Build a Smart Battery Charger Circuit Using a Single Transistor

The automatic battery charger circuit design in this article uses a circuit that can be called the simplest window comparator ever made with a single transistor (see figure). It starts charging when the voltage drops below a predetermined value and stops charging when the voltage exceeds the predetermined value.

With the help of a precise variable voltage power supply, the upper and lower voltages can be set. The normally connected relay leads are not connected to the 15V dc power supply, which prevents the voltage from passing to the battery leads. This allows the upper and lower voltages to be set accurately. However, the 15 V dc charging power supply is connected to the circuit.

First, the variable voltage source is fixed at 13.3 V dc - the voltage of a fully charged battery - and connected to the battery connection point of the circuit. VR1's slider is adjusted to the top end attached to the positive terminal of the battery. VR2's slider should be adjusted toward the end connected to VR1. The transistor starts working, shunting VR1. Then, VR1's slider is adjusted to the other end, which is connected to 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 makes the transistor no longer work. The test voltage is increased to 13.3 V dc and VR1 is adjusted to make the transistor work. With the upper and lower voltages set, the NC point is connected to the circuit (15V dc charging voltage). Now the battery charger is ready.