Many low-cost chargers for 12V lead-acid batteries are little more than a step-down transformer and bridge rectifier. As a result they tend to overcharge a battery if left connected to it for an extended period.
This simple charging regulator circuit is connected between the charger and the battery and is easily adjusted to prevent the battery being overcharged beyond the optimum level between 13.8V and 14.0V.
P-channel power Mosfet Q1 is connected as a switch in the positive battery lead, controlled by LM311 comparator IC1. IC1 monitors the battery terminal voltage and compares a portion of the battery voltage from trimpot VR1 with a fixed +7.2V at the inverting (-) input. This reference voltage is derived from the battery voltage via a shunt regulator circuit using zener diodes ZD2 and ZD3.