The circuit shown in Figure 1 charges lead-acid batteries in the conventional way: A current-limited power supply maintains a constant voltage across the battery (approximately 2.4V/cell, as specified by the battery manufacturer) until the charging current decreases below a current threshold defined by the capacity of the battery. At this point, the charger is placed in a trickle-charge mode. The current threshold is typically 0.01C, where C refers to the battery capacity, specified in ampere-hours. When charging a battery, the term "C rate" refers to the current required, in theory, to charge a battery to its full battery capacity C in one hour. In actuality, power lost during the charge cycle ensures that all batteries charged at their C rate take more than an hour to reach full charge. Ideally, you could charge a 5A-hr battery in one hour if the charge current is 5A. Also, ideally, a C/10 charge rate (500mA) charges the same battery in 10 hours. However, the power loss mentioned previously increases these charge times beyond the two time spans stated above.