The following automatic battery-charger design is created with a circuit that could qualify as the simplest window comparator ever built around a single transistor . It starts charging when the battery voltage drops beyond a preset value, and it stops when an upper preset voltage is attained. source

This charger is intended for charging small 12 Volt lead acid batteries, such as the gelled or AGM batteries of capacities between about 2 and 10 Ah, using a car’s electrical system as power source, regardless of whether the car engine is running or not.source

This circuit is intended for charging sealed lead-acid batteries with a solar panel in small and portable applications. The customary diode that prevents the battery from discharging through the solar panel has been replaced by a FET-comparator combination. The charger will stop charging once a pre-set voltage (temperature compensated) has been reached, and recommence charging when the voltage has dropped off sufficiently. The load is disconnected when the battery voltage drops below 11V and reconnected when it gets back to 12.5V.source

This application note is intended to demonstrate the use of the PIC14C000 in an intelligent battery charger. The charger is designed to charge a sealed lead-acid battery (YUASA NP7-12 12V, 7AH); however, the charge parameters are easily modified to work with different lead-acid batteries. The typical method of charging lead-acid batteries is with a constant voltage, current-limited source. That method allows a high initial charge current that tapers off until the battery reaches full charge. This design uses a constant current, allowing the voltage to rise until the battery voltage reaches a full charge.source

This design implements a charger for a lead-acid battery as a subfunction in a microcontroller whose main function can be any more complex task. Furthermore, the MCU gets its power from the same battery. The charging process is so slow and uses so little processor time that it doesn’t jeopardize the MCU’s primary task.source