The batteries used in most industrial situations (other than standby emergency use) are used in what is called cyclic operation. That is, the battery is either being charged or being used (discharged). In most such applications batteries are charged about 1500 to 2000 times during their lives. Incorrect charging for a few cycles will do little harm, but incorrect charging day after day will shorten the life of the battery.
Correct charging means charging the battery sufficiently, without overcharging, overheating, or excessive gassing. To accomplish this, the charging of batteries is usually started at a high rate of amperage known as the starting rate. Later in the charge, this rate of current flow is reduced to the finishing rate. Manufacturers generally suggest as a rule of thumb that the finishing rate should not exceed 5 A per 100 A-hr of rated battery capacity. The starting rate may be four or four and one-half times the finishing rate.
Lead-acid batteries should be charged for a sufficient length of time at a rate which will introduce into the battery the same number of ampere-hours removed on discharge, plus a 5 to 15 percent overcharge. The specific value of the overcharge depends almost entirely upon the charging temperature, and the age and history of the battery. In general, it is more harmful to overcharge excessively an older battery at a high rate or a battery operating at high temperature than a freshly manufactured unit or one being charged at room temperature or lower. Any charge rate is permissible which does not produce excessive gassing or cell temperature greater than 115°F (46°C).
Four methods of charging are discussed below; they are:
- Modified constant-voltage
- Taper
- Two-rate
- Constant-current
The selection of the appropriate method will be governed by considerations such as the type of battery, service conditions, time available for charging, and the number of batteries to be charged at one time. It should be noted that in charging motive-power batteries, the end-of-charge rate (finishing rate) is extremely important and should not be exceeded. Normally, batteries can be charged in 8 hr, assuming a normal-duty discharge; however, if time permits, a longer period can be used.