries are a throw-away
item, the LWD battery not only had to use
cost-effective packaging, but also had to rely
on intrinsically rugged cells to help ensure
an overall inexpensive pack (left). After
developing the first custom cylindrical cells,
battery evolution at SPT accelerated. A range
of cylindrical cells of varying diameters,
lengths and electrode structures was developed. The next customized battery development was the unique annular cell.
Downhole tools used inside or in conjunction with oilfield tubulars are cylindrical or
annular. Depending on the tool, battery
compartments are also annular, so the battery and enclosed cells can be annular as
well (below). The advantages of annular batteries are fewer cells, higher capacity, thinner cross sections, lower usage cost and
improved reliability. A large annular cell
replacing an ensemble of small cells in a
carousel array requires fewer connections.
By using annular shapes, it is easier to maximize cell life and minimize the tool volume
that batteries take up. The annular battery also
has a relatively large ID, which minimizes
mud or hydrocarbon flowing pressure drop.
These benefits were verified in two diverse
battery types previously composed of cylindrical cells—the Schlumberger Wireline &
Testing IRIS Intelligent Remote Implementation System tool and Anadrill LWD batteries.
After annular batteries for these tools were
introduced in 1992, other custom cells followed quickly. The ARC5 Array Resistivity
Compensated and RAB Resistivity-at-the-Bit
tools have the same annular design, but differ in ID, OD, length and electrode structure.
Annular cells
■Annular cell designs. For annular shapes it is easier to maximize battery life and
minimize required volume compared to carousel-style cylindrical cells. Annular cell
packs also have fewer electrical connections and are more robust.
50
Oilfield Review
Design Considerations
Past unrealistic tool designer and field enduser expectations highlight confusion and
misunderstandings that exist about batteries.
Cell specifications have included a wide
range of requirements in unique and sometimes conflicting combinations—small,
lightweight, waterproof, long-lasting, never
deplete prematurely, work at any temperature
with all tools, easy to install and remove,
reusable and rechargeable in or out of the
tool for an unlimited number of times, measure remaining power capacity, long shelf life
and no shipping or disposal restrictions.
Tool requirements are dictated by end
users in the field. There are, however, many
specifications that should be defined by tool
designers and battery-specific considerations that should be addressed early in the
development process. Ideally, these issues
should be dealt with before tool electronics
are planned and designed in order to
decrease battery costs and make tools more
“battery friendly.”
Costs can be reduced by minimizing tool
power and voltage requirements. Lower voltage means fewer cells and less expensive
packs. By making packs with larger annular
cells, fewer cells are needed to increase
capacity, more power and longer life are
available per cell and assembly costs are
less. Future rechargeable cell designs could
also reduce cost.
Batteries can be sized for single jobs, or
packs with larger custom cells can be used
multiple times. Tools should be able to
function through the end of battery life and
then shut down safely. Innovative management and depletion techniques will help
better utilize battery capacity and allow
maximum power to be squeezed out of
each pack. Remaining battery life and
available power can be measured by
devices in the pack or the tool can track
cumulative hours of operation.
Autumn 1998
Tools can be made battery friendly by using
a constant or slowly varying load, not
exceeding battery power limits and avoiding
no-load conditions under high-temperature
operation. Battery designs should consider
the electrical load hierarchy (easy to hard) for
batteries—constant resistor, constant current
and power, and short “on” (low-duty) or long
“on” (high-duty) cycles.
Tools need to be fault-tolerant (immune to
short periods of low voltage), function at low
voltage and have a low-power delay for surface check-out or long service. Current
requirements should be reduced as voltage
goes down for benign end-of-life loads and
cumulative amp-hours as well as temperature should be recorded to efficiently manage tool power and battery life.
To make batteries easy and safe to use,
there should be external voltage access and
a disconnect mechanism when battery packs
are installed. Concentric connector terminals should be used if possible, and batteries
should not be a force- or load-bearing tool
component. A pressure seal between the battery compartment and tool electronics is
desirable, and an external leak indicator is
also needed.
In addition to SPT Power Source Products
Group experience and technical expertise,
other less visible factors like a systems design
approach positively influence battery technology implementation. Consolidating and
prioritizing user expectations are done early
in battery development, ideally before tool
electronics are designed.
The Design Process
When a battery is requested, many parameters need to be specified by both battery and
tool developers before cell design and development are initiated (below). The goal is to
establish mutually agreed, achievable objectives based on realistic input and constraints,
so user expectations can be met. Using batteries or cells developed and tested previously
is also an option. Cells from a proven battery
can be adapted to meet new requirements—
more or less cells or different packaging.
Battery specifications need to detail deliverables, schedules and testing. To initiate the
design process, a project resumé stating the
requirements that batteries must fulfill is prepared (see “LWD Battery Project Resumé,
next page). This record, which also helps
designers optimize tool electronics to use
batteries efficiently, includes operating temperatures, tool-specific power consumption,
pack and minimum operating voltage, shelf
and operating life at various temperatures,
physical dimensions, mechanical shock,
vibration and safety testing, and transportation certification. The project resumé, initially written by tool designers for battery
developers and subsequently modified by
both parties, documents the design process.
After completion of a project resumé, engineers design and test prototype batteries. The
next step is pilot testing. Batteries are built and
run on actual jobs in the field under the supervision of tool designers. After a predetermined
number of successful jobs, the batteries and
tools go into the commercialization phase.
■Battery Design Input: A System-Level Approach.
Input Responsibility
Specifications
Chemistry
Battery shape
Cell design
Tool load
Battery-tool interface
Use environment
Safety requirement
Battery cost
Shipping and disposal
Battery
developer
X
Tool
designer
End user
X
X
X
X
X
X
X
X
X
X
X
X
X
X
51
Qualification Testing
LWD Battery Project Resumé
Objective
Develop custom lithium battery to power LWD tool. Tool will be subjected to unusual shock and vibration
stimuli at temperatures exceeding 150° C.
Description
The pack will fit inside the LWD tool battery compartment (see dimensions below) and provide N watts
of tool power for Y days at 175° C.
Technical and performance advantages
This battery must provide uninterrupted power for continuous tool operation during drilling. Previous
tools suffered intermittent power outages due to mechanically induced battery short circuits. High power
capacity is required to ensure tool operation during the total life of a drill bit.
General specifications
Description
Target
Current
Verification
Dimensions
Length, in.
24
28
C, L
OD, in.
2.5
2.8
C, L
Connector termination
TBD
–
L
Power
Open circuit voltage (OCV)
29
Same
L
Minimum voltage
20
Same
L
Nominal current, amps
0.3
TBD
L, F
Independent operation, days
14
TBD
L, F
Temperature range
0 to 175° C
TBD
L, F
Mechanical shock
TBD
TBD
L, F
Price, $USD
<500
TBD
E
Legend
C: Design calculations E: Experience from previous study F: Field testing
L: Laboratory testing
TBD: To be determined
Feasibility status and development plans
In this section, battery users and designers list concerns, specific technical challenges and issues that
affect timely development of the proposed battery. Previous work that may serve as a guide is typically
included in this section.
As projects progress, this section is used to document changes to original specifications or schedules.
Updates are performed at specific milestones during product development reviews as indicated in the
schedule. Changes and input are made as needed and communicated to users or battery designers when
they occur.
Schedule
Project launch
Feasibility review
Field prototype available
Field test conclusion
Commercialization
Plan
First quarter 1998
Second quarter 1998
Third quarter 1998
First quarter 1999
Second quarter 1999
Actual
First quarter 1998
Second quarter 1998
Fourth quarter 1998
TBD
TBD
Project team
These groups represent the core team that is responsible for the development of specific batteries.
Typically, each battery developer has three to five projects, as well as field support responsibility for
batteries developed previously.
Battery developer
Assigned engineer
Mechanical engineer
Electrical engineer
Mechanical or electrical technician
Manufacturing engineer
Buyer
52
Tool designer
Project leader
Mechanical engineer
Electrical engineer
The battery qualification program was optimized after years of testing and analysis.
Upon completion of safety, electrical and
mechanical qualification testing at SPT, batteries are manufactured for field tests. Tool,
field and SPT battery engineers work closely
during testing. Any issues that arise from field
tests are addressed quickly to obtain a quality
product that meets or exceeds expectations.
Staffed with a research team, electrical and
mechanical design engineers, and technicians, SPT fully supports battery sustaining
issues anywhere in the world.
A blast-resistant building was built at SPT
specifically to perform qualification tests and
troubleshooting. The Battery Electrical and
Shock Testing (BEST) facility has 10 explosion-resistant bays with ovens, specially
designed shock machines, and a control
room in which tests in each bay are monitored by computers (next page, left). Testing
capability of the BEST facility is being
expanded due to the increased number of
tools and new business opportunities requiring special batteries. Battery manufacturing
and engineering at SPT achieved ISO 9001
certification in August 1996.
Tests are sometimes conducted to determine failure modes and severity, and to
ensure that batteries are not operated near
failure. Cells in various stages of depletion
are placed in a pressure vessel and subjected
to 15,000 psi [103 MPa] at temperatures up
to 302°F [150°C] to simulate flooding of a
battery housing. Battery packs are also
placed in a tool and heated until they
explode to verify that tools can be recovered
in the event of a downhole failure. Cells with
various types of lithium chemistry are tested
to determine the temperatures that can be
tolerated without affecting battery performance or compromising safety. Cells have
even been crushed hydraulically to simulate
being dropped or physically damaged.
A short circuit can occur accidentally during manufacturing and assembly or in severe
field conditions. Safety is of utmost importance, so tests are performed to determine if
the cells can withstand a direct short without
leaking or venting. New batteries undergo
short-circuit testing at room temperature and
122°F [50°C] to confirm that cells are safe at
all ambient temperature conditions.
To assess battery autonomy, electrical tests
are performed at the cell and pack levels.
Initial testing is performed on the cells with a
static resistive or constant current load to
determine cell performance at the maximum
rated temperature. Results are analyzed and, if
necessary, the cell is redesigned. Next, a sim-
Oilfield Review
■Battery shock and vibration testing.
■Testing. The Schlumberger Battery Electrical and Shock Testing (BEST)
facility (top) in Rosharon, Texas, is equipped with 10 blast-resistant test
bays with ovens (middle), a specialized shock and vibration apparatus,
and control room with computers to monitor each bay (bottom). This
facility is being used for both internal Schlumberger battery qualifications and outside testing services.
Autumn 1998
ulated pack consisting of the proper number
of cells is tested at the maximum rated temperature under simulated tool load. Once
autonomy as defined by the project resumé is
confirmed, mechanical qualification begins.
One of the toughest environments for a
battery in the oil field is MWD service, which
often positions the batteries directly above
the drill bit. The MWD batteries undergo rigorous mechanical and electrical qualification tests to ensure that they can withstand
drilling shocks and vibrations while supplying continuous power to operate the tool.
A battery pack is manufactured for
mechanical testing. New batteries are placed
in mock-up housings and mandrels with the
same electrical connections as actual tools.
By using a custom shock machine and specifications similar to those used for qualifying
MWD and LWD tools, every lithium battery
design is shock and vibration tested prior to
field testing. During testing, battery voltage is
monitored to check for packaging damage,
cell leaks and electrical integrity (above).
Once shock testing is complete, the battery
is placed in a test bay and cycled from maximum rated temperature to room temperature while being depleted. Thermal cycling is
repeated until the end-of-life voltage is
reached. These tests are needed because batteries are used multiple times during their
service life. The battery is checked for physical and electrical damage to determine if
there are problems with swelling, internal
wiring or cell venting. The battery is then
depleted and thermal cycled again to determine deep discharge effects.
Battery packs manufactured at SPT undergo
qualification tests to meet US and international certification for transportation of
lithium batteries in cargo aircraft. In 1997, the
53
US Department of Transportation (DOT)
revised testing requirements to incorporate
more stringent testing than was previously
done for shipping regulation compliance. Six
series of tests are now required for lithium
batteries—altitude simulation, extreme temperature and short circuit; vibration, shock
and short circuit; vibration, shock and
charge; internal short circuit; vibration, shock
and low-capacity cell; and forced discharge.
In some instances, vibration testing is
required to make sure that the resonant frequency of a battery pack is not reached,
which could result in failure during transportation. A cumulative database that is
kept with each Project Resumé includes
performance and DOT test results, design
drawings and communications, key documents, and e-mails or messages that affect
project milestones.
Oilfield Applications
Dozens of battery types are manufactured
by SPT for three service sectors—wireline
logging and well testing, measurementswhile-drilling and seismic surveys. Each
battery has different requirements and specifications (below).
Logging, monitoring and testing wells—
Tools to monitor flow, pressure and temperature are typically small and mandrel-shaped,
and fit inside production tubing. Run time
can be a few hours or more than two weeks.
Well conditions are often below 150°C,
although temperatures as high as 200°C are
encountered in limited severe service. Power
consumption is usually less than a watt.
Semipermanent monitors are small—about
1.2-in. OD—tools placed on tubing hangers
to record pressure and time for long-term
monitoring of hydrocarbon flow in producing fields. Operations can last up to 90 days,
but temperatures are usually below 320°F
[160°C]. Power requirements are low, typically less than an eighth of a watt.
Drillstem testing (DST) involves temporary
production of formation intervals. There are
two types of battery-operated DST tools. The
first type opens and closes valves to control
downhole flow remotely. The second tool
records pressure during the period of flow.
The first requires short power pulses
repeated up to 30 times, while the second,
like production logging tools, requires constant low power.
Controlling direction and measuring while
drilling—Drilling control and measurements-while-drilling are the most demanding and critical of all battery applications. In
these applications, tools are located just
above the drill bit and subjected to tremendous shock and vibration, and rugged custom battery designs offer distinct advantages.
Batteries need to last for the life of a drill bit,
up to two weeks. Temperatures of most jobs
are 212°F [100°C], but in some cases they
climb above 150°C when mud circulation
stops. These batteries are usually either short
with large diameters or long with small
diameters depending on the tool.
Acquiring seismic data—In exploration
applications, tools on streamers behind specially equipped vessels or in land equipment
require power to record seismic surveys.
Marine batteries power a clamp-on tool that
gives streamers lift for depth control and provide compass location information. Normally,
marine tools use nonrechargeable and land
tools use rechargeable batteries. Operating
temperatures range from 0 to 50°C. Critical
factors for marine batteries are operating lives
measured in months and safe handling characteristics. Marine batteries are typically
shorter than 24 inches, about 2 inches in
diameter, and weigh less than 5 pounds.
Battery disposal—In the past, disposal of
lithium batteries was left for field organizations to handle. The Power Source Products
group recently took over responsibility for
disposing of lithium cells manufactured by
SPT. For no additional cost, field locations
worldwide can now transport used batteries
to a facility in Cuyanosa, Texas for safe and
efficient disposal. This uniform process can
minimize user costs, hassles, adverse environmental effects and liability.
■Oilfield Battery Applications.
Service type
Wireline logging
Drilling measurements
Seismic data acquisition
54
Typical
battery
OD, in.
Typical
battery
length, in.
Voltage
potential,
V
Energy
capacity,
A-hr
1 to 1.5
1.5 to 4.5
2
15
25 to 100
15
7
20 to 60
7
12 to 28
28 to 30
34
Longer Life: New Power Source Directions
There are several ways to extend the usable
life of batteries. The first, managing electrical
loads, is essential in a total power source
design and development program. Two other
keys to improving battery service are the
capability to measure and track remaining
power, and innovative methods to get maximum capacity from a battery. Each method
ensures that batteries are fully depleted
before discarding.
Battery-life indicator—One way to achieve
maximum battery consumption is to measure
elapsed A-hr. A battery can be retired when a
majority—80% to allow for safety margin—of
the capacity is consumed. The best place to
record A-hr is in the tool, rather than in the
battery. In this way, electronics can be easily
altered to record time and tool load, and a
database of battery performance can be generated. Adding electronics to batteries
increases cost in addition to reducing reliability and independent operation.
A major disadvantage of tool-resident electronics is that if a battery is removed from the
tool, the A-hr record is decoupled from the
pack unless field personnel make a note on
the pack. Either way, a mark must be made
on the battery to record remaining life. The
best method is one that is integral to the battery and does not depend on recording
elapsed parameters. Ideally, a battery-life
indicator should be compatible with existing
tools and associated directly with the battery.
Battery-life indication for lithium thionyl
chloride chemistry is difficult because most
measurable parameters, like voltage and
internal resistance, do not vary appreciably
with discharge until near the end of battery
life.3 In addition, general lithium battery voltage performance at ambient temperatures
depends greatly on the growth of LiCl surface layers at the internal electrodes, which
results in cell passivation. At elevated temperatures, passivation effects are significantly
diminished. Electronic interrogation of batteries to determine remaining life is also
complicated by additional internal battery
resistance, which depends on thermal and
load history as well as temperature.
3. Milewits M: “A Novel Method to Determine Lithium
Battery State of Charge,” in Savadogo O and Roberge
PR (eds): Proceedings, Second International
Symposium on New Materials for Fuel Cell and
Modern Battery Systems. Montreal, Quebec, Canada:
Ecole Polytechnique de Montreal (1997): 358-367.
Milewits M: “Intrinsic Method to Determine Lithium
Thionyl Chloride Battery Capacity,” presented at the
38th Power Sources Conference, Cherry Hill, New
Jersey, USA, June 8-11, 1998: 65-68.
Oilfield Review
Tool run 1
Position 1
Pack A
(100 hr)
Pack A
(50 hr)
Tool run 2
Pack A
(0 hr)
Position 1
Pack B
(96 hr)
Pack B
(46 hr)
Pack B
(0 hr)
Pack C
(100 hr)
Pack C
(98 hr)
Pack C
(96 hr)
Start
Logging
Finish
Switch
pack
Position 2
Pack B
(100 hr)
Pack B
(98 hr)
Pack B
(96 hr)
Start
Logging
Finish
Position 2
■Sequential MWD and LWD battery depletion. The battery pack in first position is removed from the tool at the surface. The pack from
second position, which was operating on standby current and as a backup power source, is moved into first position. A new battery is
placed in second position. This procedure ensures that a “fresh” battery pack is available if a drill-bit run is longer than anticipated.
The anode electrode structure can be
altered to overcome these restrictions and
give an indication of the depth of discharge
upon application of a defined load at ambient temperatures. Typically, for oilfield applications, depletion is at high temperature, but
interrogation occurs between jobs at ambient conditions. Battery-life indication was
demonstrated in the field with the UNIGAGE
well-testing battery for pressure and temperature recording. This battery is scheduled to
be commercial in late 1998.
Sequential depletion—The need for constant power on MWD jobs prompted a new
method of salvaging batteries and extending
operating life. To ensure that tools would not
run out of power during a drilling job, field
engineers put in new battery packs when a
tool was tripped out of a well. This procedure
was followed even if a battery was only partially depleted. As a result, there was a large
Autumn 1998
backlog of partially used batteries in the field
that engineers were reluctant to re-use.
To overcome this problem, during a
planned upgrade of tool sensors, the power
electronics were modified to allow more efficient battery usage. This “sequential depletion” method was conceived and
implemented by Anadrill engineering. It
requires the use of two packs, one at a time,
in temporal sequence. While the first battery
provides primary power, the second battery
is on standby with a small background current load to minimize passivation.
Switching and replacement occur at the
surface. A two-battery configuration allows
maximum power to be extracted from each
pack and there is a fresh battery in case of
an extended run with a partially depleted
pack (above).
Rechargeables
Rechargeable, or secondary batteries, are
used in automobiles, power backup systems,
energy storage and consumer electronics.
Recharge capability depends on the electrochemical reaction. Primary battery reactions
are irreversible. When the active materials
are consumed during discharge, the battery
is completely depleted. For rechargeables,
reactions at the anode and cathode are
reversible. Active materials on both electrodes can almost be fully recovered by electronic recharging.
Although a secondary battery is typically
less costly than the corresponding number of
required primary batteries, there are disadvantages. Rechargeables typically have a
third to a quarter of the energy density of primary batteries. Secondary cells have higher
self-discharge rates, their capacity varies
with each recharge cycle, and special equipment and training are required to properly
recharge the battery. There are three major
55
polymer electrolyte and a layer of cathode
material. This geometry minimizes ion diffusion losses across the polymer film and
enhances charge and discharge efficiency.
It is possible that a high-temperature stable polymer will be developed for downhole use. Obstacles like thermal stability,
manufacturing feasibility and cost are currently being addressed. In addition, other
rechargeable chemistries are being monitored and evaluated for potential use in
high-temperature markets.
Smaller
400
350
300
Ni-MH
200
100
50
0
20
Ni-Cd
150
Li-Ion
250
PbAcid
Volumetric energy density, W-hr/kg
■Energy densities of
common secondary
batteries. Chemistries
with the highest volumetric and gravmetric energy content make the
smallest and lightest
rechargeable cells,
respectively. However, the energy
densities of most
practical secondary
batteries are far less
than primary electrochemistries like
lithium thionyl
chloride.
40
60
80
100
120
Gravimetric energy density, W-hr/kg
Fuel Cells
140
160
Lighter
rechargeable chemistries—lead-acid, alkaline with nickel cathode and nonaqueous
with lithium anode (above).
Lead-acid—Cells based on lead-acid
chemistry have been around since Gaston
Plante developed the first practical battery in
1859, and are familiar for starting and lighting vehicles, and uninterrupted stand-by
power. Heavy weight, and low gravimetric
and volumetric energy density limit portable
electronic applications.
Alkaline with nickel cathode—Alkaline
batteries are currently the most popular
rechargeables for portable electronics.
These batteries include nickel-cadmium
[Ni-Cd], nickel-metal hydride [Ni-MH],
nickel-zinc [Ni-Zn] and nickel-iron [Ni-Fe]
cells. The cathode is a nickel electrode
[NiOOH/Ni(OH)2] and potassium hydroxide
[KOH] is the common electrolyte. The Ni-Fe
battery with heavier iron anode and less
gravimetric energy density is not widely
used. The Ni-Zn battery, due to the cycle limitation of a Zn anode, is still in development.
The Ni-MH battery with higher energy density and similar discharge performance, but
fewer environmental concerns, is replacing
the Ni-Cd battery in portable electronics.
Nonaqueous with lithium anode—These
advanced batteries include lithium-ion and
lithium-metal polymer chemistries. Lithiumion batteries, with the highest energy density
of rechargeables, are produced at a rate of
several million per month for consumer electronics. Lithiated carbon [LiC6] is the anode,
lithium metal oxide is the cathode and the
electrolyte is a nonaqueous organic solution.
By replacing liquid electrolyte with a solid
polymer electrolyte, the lithium polymer battery achieves the versatility and safety of an
all-solid design. The dry polymer electrolyte
plays a dual role as ionic conductor for a
56
current pathway and electronic insulator—
separator—inside the battery. Solid-state
lithium polymer cells for industrial applications are under development.
For surface application at ambient temperature, commercial Ni-MH, lithium-ion and
lithium metal rechargeable batteries may
potentially replace lead-acid batteries. The
high energy density could reduce the size
and weight of battery packs. Presently, however, these advanced batteries are limited by
their relatively small size. The largest commercially available size is only about 5 A-hr
compared with 50 to 100 A-hr (typical car
battery), which are normally found in leadacid batteries. To put the same energy into a
battery pack, larger numbers of cells need to
be connected in series or parallel. The resulting battery management becomes more
complex during charge and discharge. In
addition, designing a rugged mechanical
package that houses many small cells and
survives high shock is challenging.
Designing the largest cell size possible is
clearly the way to proceed for optimal performance and reliability of both primary and
secondary cells.
For downhole application, aqueous alkaline and lithium-ion cells are not suited for
high temperature because of the low boiling
point of liquid electrolytes—aqueous and
nonaqueous. However, a lithium-metal polymer rechargeable battery with solid electrolyte adds little vapor pressure to the battery
and is less prone to volatile reaction with the
lithium anode. In addition, the plastic characteristics of the polymer electrolyte allow
thin laminates to be placed on the lithium
metal surface to enhance thermal and electrical conductivity, and uniform current distribution. Manufacturing involves winding a
tight spiral wrap of three thin electrodes—
lithium foil laminate, lithium ion-conducting
Fuel cells (FC) also generate power from electrochemical reactions, but unlike batteries,
which have a finite amount of chemicals, fuel
is stored separately from the reaction zone.
Theoretically, a fuel cell will run forever if
fuel and oxidizer are supplied continuously.
Small fuel cells (less than 500 watts) can
replace batteries in some ambient surface
applications. If power is required for long
periods of time, fuel cells have an advantage
over batteries because the fuel source is
expandable and renewable. Fuel cells can
also have a higher energy density than batteries with resident fixed chemical supplies if
designed properly. Tripling battery life
requires a cell that is three times larger, but
only the fuel tank size needs to be tripled for
a fuel cell. Three types of fuel cells are applicable at low or medium temperatures.
Proton exchange membrane (PEM) fuel
cells use an ion exchange membrane electrolyte. A polymer film with good acid functionality provides higher proton mobility for
high power density. The typical PEMFC
operating temperature is from 68 to 212°F
[20 to 100°C]. These fuel cells can be sized
for milliwatt to kilowatt power. In smaller
sizes, the systems are simple with few moving parts, start quickly and can be throttled
up and down rapidly. Presently, high cost
limits wide application.
No fuel cell is right for all applications. For
small 10-W to 1-kW portable applications,
the PEMFC offers the most advantages and a
great deal of flexibility, ranging from passive
systems with no moving parts for lowpower applications to systems with the
complexity and power output of internal
combustion engines.
These systems can use hydrogen or
methanol directly as fuel. For surface applications, an air-breathing system is used
instead of pure oxygen to supply oxidizer. A
direct methanol PEMFC permits the convenient use of a liquid fuel that is easy to transport. Refueling is like filling the gas tank on
a lawn mower. A hydrogen-fueled PEMFC is
Oilfield Review
Load
Electron flow
-
(H2O)
Water and
waste heat
e
H
+
Cathode
Anode
(H2)
Hydrogen fuel
(O2)
Air supply
Proton conducting membrane
Anode reaction:
+
H2
2H +2e
Cathode reaction:
+
O2+4H +4e
2H2O
■Battery manufacturing, packing and shipping at SPT.
■Typical fuel cell process.
refueled by pressurized tank. Because hydrogen oxidation electrocatalysts support current densities an order of magnitude greater
than methanol oxidation electrocatalysts and
oxidize hydrogen at a higher cell potential
than methanol can be oxidized, a hydrogenfueled PEMFC operates at higher power density. This leads to smaller systems to meet
given power requirements. Safer gas storage,
such as metal hydride hydrogen devices, are
available. Hydrogen stored as a metal
hydride is not under high pressure. If the tank
ruptures, release is slow and hydrogen dissipates quickly instead of pooling like propane
or butane because it is lighter than air and
has a rapid diffusion rate.
The phosphoric acid fuel cell uses liquid
phosphoric acid in an inorganic matrix as
the electrolyte and must operate at 302 to
428°F [150 to 220°C] due to the poor acid
ionic conductivity at low temperatures. The
phosphoric acid FC is the most advanced
system for commercial power generation.
High operating temperatures and low power
density—the major disadvantage—make this
system suitable only for multikilowatt stationary applications, but it is efficient in this
role. Most phosphoric acid FCs operate on
natural gas from pipelines or on-site storage.
Autumn 1998
Alkaline fuel cells, which are used in the
space shuttle, employ aqueous potassium
hydroxide [KOH] as the ionic conducting
electrolyte. The operating temperature range
is 68 to 248°F [20 to 120°C]. A fundamental
problem limiting alkaline FC use is sensitivity of the basic electrolyte (aqueous KOH) to
carbon dioxide [CO2], which forms potassium carbonate [K2CO3] and precipitates.
These systems use only pure oxygen, making
them impractical in most applications.
Fuel cells generate waste heat as a result of
chemical inefficiency. The key reaction in
most fuel cells is hydrogen oxidation. In a
fuel cell, this process is split into two halfcell reactions that are carried out separately.
In an acid-type, PEMFC for example, hydrogen is oxidized to produce protons and electrons at the anode. Protons pass through an
acid electrolyte to the cathode, where oxygen is reduced to form water. The electrolyte
is an electronic insulator, so electrons cannot
pass to the cathode; instead they flow to the
external circuit, providing electric current.
Fuel cells use pure oxygen or collect air from
the atmosphere (above left).
The full range of fuel cell applications is
being studied at SPT. Not only are physical
and mechanical constraints—confining
geometry, high shock, wide temperature
ranges—tighter, but costs are also higher
than for batteries. If extended life and safe
high power are to be realized, these obstacles must be overcome.
A Systems Engineering Approach
By utilizing modern manufacturing and testing facilities, and an expanding base of
knowledge, expertise and experience,
Schlumberger Perforating & Testing develops,
designs and manufactures battery packs for a
variety of oilfield services and measurement
applications (above). These developments
include custom mandrel packs made with
cylindrical and annular cells, which currently
use liquid cathode lithium chemistries and
unique, proprietary electrode constructions.
Higher temperature cells for greater than
200°C, solid-cathode rechargeable cells and
other new power-source technologies are currently being pursued for oilfield services and
other applications outside of the industry.
As in the case of primary batteries, the key
to successful, cost-effective, high-performance secondary batteries and fuel cells is
concurrent engineering of mechanical, electrical and operational tool factors—a systems
approach. This approach helps battery engineers, tool designers and end users meet the
challenges of supplying power to advanced
downhole and surface tools.
—MET
57