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21
AC.MAINS
CUSTOMERS SWITCH OP SWITCHES
A — Rectifier Input Switch B — Rectifier Input Fuses C — Change-over Contactor D — Ammeter E — Milliammeter F — Lamps for Trickle Charge G — Quick Charge Switch H — Voltmeter
J — Voltmeter Switch
K — Battery
L — Secondary Lights
M — Rectifier
N — Quick Charge Resistance
O — Lighting Transformer Switch and Fuses
P — Lighting Transformer
Fig. 2. Emergency Battery connected through Gravity-operated Contactors
The maintenance of a battery in good condition can be divided into four parts :
(1) Charging.
(2) Removal of internal short circuits if any should occur.
(3) Topping up. (4) Cleanliness.
Charging
A floating battery should be recharged as soon as possible after an emergency discharge, this charge being continued until the voltage and specific gravity of the acid in a pilot cell have remained constant during one hour's charging. A " freshening " charge should be given once each week, even though the battery has not been discharged. This charge should be continued until the plates gas freely and the specific gravity of the acid has remained constant for one hour.
The necessity to give freshening and equalising charges arises from the fact that a certain amount of local action takes place in the cells while they are idle. The active material of the positive plates (which consists of the coating. of lead peroxide which has been formed by electrochemical action on the pure lead casting) reacts with the lead casting to a slight extent, thereby becoming discharged to some extent. The lead sponge which forms the active material of the negative plates also reacts with the electrolyte to a slight extent. The freshening and equalising charges compensate for :this slight self-discharge and, what may be even more important, they : ensure that if the battery should have become slightly discharged during the week due, e.g., to variations in the mains voltage, it will be brought at 'least once a week into the fully charged state.
It is advisable to record the specific gravity of each cell at the beginning and end of the monthly equalising charge. If any cell is low in specific gravity and is backward in gassing, this will probably be due to the presence of an internal short circuit. Nearly all batteries used for emergency lighting are fitted with wood separators and internal short circuits should be of very rare occurrence. It is possible, however, for a wood separator to be