Charging Rates

Author: E.E. Kimberly

The rate at which a battery may be safely charged depends on the rate at which lead sulfate may be reduced on the plates and hence depends on the rating and condition of the battery. If an attempt is made to charge a battery at an excessive rate, part of the current is used in reducing the lead sulfate and the excess current causes decomposition of the water of the electrolyte. The liberated hydrogen rises to the surface. That action is called "gassing." The rate at which sulfate reduction may be accomplished decreases as the charging progresses, and hence the charging rate should be "tapered" to a relatively small value at the end of the charging period. Excessive gassing should be avoided because the scouring action of the rising bubbles tends to wear away the active material of the plates, particularly the positive plate. The scoured-off material falls to the bottom of the jar, and may eventually fill the jar up to the bottoms of the plates and short-circuit them. Furthermore, gassing indicates a waste of energy and creates a fire and explosion hazard.

The maximum safe ampere rate of charge for any lead-acid battery, regardless of its size or condition, is equal to the number of ampere-hours required to bring it to full charge. The finishing rate, however, does not need to conform to this rule, but may be as great as recommended by the manufacturer. Quick charging can be achieved by following this rule, but the operation requires constant skilled attention. A compromise consists in charging at such a rate that 63 per cent of the desired charge is received by the battery in the first hour, and 85 per cent is received in the first two hours. Such rapid rates of charging should be used only in emergency because of the attention required to prevent damage to the battery.