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Safety Design Features of High Rate Cells
- Current and Thermal Protection System
Spiral-wound lithium cells are capable of delivering high current
output due to the
high surface area of the wound electrodes. To ensure safe operation,
manufacturers
of high rate cells normally require the use of protective devices,
such as electrical fuses to limit the amount of current which
can be drawn from the battery or thermal switches to cut off the
discharge if a battery reaches high temperatures during use. These
devices provide good protection against electrical abuse. However,
they can also be troublesome--fuses must be replaced when blown,
protective devices can create dimensional problems when battery
size is critical, and protective devices increase battery costs.
Duracell was the first to design a resettable current and
thermal protection system called a PTC (Positive Temperature
Coefficient) device into its high rate cells. This integral
safety device eliminates the need for electrical fuses or thermal
devices to protect the battery against short circuits or discharge
at currents above design
limits. Under normal operating conditions, the PTC device does
not impair cell performance. However, when a cell is short-circuited
or discharged above design limits, the PTC device causes the
cell’s internal resistance to increase substantially limiting
the amount of current which can be drawn from the cell and keeping
the internal temperature of the cell well within safe limits.
Figure 8.1.1 :
Short circuit of a DURACELL DL2/3A cell.
Cell temperature under abusive conditions is a gauge by which
safety can often be measured, particularly with lithium batteries.
It is necessary to maintain cell temperatures below the melting
point of lithium, 180.5°C (357°F), to ensure safety
- Safety Vent
The safety vent in DURACELL high rate Li/MnO2 cells
serves as a back-up safety device. In instances of electrical
abuse, such as heavy-duty discharge or accidental short-circuiting,
the PTC device overrides the safety vent. In instances of severe
mechanical abuse, the vent provides a safe means of releasing
internal pressure and prevents the cell from reaching excessively
high temperatures.
As internal pressure builds, a plastic laminate vent membrane
is punctured by a vent spike incorporated in the cell top. This
provides a safe release of internal pressure
and prevents a potentially dangerous rupture of the cell casing.
The safety vent in
this Li/MnO2 cell is designed to operate when the internal
cell temperature reaches approximately 125°C (257°F
). The internal pressure at the time of venting is around 40 psi.
Figure 8.1.2 :
Heat test of a DURACELL DL2/3A cell.
- Underwriters Laboratories Recognition
DURACELL Li/MnO2 cells are recognized under the
Component Program of Underwriters Laboratories, Inc. (UL). To
receive component recognition, UL requires that lithium batteries
pass a number of tests during which the batteries are exposed
to abusive environmental, mechanical and electrical conditions.
As a result of these tests, the DL2/3A cell and batteries comprised
of this
cell type, e.g., 123, 223, 245 are recognized
for user-replaceable and technician-replaceable applications
per UL File MH12538.
UL Test Methods and Results:
Sample Conditioning Tests
- Discharge Test
The cells were completely discharged by connecting the terminals
of the cells through resistors selected
to give the desired discharge in a 60-day interval. Cells were
discharged at room temperature.
Results: No visible change.
Environmental Tests
- Heating Test
Batteries were heated in an oven to a temperature of 150°C
(302°F). The rate of heating was controlled to 5±2°C
per minute until the test chamber reached 150°C.
Results: No fires or explosions.
Mechanical Tests
- Vibration Test
This method calls for the application of simple harmonic
motion with an amplitude of 0.03 in. (0.06 in. maximum
excursion). The
frequency was varied at the rate of 1 hertz/mm. between 10
and 55 hertz and return in not less than 90 nor more than
100 minutes.
Each cell was tested in three mutually perpendicular directions.
Results: No changes observed.
- Shock Test
Cells are subjected to a total of 3 shocks of equal magnitude.
Each shock is to be accelerated in such a manner that during
the initial 3 ms the minimum average acceleration is 75g.
The peak shall be between 125g and 175g.
Results: No fires or explosions.
Electrical Tests
- Short-Circuit Test
The cells were shorted by connecting the positive and negative
terminals with a short length of copper wire. Tests were
conducted
at room temperature and at 60°C (140°F).
Results: No fires or explosions.
- Forced Discharge and Recharging Test
Completely discharged cells were force-discharged by connecting in series with fresh cells. After the completely discharged cell was connected in series with a specified number of fresh cells the resultant battery pack was short circuited.
Results: No fires or explosions.
UL Abuse Tests for User Replacement
- Flat Plate Crush Tests
The cells were crushed between two flat surfaces. In these
tests, the cells were positioned with their long axis parallel
to the
flat surfaces. The force was applied by means of a hydraulic
ram, and the cells were crushed until a reading of 3000
psi was obtained
on the pressure gauge on the hydraulic ram. The temperature
on the
exterior surface of the metal cell casing was monitored.
Results: No fires or explosions.
- Impact Test
The cells were placed on a flat concrete surface with the long
axis of the cell parallel
to the flat surface. A cylindrical bar having a radius of
curvature of 5/8 in. was positioned on the cell with the
long axis perpendicular
to the long axis of the cell. A
20 lb. weight was raised by means of a rope and pulley and
dropped from a height of
2 feet onto the 5/8 in. cylindrical bar. The temperature on
the exterior surface of
the metal cell casing was monitored.
Results: No fires or explosions.
- Fire Exposure Test
The battery was exposed to fire by placing it horizontally on
a steel wire mesh screen having 20 openings per inch (25.4 mm)
and a wire diameter of 0.017 inch (0.43 mm). The screen was mounted
11/2 inches (38.1 mm) above a 1 1/2-inch diameter laboratory Meeker
burner fueled by propane gas.
Results: The battery met the standards established by the
Underwriters Laboratories for this test.
Caution:
In illustrating these tests, Duracell does not intend to suggest
that similar abuse testing be performed on DURACELL Li/MnO2 batteries
or any other manufacturer’s lithium batteries.
As discussed throughout this site, all lithium batteries are not
the same. Performing these tests in the manner described in this
section
or other abuse tests could result in fires or rupture of some batteries
and serious personal injury and property damage.
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