Building circuits with a single transistor -Lithium - Ion Battery Equipment

The automatic battery charger circuit design in this article employs what is arguably 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 a predetermined value.

With an accurate variable voltage power supply, the voltage can be set up or down. The normally connected relay leads are not connected to the 15Vdc power supply, which prevents voltage from passing to the battery leads. This allows precise setting of the upper and lower voltages. But the 15Vdc charging power supply is connected to the circuit.(Lithium - Ion Battery Equipment)

First, the variable voltage power supply is fixed at 13.3Vdc—this is the voltage at which the battery is fully charged—and is connected to the circuit’s battery connection point. The slider of VR1 is adjusted to the very top attached to the positive terminal of the battery. The slider of VR2 should be adjusted towards the end connected to VR1. The transistor starts to work, shunt VR1. Then, the slider of VR1 is adjusted to the other end, which is connected to the end of VR2.

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