The rate of charging may be indicated while in process by an ammeter in the circuit, but usually the means of charging is arranged for a certain rate, which is thus fixed. The number of amperes permissible varies somewhat for different types of accumulator, but should always be in rela- tion to the capacity of the cells. The maker usually marks on each set as sent out the proper maximum rate of charge and discharge, and these should never be exceeded, or the life of the accumulator will be considerably shortened. As a general rule, the charging current, expressed in amperes, ought not to exceed one-fifth, and the discharging current one-fourth, of the number expressing the capacity in ampere hours; thus, a 20-ampere-hour accumulator should not be charged with a current of more than 4 amperes, nor discharge more than 5 amperes.

Before commencing to charge, see that the plates are well covered by the acid; and, if the cells be covered or sealed for transport, see that the blow-hole is left open during the process of charging.

With a knowledge of the rate and duration of discharge of the accumulator since last recharged, one may calculate the time required, with a known rate of charging, to restore the total ampere hours of its capacity. But little, if any, harm can be done by overcharging, and full time should always be allowed for completion of the process. A ready method of telling when a cell is fully charged is to listen for the hissing effervescing sound produced by escape of the free gases. The intensity of this will depend, of course, on the strength of the charging current, but a little experience soon detects the indication. The acid, at the same time, turns milky in appearance.

Frequent recharging tends to preserve the efficiency of accumulators, and they never work better than when used and recharged daily. Even when not in use, they should be occasionally recharged—at least once in every three weeks—to keep them in good order. If one goes off for a holiday, they should be fully charged before being left.

If left at rest in a charged condition for any length of time, accumulators tend to deteriorate (as explained later); but they do so much more rapidly if left standing discharged.

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