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456 Transadions of S.M.P.E., Vol. XIII, No. 38, 1929
cells it is very important that the eut-offi voltages be defined. Otherwise, the capacity comparisons are not equitable.
Apparatus that uses dry cells should be designed to operate on as wide a range of voltage as is possible. The greatest battery efficiency is obtainable when the cut-off voltage is low, and for this reason, the greatest battery economy is possible at a low cutoff voltage. For example, if a 45 volt battery is used on equipment that becomes inoperative as the battery drops to 40 volts, its service life might be 75 hours. If the equipment can be changed to work at only ten per cent lower voltage or 35 volts, the batteries will deliver 260 hours of service or an increase of almost 250 per cent.
Mechanical Handling
If dry cell batteries are used in portable equipment or in places where they are suspended or held by straps, certain suggestions can be made concerning the installation.
Dry cell batteries are an assembly of units in an insulating container, and these containers are subject to size variation. For this reason they are not always of the exact shape or size, and allowance must be made for these discrepancies. Too often, equipment is designed around a battery that is not standard, and later, much trouble results in trying to get batteries that will fit into the places allowed for them. Any battery manufacturer can give engineers the specifications of sizes for standard batteries which will usually accomodate those of other manufacture.
Batteries must not be strapped into position under excessive pressure. While they appear to be solid and substantial, a continual pressure might in time crack the sealing wax or break an electrical connection inside of the battery. Metal straps with sponge rubber cushions make satisfactory holding means for batteries.
Sometimes a bank of batteries is desired in a movable equipment. If possible, this should be so designed that the batteries can be assembled, fastened and electrically connected in a non-conducting container, which in turn is placed in the movable equipment. By having the batteries in a replacable container of this kind they can be taken into the laboratory and tested with proper instruments. Furthermore, a spare container with batteries can be assembled at leisure and quickly exchanged for one that has been in use.