How automatic cutoff battery charger circuit works

 Here a low cost automatic cutoff battery charger circuit is presented and explained. Automatic cutoff battery charger means that once we have set a preset voltage level for the battery charger, and during charging the target voltage is reached, the battery will be disconnected from the charging circuit. That is, the circuit detects preset voltage and disconnects the battery to charged from the charger circuit. The automatic cutoff helps avoiding damaging the battery. Also the circuit is very simple and just requires relay switch, a transistor, some resistors, capacitors and LEDs.

The circuit diagram of 3.7/5V/9V/12V automatic cutoff battery charger is shown below.

More details of the working principle of this auto cutoff battery charger circuit can be learnt from the following video.

First you have to set the automatic cutoff voltage. For this you have to first decide the voltage of the battery you want to charge. If for example, you want to build automatic charger for 3.7V battery like 3.7V lithium ion rechargeable battery, then you need to use 5V relay switch and use resistor R3 of 220 Ohm resistor. Similarly if you want to build 5V battery charger then you need to use 6V relay switch and resistor of 470Ohm. For 12V battery charger you need to use 12V relay switch and 1kOhm resistor for R3 in the automatic cutoff battery charger circuit diagram above.

Once you have decided for the battery voltage, then you can start setting up the automatic cutoff voltage. The voltage is tuned using the preset resistor RV1. Connect 4V to 15V DC supply into the power jack or power socket header(depending upon what you want to use). Using rotate the preset resistor or potentiometer until the LED D2 is turned on. This can be visualized from the circuit diagram above. The applied dc power gets into the LED D2 and also into the battery to be charged. 

At this stage, the transistor 2N2222 is turned off and the battery is charging state. The power supply voltage is applied and current also flows via the resistor R2, the preset potentiometer RV1, into resistor R1. From there the capacitor C1 is being charged and some current When the preset cutoff voltage is reached, the transistor is turned on and the current flows from the power supply through the relay switch into the collector and then to the emitter and finally back to the negative power supply terminal. This turns on the relay and the battery is disconnected from the battery charger.

The BJT transistor used here is 2N2222 but other popular standard transistors such as 2N3904 or BC547 etc can be used as well. MOFET transistor should be working even better.

Such auto cutoff battery charger can be useful for RC cars and Robotics which requires batteries. For example this automatic cutoff battery charger circuit can be used in the wifi controlled NodeMCU car or the autonomous robot car.