This circuit is a high-performance charger for gelled-electrolyte lead acid batteries. It quickly recharges the battery and shuts off at full charge. Initially, charging current is limited to 2A. As the battery voltage rises, current to the battery decreases, and when the current has decreased to 150mA, the charger switches to a lower float voltage, which prevents overcharge.
When the start button is pushed, the output goes to 14.5V. As the battery approaches full charge, the charging current decreases and the output voltage is reduced from 14.5V to about 12.5V, terminating the charging. Q1 then turns on the LED, as a visual indication of full charge.
From National Semiconductor’s application notes