Page 205 - Uninterruptible Power Supplies

Batteries
Batteries 203

Intermittent charging of VRLA cells is being tried with the aim of
reducing the loss of hydrogen (thus, water loss). An example of this is
to float-charge the battery for 2 to 3 days and then leave the battery in
an open condition for 2 to 3 weeks and cycle in this mode with the pro-
viso that an unacceptable voltage reading in the open cell condition will
lead to an immediate recharge. Various cycling methods have been
tried with varying success. So far as the writer can see, the jury is still
out on this method.
With VRLA cells there is the danger of thermal runaway. This condi-
tion is the result of heat generated within the cell and results in accel-
erated dry out of electrolyte and also, in extreme cases, the melting or
distortion of the plastic casing. The cause of this condition is ineffi-
2
ciencies in the cell due to I R losses and the exothermic oxygen recom-
bination cycle. To avoid this problem consideration should be given to
the following points:

1. High charging current
2. Unlimited or too high a charging current
3. Elevated float current
4. High ambient operating temperature
5. Unventilated battery enclosure
6. Badly designed battery enclosures with cells or blocks touching
7. Battery or system failures due to above conditions
8. Unrealistic life expectancy
Thermal runaway, now it is well understood, is not met as frequently
as in past designs.
As a guide to minimize the temperature rise while minimizing
recharge time, the figures below are desirable maximum figures.

Initial current limit as Depth of discharge Approx. discharge rate
a function of 20-h rated as a function of the 20-h and period resulting in
capacity in ampere/h © rated capacity in ampere/h © % depth of discharge
C/1 45% 15 min
C/2 55% 30 min
C/3 60% 3 h
C/5 90% 8 h
Where © is ambient temperature at 25°C.
Too high a charging voltage causes gassing since the amount of
oxygen emitted exceeds the rate at which it can be diffused through
the glass mat. Gassing rate is shown as a function of charge voltage in
Fig. 7.21.

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