A simple battery charger that disconnects the battery when charge voltage reaches its nominal voltage and reconnects when battery voltage falls below a predefined level, can be designed using this circuit diagram. Circuit diagram : Autoconnect Disconnect Battery Charger Circuit diagram A fraction of the battery voltage is taken from the voltage divider R1-R2-R3-R4 and compare with a reference voltage with the help of IC2b. As long as the battery voltage is 0 V. The input current of AO produces a small voltage drop on R5, so IC2c pass in "0". Therefore, the relay remains disengaged. When connecting a battery, low residual voltage provide switch of IC2c, diodes D4 and D5 are reverse biased, a voltage reference applied to the noninverting input of IC2d and relay is activated. In these conditions, the battery charge until its voltage reaches the nominal level. Calibration is performed with a voltmeter connected to the output of IC2a, then P2 is adjusted, to obtain an indi...