Lead-Acid Storage Batteries
same. The most important aspect of charging is matching the charger to the battery
application. When choosing a charger, it is necessary to consider the type of battery, the
way in which the battery will be discharged, the time available for charge, the temperature
extremes the battery will experience, and the number of cells in the battery (output
voltage). It is important to consult the battery manufacturer at the time of purchase to
determine the appropriate charging method.
In general, lead-acid batteries may be recharged at any rate that does not produce
excessive gassing, overcharge, or high temperatures. Discharged batteries may be
recharged at a high current initially. However, once the battery approaches its full
charge the current must be decreased to reduce gassing and excessive overcharging.
A wide variety of schemes exist for charging lead-acid batteries. Although a complete
discussion of various charging techniques is beyond the scope of this Primer, a general
description of the more common methods follows.
Constant-voltage (often called constant-potential) chargers maintain nearly the
same voltage input to the battery throughout the charging process, regardless of the
battery's state of charge. Constant-voltage chargers provide a high initial current to
the battery because of the greater potential difference between the battery and
charger. A constant-voltage charger may return as much as 70% of the previous
discharge in the first 30 minutes. This proves useful in many battery applications
involving multiple discharge scenarios. As the battery charges its voltage increases
quickly. This reduces the potential that has been driving the current, with a
corresponding rapid decrease in charge current as depicted in Figure 16. As a
result, even though the battery reaches partial charge quickly, obtaining a full
charge requires prolonged charging.
Given this behavior, constant-voltage chargers are frequently found in applications
that normally allow extended charging periods to attain full charge.
Constant-voltage chargers should not be used where there is frequent cycling of the
battery. Repeated discharges without returning the cell to its full charge will
eventually decrease the battery capacity and may damage individual cells.
Constant-voltage chargers are most often used in two very different modes: as a
fast charger to restore a high percentage of charge in a short time or as a float