Specifications

8.4. BATTERY SELECTION AND CHARGING
The charging of SLA batteries is well-understood and relatively simple:
the battery should be charged with a constant voltage of 13.8V with current
limiting of approximately 40% of the rated capacity in Amp. hours (Ah)[23].
Ideally, this threshold should be temperature compensated — lower temperatures
require higher charge voltages. For example, Panasonic recommends that
at 0
◦
C, the battery is charged at 14.1V, but at 40
◦
C, only 13.4V. This
relationship is approximately linear. It is important not to overcharge the
battery as this severely diminishes capacity. Figure 8.4 shows the results of
the calculations employed where V
targ
= 14.0 − 0.015T .
Figure 8.4: SLA battery target charge voltage vs temperature.
In order to achieve a standby backup time in the order of days, a large
battery is required (based on estimated consumption figures in Section 8.2).
Popular sizes include 1.2AH, 2.2AH, 4AH and 7.2AH. Price differences between
the sizes are slight and it was thus decided to use the larger 7.2AH unit. Also
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