Page 202 - Uninterruptible Power Supplies

Batteries
200 Chapter Seven

Float Voltage Per Cell vs. Temperature
2.5
2.45

2.4
Volts Per Cell 2.25 Min. V/C
2.35
2.3
Typical V/C
Max. V/C
2.2
2.15

2.1
–15 –10 –5 0 5 10 15 20 25 30 35 40 45 50 55
Degrees C
Figure 7.16 Float voltage per cell versus temperature.

amperes rms (5 amperes rms per 100 A/h of capacity) maximum. Ripple
is also affected by frequency which maximizes at approximately 100 to
250 Hz. The effect of ripple is to cause heating of the cell, and the maxi-
mum overall heating effect should not be more than 5°C. The inverter
clearly can also induce ripple onto the battery. However, the frequency of
inverter switching is much higher and a simple choke is sufficient to solve
this problem. Figures 7.17 and 7.18 illustrate, in the case of Fig. 7.17, load
being correctly supplied by the charging system and in Fig. 7.18 some
reversal of charging is occurring, this is to be avoided. Damage to cell
plates may result. This problem will be the result of an incorrectly
designed or adjusted charging system.
Most charging systems are designed to give current limit constant
voltage. Thus, a cell on recharge initially will see a high current at a volt-
age considerably below the normal float voltage. As charging continues,
voltage will rise to normal float volt level and the current will diminish
to a low constant figure.
It should also be noted that the electrochemical efficiency of charging
is high up to approximately 80 percent charge; thereafter the efficiency
falls away and electrolysis occurs, resulting in hydrogen and oxygen
evolving (see Fig. 7.19).
Life of VRLA cells is affected by the number of discharges (cycles)
and the depth of such discharges. The cycling of a cell is also affected
by the method of charging. For example, UPS systems normally float
charge, and this is preferable, under the normal UPS operating condi-
tions, to other methods. Temperature also has an effect, thus it is diffi-

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