The following schematic is a high-performance battery charger for gelled electrolyte lead acid battery. Battery charger quickly recharges lead acid battery and shuts off at a full charge. Innitially, charging current has limitations to 2 Ampere. While the battery voltage goes up, current to the lead acid battery decrease, and if the current has decrease to 150 mA, the battery charger changes to a lower float voltage protecting against overcharge.
When the switch is actually pressed, the output of the battery charger would go to 14,5 V. As the battery nears complete charge, the charging current decreases and the output voltage is lowered from 14.5 V to around 12,5 V terminating the charging. Transistor Q1 finally lights the LED as visual warning sign of full charge. (Source : National Semiconductor Corp. Voltage Regulator Hanbook.)
