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MANAGERS AND PROJECTIONISTS 1087
sulphate in both plates. The battery is discharged when the current ceases to flow, which means that the chemical action in the battery has stopped. We can now reverse the procedure, force current through the battery and convert the plates back again into their original chemical condition. This is called charging.
When the current has been forced through the battery long enough, the positive plate is again in the chemical form of lead peroxide and the negative plate is again pure lead. In other words, the lead sulphate which was in the plates when they were in their discharged condition, has been driven out of the plates by the electric charging current. The battery is then called charged.
I might say that in practice we never let the battery discharge until it will give no more current. We find that if this is done, injury is done to the plates, hence we always stop the discharge at a point which has been found safe. See "Over Discharging" further along.
Now a few words about charging. Only direct current can be used ; never alternating current. When the power supply is A. C. it is necessary to change it to direct current for charging purposes. This may be done either by means of a motor generator set having a D. C. generator or by a rectifier.
In practice, we have found that we cannot charge at too high a rate. We can carry on the chemical action called charging no faster than the plates will respond. A high rate will do no good. On the contrary, it will heat up the battery plates and work injury to them. The charging rates to use are alwrays given by the manufacturer. They must not be exceeded.