Datasheet

24
Valve Regulated Lead-Acid Batteries
25
Valve Regulated Lead-Acid Batteries
6 | Safety
Vent (One way valve)
If the internal pressure of the battery is raised to an abnor-
mal level, the rubber one way valve opens to release ex-
cessive pressure; thus the valve protects the battery from
danger of bursting. Since the rubber valve is instantly reseal-
able, the valve can perform its function repeatedly whenever
required.
Example of Valve Construction
Valve retainer
Top cover
Cover
Rubber one-way valve
Absorbent mat
7 | Safety Design
Item Test method Check point
1. Shock test (Drop test)
IEC 61056-1 and JIS C 8702
(These specifications are
harmonized each other)
A fully charged battery is allowed to drop in the
upright position from the height of 20 cm onto a hard
board having a thickness of 10 mm or more. Test is
repeated three times.
The battery should bee free from noticeable
breakage or leaks; and its terminal voltage should
be held higher than the nominal voltage.
2. Vibration test
IEC 61056-1 and JIS C 8702
(These specifications are
harmonized each other)
A vibration frequency 1000 times/minute and
amplitude 4 mm is applied to the X-, Y- and Z-axis
directions of a fully charged battery for 60 minutes
respectively.
No battery part should be broken; the battery
should be free from leaks; and its terminal voltage
should be held higher than the nominal voltage.
3. Oven test
Panasonic internal standard
A fully charged battery is left standing in an
atmosphere of 70°C for 10 hours.
The battery case should not be deformed;
the battery should be free from leaks.
4. Coldproof test
Panasonic internal standard
A fully charged battery is connected to a resistor
equivalent to 60 hour rate discharge and left
for 4 days; than the battery is left standing in an
atmosphere of -30°C for 24 hours.
No crack should develop in the battery case;
the battery should be free from leaks.
5. Heat cycle test
Panasonic internal standard
A fully charged battery is exposed to 10 cycles of
2 hours at -40°C and 2 hours at 65°C.
No crack should develop in the battery case;
the battery should be free from leaks.
6. Short circuit test
Panasonic internal standard
A fully charged battery connected with a small
resistor of 10 ohms or less is allowed to discharge.
The battery must not burn nor bust.
7. Large current
discharge test
Panasonic internal standard
A fully charged battery is allowed to discharge at
3CA to 4.8V / 6V battery level. (This test is not
applicable to batteries having built-in thermostat.)
The battery should not burn or bust, and it should
be free from battery case deformation, leaks and
any irregularity internal connections.
8. Vent valve function test
UL1998
A fully charged battery is submerged in liquid paraffin
in a container, then overcharged at 0.4CA. (UL1989)
Release of gas from the vent should be observed.
9. Overcharge test
Panasonic internal standard
A fully charged battery is overcharged at 0.1CA for
48 hours, left standing for one hour, and allowed to
discharge at 0.05CA to 5.25V / 6V.
No irregularity should be noticed in the battery
appearance; the battery should retain 95% or more
of the initial capacity.
VRLA battery safety test items
(Note) The above safety notes apply only to standalone batteries, not to embedded batteries.
7 | Safety Design
VRLA batteries are inherently safe. However, there are some
risks when VRLAs are used beyond a reasonable replacement
time span, misapplied or abused. There are two main failure
mode of VRLA battery used for trickle (float) application. In
high temperatures and/or high voltage charging, dry-out is
accelerated. This leads to loss of capacity and eventually
the cell will fail open. Grid growth due to grid corrosion
causes loss in mechanical strength and eventually leads
to loss of contact with the grid. Battery should be replaced
before these failures. If VRLA batteries are used after the
end of life, the grid growth may cause a crack of container.
Capillary action can result in a slight film of conductive
electrolyte forming around the crack even though VRLA
batteries contain significantly lower volumes of electrolyte
and the electrolyte is immobilized. This electrolyte film will
be in contact with an un-insulated metal component and
this ground fault current could result in thermal runaway of
a portion of the string or even a fire. And the grid growth
may cause internal short between positive grid and negative
strap in a cell. Continuing to charge a string of cells when
one or more of the cells exhibit internal shorts, can result in
thermal runaway. For example, assume a string of 12 cells is
being charged at 27.3V (2.275V/cell) and the string continues
in operation with two cells shorted. In this situation the
average charging voltage on the remaining 10 good cells is
2.73V/cell. This will result in very high float current and cause
thermal runaway.
Figure 1 is the mechanism of above phenomena.
Panasonic VRLA battery minimizes these risks by using less
corrosive lead alloy and expanded positive grid.
Figure 2 shows an example of cast grid and expanded grid.
Expanded grid does not have enough strength to crack
container case by grid growth. And an insulator between
positive grid and negative strap is installed in the models as
necessary.
Furthermore, Panasonic uses flame retardant battery
container
case for the models used for stand-by application.
The cases are designed to be self-extinguishing and meet
minimum flammability standards of UL94 V-0 and 28 L.O.I.
(limiting oxygen index).
Figure 3 is the picture of self-extinguishing phenomenon.
Fig. 1 Mechanism of thermal runaway caused by grid growth
Fig. 2 Cast grid and expanded grid
Fig. 3 Flame retardant case (Self-extinguish phenomenon)
Expansion of positive grid
Container case crack
Electrolyte leakage
Ground fault current
Thermal runaway
Container case crack
Expansion of positive grid
Short circuit between
negative strap and
positive grid
Several call´s short-circuit
Continuous operation at
high voltage
Thermal runaway
Internal short circuit
strap strap
Expansion of positive grid
(In case of continuous operation after the life end)
Cast grid
Flame retardant ABS
(UL94 V-0)
Standard ABS
(UL94 HB)
Burnt
Expanded grid
New Technology
Safe & Reliable
1. Safety & reliability (improved)
2. Corrosion resistance (improved)
3. Small discharge dispersion