Using a single transistor to build a smart battery charger circuit diagram

This article's automatic battery charger circuit design uses a circuit that is arguably the simplest window comparator ever built from a single transistor (see figure). It starts charging when the voltage drops below a predetermined value and stops charging when the voltage exceeds a predetermined value.

With a precise variable voltage power supply, the voltage can be set up or down. The normally connected relay leads are not connected to the 15V dc supply, which prevents voltage from reaching the battery leads. This allows precise setting of the upper and lower voltages. But a charging power supply of 15 V dc is connected to the circuit.

First, the variable voltage power supply is fixed at 13.3 V dc - which is the voltage at which the battery is fully charged - and is connected to the circuit's battery connection point. The VR1 slider is adjusted to attach to the very top of the positive battery terminal. VR2's slider should be adjusted toward the end connected to VR1. This transistor starts working, shunting VR1. Then, adjust the slider of VR1 to the other end, which is connected to the end of VR2.

Now set the test supply voltage to 11.8 V dc, which is the voltage when the battery is exhausted. Then, adjust VR2 so that it stops the transistor from working. The test voltage is then increased to 13.3 V dc, and VR1 is adjusted to make the transistor work. With the set upper and lower voltages, the NC point is connected to the circuit (15V dc charging voltage). The battery charger is now ready.