Designing Battery Packs for Thermal Extremes

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Designing Battery Packs
for Thermal Extremes
By Jeff VanZwol, Marketing Manager, Micro Power
Electronics, Beaverton, Ore.
While battery packs enable individual cell capacities to be scaled in series/parallel configurations to
suit portable applications, they must also provide
thermal management in extreme environments.
B
attery packs for ruggedized portable devices must
operate in both extreme hot and cold environments. Many of these devices, such as handheld
radios, telemetry monitors, weather stations,
test equipment, missiles, rockets and satellites,
are used in harsh environments. There are unique design
considerations and techniques that must be considered when
designing and manufacturing a pack that will be operated in
extreme environments from -40°C to +80°C.
The main components of a typical battery pack are shown
in Fig. 1. The cells serve as the primary energy source. The
printed circuit board provides the intelligence of the system for advanced functions such as fuel-gauge calculations
on remaining cell capacity, protection circuitry, thermal
sensors used to monitor internal pack temperature, LEDs
that indicate pack or cell status, and a serial data communications bus that communicates with the host device. A custom
plastic enclosure is typically produced in an injection mold.
External contacts provide a physical electrical interface, and
insulation is used to absorb external shock, as well as retain
or dissipate heat generated within the pack.
All of these elements can be customized when designing a battery pack for high- or low-temperature operation.
However, the cells are the elements critically affected by
extreme temperatures.
subsequently, expose the user to harmful chemicals or even
flames. Therefore, each rechargeable chemistry has its own
set of risks in addition to its desirable attributes.
For example, sealed lead acid (SLA) cells use concentrated
Top plastic enclosure
(cover)
PC board
Cells
Temperature
sensors and
adhesive
tape
Insulation
Optimizing Battery Chemistry
External
contacts
Advances in battery technology have led to increased
energy densities over the last few decades. More reactive
materials have been employed to achieve these advances,
and active safety circuits are now required to ensure that
certain battery chemistries are kept in a stable condition.
With careful design, incidents involving battery rupture or
explosion are rare. Nevertheless, it should be recognized
that under certain conditions, such as high temperature
or punctured cells, the pack integrity can be breached and,
Power Electronics Technology July 2006
Bottom
plastic
enclosure
Fig. 1. While all of the elements within a battery pack can be customized when designing for high- or low-temperature operation, the cells
are most critically affected by extreme temperatures.
40
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Charge condition: 4:20 V-CVCC, 1430 mA max, 100 mA cutoff at 20°C
Discharge condition: 2040 mA-CC, 3.0 V cutoff at 20°C, 45°C
4.5
2.900
2.700
2.500
Voltage (V)
Voltage (V)
4.0
3.5
45°C
20°C
0°C
3.0
2.5
0
Li/MnO2
Li/SO2
2.300
2.100
1.900
1.700
400
800
1200
1600
Discharge capacity (mAh)
2000
1.500
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
Time (s)
2400
Fig. 2. Li-ion cells operating at lower temperatures exhibit voltage
droop and reduced capacity.
Fig. 3. Voltage delay at lower temperatures is less in primary Li/MnO2
cells than in Li/SO2 cells. (Courtesy of UltraLife Batteries.)
sulfuric acid electrolyte and toxic heavy metal electrodes,
and provide a nominal voltage of 1.5 V. SLA cells are costeffective, but are too bulky and heavy for most portable
applications. SLA cells have a wide operating temperature,
ranging from -40°C to +70°C.
Nickel metal hydride (NiMH) cells include a nominal
voltage of 1.25 V, 500 duty cycles per lifetime, an optimal
load current of less than 0.5 C, an average energy density of
100 Wh/kg, a charge time of less than 4 hours, a typical
discharge rate of approximately 30% per month when in
storage and a rigid form factor. NiMH cells operate effectively
between -20°C and +60°C.
Lithium-ion (Li-ion) cell characteristics include a
nominal voltage of 3.6 V, 1000 duty cycles per lifetime, a rate
load current of less than 4 C, an average energy density of
160 Wh/kg, a charge time of less than 4 hours and a typical
discharge rate of approximately 1% to 3% per month when
in storage. Li-ion cells operate effectively between -20°C and
+60°C. However, new chemical formulations are extending
that range to -30°C and +80°C.
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41
Power Electronics Technology July 2006
BATTERY PERFORMANCE
Remaining capacity (%)
100%
80%
T1 = 65°C
60%
0°C
20°C
45°C
60°C
40%
20%
0%
0
2
4
6
8
10
T2 = 64°C
12
Storage time (month)
TCORE = 66°C
Fig. 4. The negative effects of self-discharge increase with higher
temperatures.
Fig. 5. Thermistors registered operating temperature at the core
and edge of the pack, as well as outside the enclosure, during an
operational test.
Among these chemistries, Li-ion requires the greatest
degree of protection, including a thermal shutdown separator and exhaust vents (within each cell) to vent internal
pressure, an external safety circuit that prevents overvoltage
during charge and undervoltage during discharge, and a
thermal sensor that prevents thermal runaway. However,
with the appropriate level of safety designed into a Li-ion
pack, Li-ion offers the most attractive method of portable
battery power. Many of the portable devices using the older
chemistries have migrated to Li-ion in recent years.
If rechargeable chemistries cannot perform in the
required temperature ranges, disposable, one-time-use
lithium cells—known as lithium-primary cells—should be
considered. These lithium-primary cells feature a nominal
voltage of 3.6 V, an optimal load current of less than 5 C, an
average energy density of 160 Wh/kg and a negligible selfdischarge rate supporting years of storage. The operating
temperature for lithium-primary cells ranges from -40°C
to +80°C. Examples of lithium-primary chemistries include
lithium thionyl chloride (Li/SOCl2), lithium sulfur dioxide
(Li/SO2) and lithium manganese dioxide (Li/MnO2).
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Temperature Effects
Much like humans, temperature affects a battery’s performance during both rest and work. How Li-ion cells react and
perform depend on the temperature under which they are
operating. Many of the principles that hold true for lithium
also apply to other rechargeable chemistries such as nickel
cadmium, NiMH and alkaline.
The performance of rechargeable Li-ion chemistry starts
to suffer as the temperature drops below 0°C, causing the
internal impedance of the battery to increase. The result of
this effect is shown in Fig. 2, where a 2-A load causes the
voltage to droop. This voltage droop is more pronounced
at -20°C, and the electrolytic material within the cell will
freeze with further temperature declines. Cell capacity is also
reduced during the lower temperatures. If these cells are used
or stored at or below -50°C, irreparable damage may occur
under certain conditions to internal separators within the
cells, making the cells a safety hazard.
If extremely low temperature requirements eliminate
Li-ion as a viable chemistry, one should consider using
lithium-primary cells, because they operate down to -40°C.
Cells based on Li/MnO2 chemistry use a solid cathode, while
Li/SO2 cells use a liquid cathode.
Liquid cathode systems suffer from a voltage-delay phenomenon, which causes the resulting voltage to be momentarily suppressed when a load is applied, particularly after
extended periods of storage. Voltage delay is related to the
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Power Electronics Technology July 2006
TCASE = 54°C
42
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BATTERY PERFORMANCE
growth of a cell’s passivation layer and is exacerbated when
the cell is stored at a high temperature and then a load is
placed on it at a low temperature. As shown in Fig. 3, this
condition results in the possibility that a liquid-cathode battery may not perform at low temperatures when required.
The Li/MnO2 system is less susceptible to the voltage-delay
phenomenon at lower temperatures.
Storage temperature affects the subsequent performance
of Li-ion cells. As Fig. 4 shows, under
optimal storage conditions of 20°C, a
fully charged Li-ion cell has a natural selfdischarge of 1% per month. However, with
an elevated storage temperature of 60°C or
a 12-month period, the capacity naturally
discharges down to 40% of the original
capacity. This drastic self-discharge substantially limits the
run time of the cells after storage.
Additionally, after storage this fully charged cell would
only have a recoverable capacity of 70% of the original
capacity. Although not shown in Fig. 4, a cell stored at 60°C
for 12 months at 50% state-of-charge would have a recoverable capacity of 90%. This demonstrates the wisdom of
storing cells at a 40% to 50% state-of charge during transport
and prior to regular use.
Extremely high temperature operation provides equal
challenges for cells based on lithium chemistry. As mentioned
previously, the upper range of safe operation for Li-ion and
lithium-primary cells is 60°C. Cells provide energy through
the electrochemical shuttling of Li-ions between the anode
and cathode materials. However, at high discharge rates this
chemical reaction generates heat, and the effects of this heat
must be factored into a sound battery-pack design. The effect
of the generated heat is compounded in a multicell pack.
The Li/MnO2 system is less susceptible to the voltagedelay phenomenon at lower temperatures.
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To demonstrate the typical temperature rise in a multicell
pack, Micro Power has assembled a pack and monitored the
temperature rise using environmental and electrical test
chambers. This testing should be performed on packs that
must operate near the upper or lower boundaries of the cell
specifications. The pack assembled for the test was a 4S6P
(four cells in series, six strings in parallel) Li-ion pack using
18650 (18-mm diameter, 65-mm length) 2.4-Ah cells, resulting in a pack that provides 14.4 V and 14.4 Ah of capacity.
Testing was conducted at a condition of 145-W discharge
43
Power Electronics Technology July 2006
BATTERY PERFORMANCE
Fig. 6. Thermal imaging of the battery pack shows uniform heat generation and distribution.
at 45°C ambient temperature. Thermistors were placed within the pack core
and around the outside edge of the
cells. The pack was wrapped in packing
material to simulate a plastic enclosure,
and an additional thermistor was
placed on the outside of the packing
material to capture the temperature
outside the simulated enclosure.
Temperature and performance data
Power Electronics Technology July 2006
were recorded on automated Maccor
battery-testing equipment.
Results of the testing are presented
in Fig. 5. The TCORE thermistor placed
within the cells in the center of the
pack registered a core temperature of
65°C. The two thermistors (T1 and T2),
placed at the edge of the assembled
cells, registered a perimeter temperature of 64°C and 65°C. Note that the
44
temperature at the core and edge of the
pack is similar. The cylindrical 18650
cells used in this test are efficient at
distributing heat generated from the
center of the cell to the cell exterior.
Additionally, these cells are resistant
to absorbing heat from external sources.
Hence, cells located in the center of
the pack assembly and surrounded by
other cells had a similar temperature to
cells located on the edge of the pack,
which have fewer neighboring cells.
Fig. 6 shows a thermal image of the
pack during the test, and one can see
that the temperature of the interior
and edge cells are similar. Finally, the
TCASE thermistor outside the packing
material registered a temperature of
54°C.
This test demonstrates that one can
expect up to a 20°C rise in temperature
when the pack is in operation, which
may result in a 9°C temperature rise
of the pack enclosure. Note that the
temperature changes are dependent
on the amount of current drawn from
the pack (i.e. greater current results in
greater heat generation). These temperature increases, both within and
outside the pack, must be factored
into the design of the battery pack and
portable device.
This level of testing and analysis
should be performed on all pack
designs that must operate in low- or
high-temperature conditions. Intimate
knowledge of the usage profile allows
the pack to be designed in a manner
that avoids thermal stress to the pack.
Temperature extremes, especially
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BATTERY PERFORMANCE
high temperature, may pose design
challenges for battery packs. Mil-spec
requirements may specify extended
operating temperatures up to 80°C.
However, most commercial Li-ion cells
are specified to operate from -20°C to
+60°C; therefore, thermal monitoring
and heat dissipation within the battery
pack is critical for high-temperature
operation. If a pack will be used in
high-temperature environments, then
specific design principles should be
applied to that pack.
First, when charge is introduced
(via charging current) or removed
(during discharge) from a battery
at high rates, there is an associated
temperature increase, which can be
dangerous. The pack circuitry should
use a thermal sensor to disconnect
the cells at a specified temperature.
This eliminates thermal runaway and
overheating.
Second, the placement of circuitry
within the pack is critical. The circuit
board may have heat-generating
components, such as a FET, and improper placement could result in the
FET heating the cells. The application
of heat to select cells within a pack
erodes the longevity and safety of
that pack.
Third, packs may be designed with
vent holes to dissipate generated heat
or exhaust vented gases from cells.
Multiple vent holes may be used to increase airflow in and out of the pack.
A final consideration is the position of the pack in relation to any
heat-generating components, such as
high-performance processors, operating within the host device. Uneven
heating may cause the cells to behave
differently from their companions in
the pack, thus shortening the pack life
and compromising safety.
If these design techniques cannot
safely extend a rechargeable Li-ion pack
up to the target (high) temperature,
one should consider using lithiumprimary cells to power the device
because lithium-primary cells have an
operational range of -40°C to +80°C
with very low self-discharge.
Environmental requirements may
specify extended operating temperawww.powerelectronics.com
tures down to -40°C. Rechargeable
Li-ions operate at -20° to +60°C. When
challenged with this requirement, there
are several design options to maximize
electrical output at low temperatures.
A heater embedded with the pack
can warm cells prior to use. The embedded heater can be powered from
the main cells within the pack or by
an external source such as a charger or
another battery pack. Embedded heaters can heat cells, reduce electrolyte
viscosity and lower voltage droop or
delay prior to use.
The host device may be designed
to pulse discharge cells prior to primary discharge; this self-warms the
cells via the I2R heating effect. This
technique is applicable when the duty
cycle is predictable and cyclical (i.e.,
periodic transmission of telemetry
report), rather than a random or haphazard duty cycle (i.e., handheld radio
transmission).
Super capacitors embedded within
a pack may provide immediate energy to the host device while cells
warm up to their optimal electrical
performance.
Typically, vent holes are unobstructed openings that expel potential
gas vented from cells (when the cells
are stressed or misused). High-pressure vent holes may relieve and exhaust
warm air only after a specific pressure
has been reached within the pack. As
long as the pressure is not increased
to a dangerous level with the pack,
high-pressure vent holes can retain
heat generated within the pack.
Lastly, if these design techniques
cannot extend operations of a rechargeable Li-ion pack down to the
target (low) temperature, one should
again consider using lithium-primary
cells to power the device. When assessing lithium-primary formulations,
Li/MnO2 provides less voltage droop
than Li/SO 2 and Li/SOCl 2 in cold
temperatures.
PETech
www.bussco.com/busbar
Visit
to download our design guide or brochure. Call toll free: 866-481-5137
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Power Electronics Technology July 2006
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