Chapter 6
Batteries
Types and Characteristics ● Functions and Features ●
Specifications and Ratings
 2012 Jim Dunlop Solar
Overview
 Describing why batteries are used in PV systems.
 Identifying the basic components of battery construction.
 Defining battery terminology for battery specifications, ratings
and operating parameters.
 Classifying common types of storage batteries and their
performance characteristics.
 Understanding the NEC and OSHA requirements for battery
installations and safety.
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Batteries: 6 - 2
Battery Fundamentals
 A battery is an electrochemical cell that stores energy in
chemical bonds. Chemical energy is converted to DC electrical
energy when a battery is connected to a load.
 Batteries are used in PV systems for the following purposes:

To store energy produced by the PV array and supply it to electrical loads
as needed.
 To operate the PV array and electrical loads at stable voltages.
 To supply surge currents to electrical loads or appliances.
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Batteries: 6 - 3
Battery Cell Design
 A cell is the basic electrochemical unit in a battery.
+
Positive plate
-
Separator
Electrical load
Negative plate
Case
Electrolyte
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Batteries: 6 - 4
Battery Components:
Definitions
 Plate: The positive or negative electrode in a battery cell,
consisting of a grid and active material.
 Active Material: The reactant materials that comprise the positive
and negative plates.
 Grid: A metal alloy framework that supports the active material
on a battery plate.
 Separator: A porous, insulating divider between the positive and
negative plates.
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Batteries: 6 - 5
Battery Components:
Definitions (cont.)
 Electrolyte: A conducting medium which allows the flow of
current via ionic transfer between the battery plates.
 Case: A container which encloses the plates, separators and
electrolyte in a battery.
 Vent: a cap on flooded battery cells that also serves as a means
to fill the battery. For sealed batteries, the vent is also a pressure
regulating valve.
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Batteries: 6 - 6
Battery Construction
 The major components of battery construction are shown in this
cut-away view.
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Batteries: 6 - 7
Battery Capacity
 An ampere-hour (Ah) is the
common unit of battery energy
storage capacity, equal to the
transfer of one ampere for one
hour.
 Capacity depends on the battery
temperature, discharge rate and
cut-off voltage.
 2012 Jim Dunlop Solar
Voltage (V)
 Capacity is a measure of the
stored electric charge or stored
energy that a battery can deliver
under specified conditions.
Low discharge rate
High discharge rate
Cut off voltage
Capacity (Ah)
Batteries: 6 - 8
Battery Discharging
 Discharging is the process when
a battery delivers current under
the application of an electrical
load.
110
100
90
80
o
Percent of 25 C Capacity
 The discharge rate is the time in
hours required to fully discharge
a battery at a given current to a
specified cutoff voltage.
120
C/500
C/50
C/0.5
70
60
C/120
C/5
50
40
30
 Lower temperatures and high
discharge rates reduce available
battery capacity.
 2012 Jim Dunlop Solar
-30
-20
-10
0
10
20
30
40
50
o
Battery Operating Temperature ( C )
Batteries: 6 - 9
Battery State-of-Charge
 State-of-charge (SOC) is the percentage of energy stored in a
battery compared to a fully charged condition.
 Depth-of-discharge (DOD) is the percentage of capacity that has
been withdrawn from a battery compared to the total fully
charged capacity:

DOD = 100% - SOC
 Allowable depth-of-discharge is the maximum limit of battery
discharge in system operation.

Cut-off voltage is the lowest voltage that a battery is allowed to operate
and defined by the charge controller or equipment low voltage disconnect
set point. The cut-off voltage defines the allowable depth-of-discharge and
usable battery capacity at a specific discharge rate.
 2012 Jim Dunlop Solar
Batteries: 6 - 10
100
0
80
20
Average daily DOD
60
Max allowable DOD
30
Summer
 2012 Jim Dunlop Solar
Depth of Discharge (%)
State of Charge (%)
Battery State-of-Charge
Winter
Batteries: 6 - 11
Self-Discharge Rate
 Higher temperatures result in
higher self-discharge rates.
 Lead-antimony types and older
batteries have higher selfdischarge rates.
Self Discharge Rate (%/month)
 Self-discharge is due to internal
losses within a battery that
reduce state-of-charge over time.
20
15
Lead-Antimony Grid
(end of life)
Lead-Antimony Grid
(new)
Lead-Calcium Grid
(typical)
10
5
0
-50
-25
0
25
50
o
Operating Temperature ( C)
 2012 Jim Dunlop Solar
Batteries: 6 - 12
Battery Charging
 Charging is the process when a battery receives or accepts
current from a charging source, such as a PV array, and
quantified by the charge current or rate.
Bulk Stage
Battery
Voltage
Battery
Current
Increasing
Voltage
Maximum
Charge
Current
Absorption Stage
Float Stage
Bulk Charge Constant
Voltage
Float Voltage
Reducing
Absorption
Current
Float Current
Time 
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Batteries: 6 - 13
Effects of Charge and Discharge
Rates on Battery Voltage
Battery Voltage (V)
16
15
C/5 Charging
14
C/20 Charging
13
C/20 Discharging
12
11
C/5 Discharging
10
0
20
40
60
80
100
Battery State of Charge (%)
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Batteries: 6 - 14
Lead-Antimony Charging
Voltage vs. Battery SOC
3.0
Cell Voltage (volts)
2.9
Lead-Antimony Grids
Charge Rate
2.8
C/2.5
2.7
2.6
Gassing Voltage at 0 oC
C/5
2.5
C/20
Gassing Voltage at 27 oC
2.4
Gassing Voltage at 50 oC
2.3
2.2
2.1
2.0
0
20
40
60
80
100
Battery State of Charge (%)
 2012 Jim Dunlop Solar
Batteries: 6 - 15
Chemical Reactions for the
Lead-Acid Cell
At the positive plate or electrode:
PbO2 + 4 H + + 2e − ⇔ Pb 2 + + 2 H 2 O
Pb 2 + + SO42 − ⇔ PbSO4
At the negative plate or electrode:
Pb ⇔ Pb 2 + + 2e −
Pb 2 + + SO42 − ⇔ PbSO4
Overall lead-acid cell reaction:
PbO2 + Pb + 2 H 2 SO4 ⇔ 2 PbSO4 + 2 H 2 O
 2012 Jim Dunlop Solar
Batteries: 6 - 16
Sulfation and Stratification
 Sulfation is the process where lead-sulfate crystallizes on battery
plates when left at partial state-of-charge, and reduces capacity.
 Stratification is a condition that can occur in taller batteries when
electrolyte concentration varies vertically in the battery cell.
Resulting higher concentrations at the bottom of the cell
accelerate battery degradation and loss of capacity.
 Proper charging can minimize the effects of sulfation and
stratification.
 2012 Jim Dunlop Solar
Batteries: 6 - 17
Battery Types
 Primary batteries are not rechargeable.
 Secondary batteries are rechargeable.
 Lead-acid batteries are classified based in their design and
intended service:

Starting, Lighting and Ignition (SLI) – not typically used in PV systems
 Motive Power or Traction
 Standby or Stationary
 Batteries are also classified as either flooded or sealed valveregulated types.
 2012 Jim Dunlop Solar
Batteries: 6 - 18
Flooded and Sealed Batteries
 Flooded batteries have a liquid (fluid) electrolyte.

Open-vent types have removable caps that permit electrolyte additions.
 Sealed-vent types have non-removable caps and do not permit electrolyte
additions.
 Valve-regulated lead-acid batteries have an immobilized
electrolyte and sealed pressure-relief vents.

Gelled types immobilize the electrolyte by the incorporation of additives.
 Absorbed glass mat (AGM) types immobilize the electrolyte in glass
separator mats.
 2012 Jim Dunlop Solar
Batteries: 6 - 19
Types of Lead-Acid Batteries
Flooded Lead-Acid
Batteries
Valve-Regulated LeadAcid Batteries
Gelled
Absorbed Glass Mat
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Batteries: 6 - 20
Flooded Nickel-Cadmium
Batteries
 Flooded pocket-plate nickel-cadmium batteries are used in some
critical and low temperature PV applications.
 Advantages include long life and low maintenance, and excellent
deep discharge and low temperature performance.
 Disadvantages include high cost and limited availability.
 Electrolyte is a flooded potassium-hydroxide solution.
 2012 Jim Dunlop Solar
Batteries: 6 - 21
Battery Characteristics
BATTERY TYPE
ADVANTAGES
DISADVANTAGES
FLOODED LEAD-ACID
Lead-Antimony
Lead-Calcium Open-Vent
low cost, wide availability, good deep cycle
and high temperature performance, can
replenish electrolyte
low cost, wide availability, low water loss,
can replenish electrolyte
high water loss and maintenance
low cost, wide availability, low water loss
medium cost, low water loss
poor deep cycle performance, intolerant to
high temperatures and overcharge, can not
replenish electrolyte
limited availability, potential for stratification
medium cost, little or no maintenance, less
susceptible to freezing, install in any
orientation
medium cost, little or no maintenance, less
susceptible to freezing, install in any
orientation
fair deep cycle performance, intolerant to
overcharge and high temperatures, limited
availability
fair deep cycle performance, intolerant to
overcharge and high temperatures, limited
availability
wide availability, excellent low and high
temperature performance, maintenance free
only available in low capacities, high cost,
suffer from ‘memory’ effect
excellent deep cycle and low and high
temperature performance, tolerance to
overcharge
limited availability, high cost, water additions
required
Lead-Calcium Sealed-Vent
poor deep cycle performance, intolerant to
high temperatures and overcharge
Lead-Antimony/Calcium Hybrid
VALVE-REGULATED
LEAD-ACID
Gelled
Absorbed Glass Mat
NICKEL-CADMIUM
Sealed Sintered-Plate
Flooded Pocket-Plate
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Batteries: 6 - 22
Effects of Temperature
on Battery Life
 Lower operating temperatures
reduce battery capacity but
increase cycle life.
Effects of Temperature on Battery Life
 Higher temperatures accelerate
corrosion of the grids and result
in greater gassing and
electrolyte loss.
 2012 Jim Dunlop Solar
1000
Battery Life
(% life at 25 oC)
 For vented batteries, a 10°C
increase in average operating
temperature above 25°C reduces
battery life by 50%. This is worse
for VRLA batteries.
Lead-Antimony Grids
Lead-Calcium Grids
Nickel-Cadmium
100
10
5
10
15
20
25
30
35
40
45
o
Battery Operating Temperature ( C)
Batteries: 6 - 23
Electrolyte Properties
 Batteries must be protected from
freezing at low state-of-charge.
1.35
Electrolyte Specific Gravity
 Electrolyte concentration is
measured by its specific gravity,
and related to battery state of
charge.
1.3
1.25
1.2
1.15
1.1
1.05
1
0
-3.3 -7.8
-15
-27
-52
-71
Electrolyte Freezing Temperature (oC)
o
Specific Gravity
H2SO4 (Wt%)
H2SO4 (Vol%)
Freezing Point ( C)
1.000
1.050
1.100
1.150
1.200
1.250
1.300
0.0
7.3
14.3
20.9
27.2
33.4
39.1
0.0
4.2
8.5
13.0
17.1
22.6
27.6
0
-3.3
-7.8
-15
-27
-52
-71
 2012 Jim Dunlop Solar
Batteries: 6 - 24
Battery Design and
Selection Criteria
 Electrical properties

Voltage, capacity, charge/discharge rates
 Performance

Cycle life vs. DOD, system autonomy
 Physical properties

Size and weight
 Maintenance requirements

Flooded or VRLA
 Installation

Location, structural requirements, environmental conditions
 Safety and auxiliary systems

Racks, trays, fire protection, electrical BOS
 Costs, warranty and availability
 2012 Jim Dunlop Solar
Batteries: 6 - 25
Battery Connections
 Batteries are first connected in series to achieve the desired DC
system voltage for utilization equipment.
 Series connections of batteries are connected in parallel to
increase energy storage capacity (amp-hours).
 2012 Jim Dunlop Solar
Batteries: 6 - 26
Series Battery Connections
Battery 1
+ 12 volts 100 amp-hours
Battery 2
+ 12 volts 100 amp-hours
Total:
+
24 volts
-
100 amp-hours
 2012 Jim Dunlop Solar
Batteries: 6 - 27
Parallel Battery Connections
+
Battery 1
+ 12 volts 100 amp-hours
Battery 2
+ 12 volts 100 amp-hours
 2012 Jim Dunlop Solar
Total:
12 volts
200 amp-hours
-
Batteries: 6 - 28
OSHA Requirements for
Battery Installations
 Unsealed batteries must be installed in ventilated enclosures to prevent
fumes, gases, or electrolyte spray entering other areas, and to prevent
the accumulation of an explosive mixture.
 Battery racks, trays and floors must be of sufficient strength and
resistant to electrolyte.
 Face shields, aprons, and rubber gloves must be provided for workers
handling acids or batteries, and facilities for quick drenching of the eyes
and body must be provided within 25 feet of battery handling areas.
 Facilities must be provided for flushing and neutralizing spilled
electrolyte and for fire protection.
 Battery charging installations are to be located in designated areas and
protected from damage by trucks. Vent caps must be in place during
battery charging and maintained in a functioning condition.
 2012 Jim Dunlop Solar
Batteries: 6 - 29
NEC Requirements for
Battery Installations
 Battery installations in dwellings are limited to less than 50 volts,
nominal unless live parts are not accessible during maintenance
[690.71(B)].
 Live parts must be guarded for all battery systems in dwellings
regardless of voltage [690.71(B)(2)].
 Live parts on any battery installations 50 volts and greater must
be guarded [490.9, 110.27].
 Sufficient working spaces and clearances must be provided for
any battery installations [110.26].
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Batteries: 6 - 30
Battery Overcurrent Protection
 Battery circuit conductors must
be protected from overcurrent in
accordance with Art. 240 [690.9].
 Current-limiting overcurrent
devices may be required for
large battery banks with high
fault currents [690.71(C)].
 Fuses energized from both
directions must be able to be
disconnected from all sources
[690.16].
 2012 Jim Dunlop Solar
Batteries: 6 - 31
Battery Disconnects
 A disconnecting means must be
provided for all ungrounded
battery circuit conductors
[690.15, 480.5].
 Disconnecting means must be
provided for battery systems
greater than 48 volts to isolate
the battery system to sections
no more than 48 volts for service
and maintenance [690.71(E)].
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Batteries: 6 - 32
Grounding Battery Systems
 Battery systems are considered to be grounded when a currentcarrying conductor of the connected PV source is grounded
[690.71(A), 690.41].
 Battery systems over 48 volts are permitted without a grounded
circuit conductor where all of the following apply:

The PV and load circuits are grounded,
 Both ungrounded battery circuit conductors have overcurrent protection
and disconnecting means, and
 A ground-fault indicator is required for the battery system [690.71(G),
690.35].
 2012 Jim Dunlop Solar
Batteries: 6 - 33
Battery Wiring Methods
 Flexible cables are permitted to facilitate battery connections
[690.74, 400].
 Cables must be rated for hard service and moisture resistance.
Welding cables are not allowed [690.74].
 Fine stranded cable must use lugs and terminals approved for
such cables [690.74, 110.3].
 2012 Jim Dunlop Solar
Batteries: 6 - 34
Battery Racks and Trays
 Metal racks must be painted or
otherwise treated to resist
degradation from electrolyte and
provide insulation between
conducting members and the
battery cells [480.9].
 Conductive racks are not
permitted to be located within
150 mm (6 in.) of the tops of the
nonconductive battery cases
[690.71(D)].

Does not apply to sealed batteries
that are manufactured with
conductive cases.
 Conductive battery racks, cases or
trays must also have proper
equipment grounding.
 2012 Jim Dunlop Solar
Batteries: 6 - 35
Battery System Ventilation
 Ventilation of explosive battery gasses is required [480.9].
 Vented battery cells must incorporate a flame arrestor, and
sealed batteries must have pressure relief vents [480.10].
 2012 Jim Dunlop Solar
Batteries: 6 - 36
Summary
 Batteries are used in stand-alone PV systems to store energy
produced by the PV array for use by electrical loads as required.
 Batteries also establish the operating voltage for PV arrays and
DC utilization equipment, such as charge controllers, inverters or
DC loads.
 The types of batteries and their performance characteristics vary
widely.
 Battery energy storage capacity is a function of temperature,
discharge rate, cutoff voltage and age of the battery.
 Battery installation and safety requirements are covered in the
National Electrical Code® and OSHA safety standards.
 2012 Jim Dunlop Solar
Batteries: 6 - 37
Questions and Discussion
 2012 Jim Dunlop Solar
Batteries: 6 - 38