Lithium cell safety is a major issue with both manufacturers and end users. Most manufacturers have taken great strides to develop the safest cells possible while still maintaining performance characteristics. The combining of lithium cells for higher voltages, currents, and capacities requires the fabricator of lithium battery packs to be knowledgable about the specific electrochemical system being used. Relatively high rate, spirally wound (large surface area) sulfur oxychloride cells systems, such as Li/Thionyl or Sulfuryl chloride are considered. Prior to the start of a design of a battery pack, a review of the characterization studies for the cells should be conducted. The approach for fabricating a battery pack might vary with cell size

Ciesla, J. J.

English

NASA Technical Reports Server

N87-II079
Safety Considerations for Fabricating
Lithium Battery Packs
James J. Ciesla
Electrochem Industries, Clarence, New York
ABSTRACT
Lithium cell safety has been a major issue with both manufacturers and
end users since first discussed by Brooks (I) and Warburton (2) at the
1974 Power Sources Symposium. Host manufacturers have taken great
strides to develop the safest cells possible while still maintaining
performance characteristics. The combining of lithium cells for higher
voltages, currents, and capacities requires the fabricator of lithium
battery packs to be knowledgeable about the specific electrochemical
system being used.
The following will refer to relatively high rate, spirally wound (large
surface area) sulfur oxychloride cell systems, such as Li/Thionyl or
Sulfuryl Chloride. Prior to the start of a design of a battery pack, a
review of the characterization studies for the cells should be
conducted. The approach for fabricating a battery pack might vary with
cell size.
INTRODUCTION
All lithium cell manufacturers recommend limits on the use of a cell for
any given application. These limits may or may not have an incorporated
safety factor. For the purpose of building a battery pack from any of
the cells that are available, it would be prudent to treat the
advertised specifications as a working limit. Battery pack designers
must look at the same abusive conditions as cell designers. These are
high-rate discharge, shorting, forced discharge, charging, overheating,
and mechanical abuse. Although single cells may react well under one or
more of the above abusive conditions, a battery pack, when subjected to
the same conditions, may present a hazard or not function as expected.
As an example, a cell discharged at a maximum continuous current of
three amperes will dissipate heat at a rate that will keep the cell
below its maximum rated temperature. Should a battery be discharged at
the same 3 A rate, in a cluster, the internal pack temperature may
easily exceed the cell temperature limits. Although the battery pack's
intended use may require low current drains at room temperature, the
designer must account for all conditions that might compromise battery
safety.
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The following steps are a precursor to a functional and safe battery
design. Knowledge of the performance, storage, environmental, and abuse
requirements for the system into which a pack is going will enable the
battery pack designer to foresee safety problems about which the system
designer may be unaware.
APPLICATION DESCRIPTION
A detailed description of the intended use of a battery will give
direction for the successful design of the product. The description
may help the designer to move in a more positive manner with respect to
the overall safety of the pack design and its fabrication. This method
of approach will lead to specific questions that are germane to the
application.
GENERAL PERFORMANCE REQUIREMENTS
These will include minimum voltage, maximum voltage, average current,
peak current, capacity, minimum temperature, maximum temperature, duty
cycles (intermittent or continuous), motion, and orientation.
SPECIFIC REQUIREMENTS
These include available space, weight, method of attachment and
termination.
DISCUSSIOI
With all of the above data taken into account, a preliminary battery
pack design should now be analyzed for safety. All precautions should
be taken to keep the cells in the pack from cresting the cell's upper
temperature limits.
The battery construction should take into account all possible adverse
conditions. Some of these scenarios could be venting, shorting,
charging, cell reversal, mechanical abuse and overheating. Should a
single cell in a battery pack vent, there should be an unrestricted
escape path leading away from the cell's venting mechanism - preferably
to the pack's exterior or to an absorbent material.
The major protective method for an external short circuit of the pack is
through use of fuses. These fuses can be placed internal to the pack
or externally in the power supply wires. A replaceable, fast acting
fuse will offer the greatest protection and minimize capacity loss. If
the fuse is external, it may be replaced after the short is cleared. By
far, the highest occurrence of short circuiting in a battery pack is
during the installation of a connector. Slow blow fuses are sometimes
94
used when surge or pulse currents are required. Fuse values should be
set at the lowest levels possible for the application.
To protect against cell or series string charging from external power
supplies or parallel strings, blocking diodes are used in each series
string. Diode placement is usually within a battery pack, but can be
external, if standard series packs are assembled into a larger power
source. Cells of the same age and history should be used to assemble
power sources.
A partially discharged cell in series with a number of fresh cells has
been shown to eventually overheat due to forced discharge by the fresher
cells. Reversed biased diodes on each cell in a series string may offer
some protection against cell voltage reversal. In this case, other
factors such as diode leakage current and cell or series string capacity
loss must be considered.
Mechanical abuse may happen after the battery pack has been installed in
a system or while it is being handled during installation or removal.
Each application will dictate the amount of safeguarding necessary
during battery design and building.
Overheating cells or battery packs will lead to venting or, in more
extreme cases, cell rupture. The following will deal with internally
generated heat due to cell discharge.
A current limiting resistor is often used in series with a battery pack,
and will, depending on the application, restrict the current level of
the battery to a safe level. In most cases a thermally activated device
will be utilized to keep the pack from exceeding the safe temperature
limit of the cells. Items such as thermal cutoffs (TCO) and polymeric
positive temperature coefficent (PTC) resistors are in common use. The
PTC can be either temperature or current activated. It will handle
transients or brief current overloads. Trip time may typically run from
about 0.1 to 1000 seconds dependent upon the degree of overload. Once
tripped, the PTC resistor remains in a high resistance state, so long as
the source voltage is maintained.
A TCO is primarily a thermal trip device but can be activated due to
high current self-heating. A thermal pellet inside the TCO melts when
overheated. TCO's are not able to be reset and must be replaced after
being tripped. The outer case of the TCO is electrically live, and
consideration must be given to this fact when it is being installed in a
battery pack. Selection of a TCO to correspond to the maximum
recommended temperature of a cell may not offer the protection desired.
TCO's are often covered with insulation due to the "electrically live"
case.
95
With this in mind, a series of tests were conducted to characterize the
temperature lag between the cell wall and the TCO temperature.
TEST PROCED_E
A slip fit copper slug with a pencil heater connected to a Variac was
inserted into a double D case. Two layers of PVC shrink-wrap were
placed on the exterior of the DO case along with two TCO cases. The
internal mechanisms of the TCO's were removed (Figure I). Thermocouples
were placed at the inside and outside of the metal can walls, on each
layer of shrink-wrap, and in each TCO case. Thermal grease was used
between the copper slug and the cell case and in each TCO case to
facilitate heat transfer. One TCO case had been covered with one layer
of shrinktube. This was done to account for the fact that the TCO case
is electrically live in normal use. TCO attachment to the cell was
accomplished with cyanoacrylate adhesive. The test setup was covered
with a 17cm H x 17cm W x 12cm L box to simulate a dead air environment.
All test runs were begun at 24°C. The cell wall temperatures were used
to determine the heating rate. Although the heating rate for each test
run was not linear, an average heating rate was calculated over the
total run time.
4¢°C/minute
23°C/minute
lOt/minute
10°C/minute
(Figure 2)
(Figure 5)
(Figure 4)
(Figure 5)
In all cases there is an expected lag between the cell wall and TCO
temperatures. The thermal cutoff with no shrinktube showed a marked
improvement in response to the cell wall temperature, over the thermal
cutoff with shrinktube. A closer look at the data for the 1°/minute
test run, and the temperature of the non-insulated TCO at the 73°C mark,
showed a corresponding cell wall temperature of 82°C. A 68°C TCO
temperature had a corresponding 76°C cell wall temperature. These nine
and eight degree temperature lags correspond to fourteen and thirteen
degrees respectively during the 10°C/minute heating rate.
An increase in heat transfer between the cell and TCO was facilitated by
mixing copper metal dust with the cyanoacrylate adhesive. The
differential for the 10°C/minute run was now eight degrees. This is a
forty-five percent increase in heat transfer for a non-insulated TCO.
The increase over the insulated TCO was sixty percent.
Table I presents a comparison between an insulated TCO and the cell wall
temperature at the various heating rates. The non-insulated TCO with
conductive adhesive for the 10°C/minute rate is at the far right. Table
2 presents the temperature lag for the same conditions.
96
One other non-insulated TCO was attached to the cell wall with the same
conductive adhesive. This was a functional TCO. A multimeter was
attached to the leads of the TCO and monitored for loss of continuity
during a IOOC/minute run. The trip temperature for this TCO was 72°C
(+O, -4°C). This TCO tripped 4.4 minutes after the cell wall
temperature hit 72°C. The thermocouple in the non-insulated TCO case
with conductive adhesive indicated a temperature of IOO°C. The cell
wall temperature at that time was 111oc.
TEST SUMMARY
Conductive adhesive will greatly improve the heat transfer for thermal
cutoffs. Appreciable temperature lag can result if a thermal cutoff is
insulated prior to installation. Careful consideration must be given to
the trip temperature of a TCO if it is to act as a protection device in
the event of cell self-heating.
CONCLUSION
The design and construction of a lithium battery pack must be
accomplished methodically if it is to function in a safe manner.
Devices which are added to a cluster of cells have their own distinctive
characteristics and these should be understood if they are to be relied
upon as a safety item.
The reaction time of a thermal cutoff will be different than the above
test if it is in a cluster of cells, in a colder or hotter environment,
has air movement by it, or is from a different manufacturer. A battery
pack becomes a system in itself that must be looked at from every
perspective. Overall safety depends on it!
97
REFERENCES
I. Brooks, E.S., Proc. 26 th Power Sources Conf., p. 31, 1974
2. Warburton, D.L., Proc. 26 th Power Sources Conf., p. 34, 1974
98
Table 1. CELL WALL AND TCO TEMPERATURE COMPARISON
CELL VALL vs INSULATED TCO TEMPERATURE Conductive
Adhesive
Cell Vall (°C)
55
75
85
95
I05
115
Too (oc)
Heating Rate (oc/Min)
' II,oII_,
47 ¢5 40
63 55 50
72 60 55
80 70 64
87 79 69
- 85 79
44
31
43
49
55
6O
I0
49
67
76
85
95
I03
99
Table 2. TCO TEMPERATURE LAG
CELL WALL vs IWSULATED TCO (TEHPERATURB LAG) Conductive
Adhesive
Cell Wall (°C)
55
75
85
95
105
115
_co (oc)
Heating Rate (°C/fin)
8
12
13
15
18
I II,o
10
20
25
25
26
30
15
25
30
31
36
56
24
32
36
40
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Safety considerations for fabricating lithium battery packs

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