In a full charged, healthy battery the positive plates have a chocolate colour, and the negative plates are of light or bluish grey. Any deposit at the bottom of a cell should be brown. The cells will not give off gas during discharge or when idle – except for a short while after a full charge: and during charging they will all begin to gas at about the same time.
Bad signs are: a whitening of the plates (sulphating); premnture gassing of the plates during a charge at normal rate; excessive shedding of deposit. The first is due to insufficient charging, or leaving the battery idle too long.
Premature gassing may be caused either by the plates being worn out by use, or by sulphating checking the chemical changes which take place during charging. Deposit is shed at an abnormal rate if the charging-ratc is too lugh or the cells are allowed to exhaust themselves too far during discharge.
Like a living creature, a battery requires some exercise to keep in good condition. Sulphating, which is the cause of most battery troubles, can be prevented if the battery, during normal use, is charged at regular intervals, given an extended charge – with full gassing for 45 minutes – once a fortnight and never allowed to over-discharge itself or stand quite idle for long periods.
During the summer, when lights are little used, and radiators not at all, it will do the battery good to make it discharge for periods through an artificial resistance at at least half its full ten-hour discharge rate. The chief engineer to a leading firm of accumulator makers recommends the following simple method.
Two lead plates, having an area of about half a square foot each, are suspended from wooden bars in a barrel filled with water. The plates are connected by cables with the terminals on the double pole discharge switch, on the side remote from the battery cable connections. The plates having been set about 6 inches apart, the switch is closed.
An alternative is to pass the current through resistance wire wound on some heat-resisting substance such as firebrick.
E The formula to be used is R= – where
R=resistance, in ohms: E = the voltage of the battery; and C = the current desired in amps. Thus, assume E to be 50 volts, and half the maximum discharge rate to be 20 amps, the resistance required would be one of 2i ohms.
This would be provided by 25-ft. Of a resistance wire with a resistance of 1 ohm per foot.
Testing by Hydrometer
While a cell is being charged, the electrolyte becomes denser, and conversely during discharge it becomes more dilute, and lighter. These changes are due to chemical changes and the interaction of plates and electrolyte: and they afford a ready means of deciding how far a cell has been charged or discharged, or, in other words, how much useful charge it has left in it. 7
At the beginning of a charge the hj-dro-meter floats, with, say, the 1175 mark on its neck level with the surface. The electrolyte has thus gained 40 points in density. During discharge, by the time the specific gravity has fallen to 1195, half of the charge put in has been used; at 1185, three quarters; and at 1175 the cell is back to its pre-charging condition.
When the specific gravity sinks to a certain point – which varies with different batteries – the battery should at once be recharged. Once the full charging range in specific gravity is known, a hydrometer is, under normal conditions, an accurate index to charge state. The whole battery should be gone over with the hydrometer now and then, to find out whether all the cells are behaving alike.
Teat by Voltmeter
The switchboard voltmeter, which can be connected with the battery discharge circuit, also gives some indication of the batterys condition as a whole. (But it is of value only when the battery is discharging at a fair rate. At no load an almost exhausted battery may show full voltage.) If the reading falls to an average of 185 volts per cell, re-charging is needed.
The voltage of each cell should be tested occasionally (again, during discharge) with a special cell-testing voltmeter (the ordinary cheap instrument is useless), and a record kept. In this way any weak cell will be detected, even though the specific gravity may be correct.