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19
USES, CARE, AND MAINTENANCE OF LOW TENSION BATTERIES
J. D. Wright, B.Eng.*
Read to meetings of the 8./C.S. Sections at Manchester on February II, 1947, and Newcastle-on-Tyne on March 4, 1947.
THE most important task which batteries perform in the kinema, is that of supplying the emergency lighting in case of a shut-down of the mains supply. The present seems to be a suitable time to consider how those responsible for the maintenance of batteries can best assure themselves that their emergency lights will, in fact, get an adequate supply from the battery in case of a mains failure.
The Home Office has, since 1934, limited the manner of employing the battery in kinemas by insisting that connection of the battery to the special circuits in case of emergency shall not depend on the operation of an automatic switch.
Battery Arrangement
The usual arrangement for using a battery, therefore, employs a rectifier of comparatively low voltage, usually 50 or 100 volts, which supplies the emergency lights under normal conditions. The battery floats in parallel with the rectifier, neither charging nor discharging. In case of a mains failure, the battery discharges on the emergency load at a slightly lower voltage than the normal one. Since a rectifier presents a high resistance to the discharge current, it is unnecessary to include any reverse current cut-out between the battery and rectifier. Fig. 1 shows diagrammatically the arrangement of the circuit.
The disadvantage of this system is, that if the mains voltage varies, the rectifier voltage varies approximately in proportion. If the mains pressure falls, the battery may begin to discharge on to the load, with the consequence that it may be partially discharged when called on to give a discharge in case of a shut-down. If the mains pressure rises, the rectifier voltage will rise above its normal value and lamps may be burnt out. If any lamps should be burnt out, the consequent reduction in the load will tend to cause a further rise in the rectifier voltage. The relays which illuminate lamps to indicate to the attendant that the battery is discharging usually operate on approximately 25% of the rated output of the rectifier, and, therefore, do not give any indication of a slight discharge current from the battery.
If the mains supply is D.C., a motor generator set, consisting of a D.C. motor driving a low-voltage D.C. generator, can be employed in place of the rectifier.
Constant Rectifier Voltages
These difficulties may be avoided by employing a more elaborate floating system whereby chokes are incorporated in the rectifier circuit so as to maintain almost constant rectifier voltage, irrespective of variations in load current and mains voltage within wide limits.
Later on it may be permissible to employ an automatic switch between the battery and the emergency circuits. If a scheme of this type should become permissible, it would have the advantage that the state of charge of the* | battery would be independent of the usual fluctuations in mains pressure and the operator could always depend on his battery being fully charged and
* The Chloride Electrical Storage Co., Ltd.