This paper will delineate the criteria required for the selection of batteries as a power source for space experiments. Four basic types of batteries will be explored, lead acid, silver zinc, alkaline manganese and nickel cadmium. A detailed description of the lead acid and silver zinc cells while a brief exploration of the alkaline manganese and nickel cadmium will be given. The factors involved in battery selection such as packaging, energy density, discharge voltage regulation, and cost will be thoroughly examined. The pros and cons of each battery type will be explored. Actual laboratory test data acquired for the lead acid and silver zinc cell will be discussed. This data will include discharging under various temperature conditions, after three months of storage and with different types of loads. A description of the required maintenance for each type of battery will be investigated. The lifetime and number of charge/discharge cycles will be discussed

Francisco, David R.

English

NASA Technical Reports Server

N93-13 84
Battery Selection for Space Experiments
David R. Francisco
NASA-Lewis Research Center
Abstract
This paper will delineate the criteria required for the selection of batteries as a power source for
space experiments. Four basic types of batteries will be explored, lead acid, silver zinc, alkaline
manganese and nickel cadmium. A detailed description of the lead acid and silver zinc cells while a
brief exploration of the alkaline manganese and nickel cadmium will be given. The factors involved in
battery selection such as packaging, energy density, discharge voltage regulation, and cost will be
thoroughly examined. The pros and cons of each battery type will be explored. Actual laboratory test
data acquired for the lead acid and silver zinc cell will be discussed. This data will include
discharging under various temperature conditions, after three months of storage and with different
types of loads. A description of the required maintenance for each type of battery will be investigated.
The lifetime and number of charge/discharge cycles will be discussed.
Space Experiment Battery Selection Overview
When designing a space experiment (for the purpose of this paper a space experiment is defined as
hardware that is flown on the shuttle or any other vehicle that provides a platform for microgravity
research) one of the primary concerns is the power source for the experiment. If a battery is
selected, many factors contribute to this selection. Complete load characteristics, power
requirements, energy requirements, and storage period must be considered in order to choose the
correct battery. The available weight, volume restrictions and the environment in which the
experiment will be operated is also critical in determining the correct battery for a space
experiment.
Other factors that contribute to the selection of a battery is the location in which the experiment
will operate. If the experiment will operate in an inhabited environment, then additional precautions
must be taken. The cost of the.battery and the maintenance required also plays a major role in
selection of a battery. In this paper all of these factors will be considered in the description of each
battery type.
Battery Terminology
Before a detailed description of each battery type can be started, a understanding of battery
terminology is necessary. The following sections explain the relevant terms needed in understanding
the application of batteries.
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TThe capacity (C) of a battery is usually defined as the number of amp-hours that a fully charged cell
can supply. The capacity is affected by the rate at which the battery will be discharged, the
temperature of the battery when discharging, and the state of charge. The capacity listed on a battery
is valid only when the the battery is discharged at 1/10 (C/10) of the listed amp-hours (i.e. a 15
amp-hour battery discharged at 1.5 amps for 10 hours). If the battery is discharged at a faster rate,
the capacity will significantly decrease. Different types of batteries perform better at higher
discharge rates than others, this will be discussed in each of the battery type descriptions.
A cycle on a battery is one sequence of charging and discharging. Only secondary batteries have
cycles.
The process of returning electrical energy to a battery. Secondary cells require charging.
Discharge Voltage Regulation
Discharge voltage regulation is defined as the stability of the batteries voltage during discharge. Some
batteries, such as lead acid cells, will slowly decrease in voltage as they are discharged until the
voltage drops quickly at the end of capacity. Other cells such as silver zinc cells will remain at one
voltage level for a majority of the discharge cycle, then it will drop dramatically.
De oth of Discharge (DOD_
This is the percent of the capacity discharged in relation to rated capacity from a battery.
Electrolyte is the medium used to transport ions within the battery.
E[m.r. 
Energy is the total amount of power delivered by the battery over time. This is calculated by taking
the capacity (amp hours) times the average discharge voltage (= watt-hours).
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Open Circuit Voltage (OCVI
The OCV of a battery is voltage measured when no load is applied to the battery. Measurement of the
OCV is an indicator of the state of charge for many types of batteries.
Power
Power is defined as the number of watts discharged from a battery at a specific instance in time (such
as the peak power was 150 watts).
Primary Battery
A primary battery can not be charged and is only used once.
Sealed cells do not require maintenance and are usually equipped with a high pressure vent. Most of
these types of cells contain the minimal amount of electrolyte (see the section on Lead Acid Cells).
Secondary_ Battery_
A secondary battery is one that can be recharged to the original full capacity.
rated for the maximum number of charge/discharge cycles.
This type of battery is
Self-Discharge Rate
Self-discharge rate is the rate at which a battery (or cell) loses capacity while standing idle (in
storage). This rate is affected by the temperature at which the battery is stored. This parameter is
critical when selecting a type of battery for a space experiment.
This concludes the discussion of battery terminology. The following sections will discuss the use of
lead acid, silver zinc, alkaline manganese and nickel cadmium cells with respect to space
experiments.
Sealed Lead Acid (SLA) Cells
Lead acid cells are secondary cells that are commonly used in the automotive industry. The cells used
in autos have a liquid electrolyte (H2SO4 - sulfuric acid), where as the cells considered for space
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experiments are sealed lead acid cells (SLA). The SLA cells have a minimum amount of electrolyte
and are maintenance free because they operate on a gas recombination principle (the oxygen and most
of the hydrogen generated within the cell are recombined, therefore no additional water is needed).
The anode is made of lead and the cathode consist of lead dioxide (PbO2). The cases are made of metal
(isolated) and are sealed with a resealable pressure valve.
k
Lead acid cells have a low internal resistance (on the order of 5 m_) which allows the cells to source
high current. A 12.5 amp-hour cell (rating at C/10 discharge rate) is rated at 9.0 Amp-hours when
discharged at 12.5 Amp/hour rate (1C). In comparison to most other batteries SLA cells are much
heavier for the same capacity.
The number of cycles that can be obtained from a lead acid cell can vary between 200 to 2,000
depending on the use of the cell. The two factors that affect the life are the depth of discharge and the
charging method. If the cells are discharged 100% during each cycle and charged without a current
limit, then the life will end at about 200 cycles. If the cell is only discharged 25% and charged with
a current limit of 2 Amps the life will be closer to 2,000 cycles.
Stored SLA cells should be trickle charged with a voltage approximately 2.25V to 2.30 V per cell. If
the cells are not trickle charged, then they will loose a noticeable amount of capacity during storage.
At room temperature the specifications indicate that these cells will loose 20% of their capacity after
6 months. Test data reveals that SLA cells stored at room temperature decreased in capacity by 35%
after 5 1/2 months. At lower temperatures the cells will retain more capacity.
TemDerature Performance
Lead acid cells perform better than other types of cells at lower temperatures. As per the
specifications of sealed lead acid cells, a 15% reduction in energy is seen at a temperature of 0 C for a
discharge rate of C/10. If the cells are discharged at a rate of 1.2C the reduction energy is
approximately 35 %. A battery made of 16 cells with a capacity of 12.5 Amp hours was discharged at
32 F. These cells were discharged at a C/6 rate. Approximately a 8 % reduction of energy from a
room temperature discharge was documented during this test.
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Voltaae Characteristics
The discharge voltage of lead acid cells varies from approximately 2.15 volts when fully charged to
1.6 volts when completely discharged. The temperature at which the discharge occurs will affect the
discharge voltage. At 0 C the discharge voltage will be slightly (0.05 volts) lower.
Maim  alzc 
These cells required very little maintenance. No additional electrolyte has to be added during the life
of the cells. Charging of the cells requires a constant voltage between 2.4-2.6 volts per cell.
Standard battery chargers may be readily purchased. Determining the state of charge of SLA cells is
easily achieved by taking the open circuit voltage (OCV) and substituting this voltage in the following
equation: % charge = (OCV-1.98V)500. The only characteristics of the cells that may require
additional maintenance is that these cells may need to be cycled (discharged deeply -90% of rated
capacity) after an extended storage in order to achieve full capacity.
SLA cells are very inexpensive, require very little maintenance and can withstand many cycles. The
voltage regulation and the storage charge capacity retention is average. The major disadvantage to
these cells is the weight and volume required to obtain the desired energy. Relative to other battery
types, these cells require the most weight and volume to achieve the same capacity. Therefore if
minimal energy (< 300 watt hours) is required and considerable weight and volume is available
(30Kg and .36 m2) then SLA cells should be considered.
Silver Zinc
Silver zinc cells are secondary cells that are usually used in military and aerospace applications.
Silver zinc cells have a liquid electrolyte of potassium hydroxide (KOH) with the negative electrode of
zinc and the positive electrode as silver. For space experiment applications the cell has a resealable
vent valve. The case is made of a styrene plastic that is very durable.
These cells are purchased "dry, uncharged" or "dry, charged." Either type requires filling the
cells with electrolyte. The dry uncharged cells require a formation process (see the maintenance
section) and the dry charged cells may be used immediately after the filling of electrolyte. Both types
of cells have the same characteristics, which are described in the following sections.
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Silver zinc cells have a low internal resistance (on the order of 3.5 m_) which allows the cells to
source high current. A 15 Amp-hour cell is rated at 20 Amp-hours when discharged at C/10 at the
beginning of life. The capacity at C/3 rate was tested at the beginning of life (initial cycle), a 15 amp
hour cell supplied approximately 20 amp hours. Subsequent cycles showed a slight decrease (10%)
in capacity. In comparison to most other batteries silver zinc cells are much lighter and much more
energy dense (5 times greater than SLA cells).
i
The number of cycles that can be obtained from a silver zinc cell can vary between 100 to 200
depending on the use of the cell. The major factors that affect the life are the depth of discharge,
maintenance and storage conditions.
Silver zinc cells may be stored in the dry state (no electrolyte) for at least five years in a dry
environment. Once the cells are wetted (filled with electrolyte), the active life is usually 1 to 2
years depending on the number of cycles.
Once the cells are wetted and charged, they are specified to retain 85% of their capacity when
stored at room temperature for 3 months. Testing has verified this specification.
Temperature Performance
Silver zinc cells do not perform as well as lead acid cells at lower temperatures. As per the
specifications of silver zinc cells, a 22% reduction in energy is seen at a temperature of 0 C for a
discharge rate of C/10. Testing of a 19 cell 15 amp-hour battery at 0 C indicated a 18% reduction in
capacity.
=
z
Voltage Characteristics
The discharge voltage characteristic of silver zinc cells is a dual plateau (two separate and distinct
voltage levels during discharge). The first plateau is 1.75 volts/cell for approximately 20% to 30%
of capacity and for the remainder of the capacity the voltage is at 1.5 volts/cell. Once the second
plateau is reached the voltage is very stable for silver zinc cells.
Maintenance
These cells required a considerable amount of maintenance.
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Initially the cells must be filled with
electrolytewhich requires safety precautions (KOH is a caustic) and is time consuming. If the
uncharged cells are used, then a formation process is required. This process requires the cells to be
charged initially and then discharged at a C/3 rate. If the rated capacity is not achieved then the cycle
must be repeated. These cells have a resealable vent valve and they do vent during their lifetime.
This requires the box that encloses the cells (and any components in contact with the cells such as an
electrical connector) to be impervious to KOH. In most cases an epoxy is used to pot the cells with a
opening for the vent valve. Charging of the cells requires a constant current with a shutoff of
approximately 2 volts per cell. Usually a custom battery charger is required. Some manufacturers
produce battery chargers that may be readily used. Determining the state of charge of silver zinc
cells from their open circuit voltage (OCV) may be done. The OCV will tell if the cells are greater
than 70% capacity (cell voltage approximately 1.85 V) or less than 70% (cell voltage
approximately 1.6 volts).
Silver Zinc Conclusion
Silver zinc cells are very expensive, require considerable maintenance, and can withstand a limited
number of cycles. Packaging also requires additional provisions. The voltage regulation and the
storage charge capacity retention is excellent. The major disadvantages to these cells is the
maintenance required and the cost. Relative to other battery types, these cells require the least
weight and volume to achieve the same capacity. Therefore if weight and volume is a concern and
considerable energy is required, silver zinc cells should be considered.
Alkaline Manganese Dioxide
Alkaline cells are primary cells that are used for most household electronic appliances. The type of
alkaline most widely used is alkaline manganese dioxide (Zn-MnO2). These cells utilize zinc as the
anode and manganese dioxide as the cathode and potassium hydroxide as the electrolyte. These cells
are cylindrical and the case is usually made of steel.
Electrical Characteristics
Alkaline cells have an internal resistance of less than 1 _ and are suited for low current draw over a
long period of time. A nominal D-cell will deliver 0,8 amps for 15 hours. The starting voltage will
be 1.5 volts/cell and its end of life will be at approximately 1.0 volts/cell. The charge retention
after storage for alkalines is excellent and is not a concern when used for space experiments. These
cells are specified to retain less than 75% of their capacity at 0 C. The overall cost is low and the
maintenance required is minimal.
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_kaline Cell Conclusion
Alkalines offer excellent storage capabilities and are very inexpensive. One disadvantage is that if
substantial amounts of current (> 1.0 Amps) is required, parallel batteries are needed which adds
weight, volume and complexity to the fusing and wiring of the batteries. The voltage regulation
during discharge varies significantly and the loads must be able to accommodate this variation. The
operation of these cells at lower temperatures is a concern and should be thoroughly tested.
Nickel Cadmium
Nickel cadmium cells are secondary cells that use KOH as the electrolyte, cadmium as the anode and
nickel (NiOOH) as the cathode. These cells are commonly used for household electrical appliances that
are rechargeable. The case is made of nickel plated steel and is very rugged.
Electrical Characteristics
The internal resistance of nickel cadmium cells is on the order of 15 m_. These cells are suited for
moderate rates of discharge. Most cells have a capacity (C) in the range of 1 to 3 amp hours. The
discharge voltage ranges from 1.35 volts/cell at full capacity to 1.05 volts/cell at the end of
capacity. Nickel cadmium cells lose approximately 1% of capacity per day. The capacity retention
due to storage is not suitable for space experiments.
Nickel Cadmium Conclusion
Even though new higher capacity nickel cadmium cells have been developed, the limiting factor with
nickel cadmium for space experiment applications is their storage retention. Since most space
experiments are stored for 3 months before flight with no access, nickel cadmium cells would not
have sufficient capacity to operate an experiment. Nickel cadmium cells should not be considered for
space experiments with an extended storage period.
Conclusion
Table I summarizes the characteristics of each type of battery discussed. The most important
consideration when choosing a battery type for a space experiment application is the conditions in
which the battery will operate. Details of the loads (power profile) of the experiment, the
environment (temperature) of operation, and the available weight, volume and funds available are
required to accurately choose the correct battery. Detailed testing under the worst case conditions
that the space experiment will experience should also be conducted before choosing a specific battery
type. It is also worth noting that there are many other types of batteries that may be used for space
experiments.
tL--
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REFERENCES
1. Battery Energy Data Manual, Whittaker-Yardney Power Systems.
2. DuraceU Battery Data Guide For Designers, Duraceli U.S.A., Tarrytown, New York, 1981.
3. Duracell Technical Bulletin, Alkaline, Duracell Inc. Bethel, CT, October 1991.
4. Gates Energy Standard Cyclon Battery Catalog, Gates Energy Products, Denver, Colorado, January
1986.
5. Gates Sealed Nickel Cadmium Rechargeable Cells, Gates Energy Products, Gainesville, Florida,
October 1987.
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