Ques. How are internal short circuits indicated?
Ans. Short circuits in a cell are indicated by short capacity, low voltage and low specific gravity, excessive heating and evaporation of the electrolyte.
Ques. How are internal short circuits located?
Ans. If the trouble cannot be located by the eye, the battery should be connected in series and discharged at the normal rate through suitable resistance. If a suitable rheostat be not available, a water resistance may be used.
This consists of a receptacle (which must not be of metal) filled with very weak acid solution, or with salt water in which are suspended two metal plates, which are connected by wires through an ammeter. The current may be regulated by altering the distance between the plates, or by varying the strength of the solution. As the discharge progresses the voltage will gradually decrease, and it should be frequently read at the battery terminals; as soon as it shows a sudden drop, the voltage of each cell should be read with a low reading voltmeter.
While the readings are being taken, the discharge rate should be kept constant and the discharge continued until the majority of the cells read 1.70 volts; those reading less should be noted. The discharge should be followed by a charge until the cells which read 1.70 volts are up, then the low cells should be cut out, examined, and the trouble remedied.
Overdischarge: Buckling.—On account of unequal expansion of the two sides of a plate, or certain portions thereof, the strains thus set up may distort it and cause it to assume a buckled shape, that is, bent so one side is concave.
Fig. 1,164.—Method of straightening a buckled plate. Buckling is caused by the unequal expansion of the plates which is due to the sulphate lodging on the plates, thus preventing action taking place at that point; and by excessive charging. If the plates be not badly buckled, they can be placed between 2 boards and with a little pressure, can be straightened out.
Buckling is due always to over discharge on either the whole, or some portion of the plate. Occasional buckling may occur with too rapid charge and discharge.