charge other voltage batteries too by adjusting the 10K variable resistors & 250K potentiometer. 10 LEDs are used to show the voltage level of battery, the charging indicator LED shows battery under charge and goes off when battery is full.
The automatic charger circuit is built around four transistors, base of the transistor Q1 should be attached to the desired level of battery where you want to start the charging, we have connected it with the pin15 of the IC, normally a 12V battery shows 12.6V when it is fully charged and should be charged when its voltage reached to 12V, therefore when you calibrate the circuit check which LED turns ON at 12V and connect base of the Q1 to that pin of the IC. You can also connect it with pin 11 or 12 of the IC to not let it go down to 12V and start charging on 12.4V or 12.3V etc.
The circuit requires a little calibration for the first time that can be done by using an adjustable power supply, set 15V in the adjustable power supply and remove the battery from the circuit and connect the adjustable power supply in the place of battery, now adjust the 10K variable resistor until all LEDs light up, the charge indicator LED will go off when the last LED (Connected with pin10) will become active.
Then set 11.5V in the adjustable power supply and adjust the 250K potentiometer until the first LED connected with pin1 goes off. After these adjustments the circuit will be ready to use.
LM3914 IC is made to sense voltage levels of power supplies and batteries, but it can easily be modified to a very smart automatic battery charger that can be used to charge just about any battery. In the schematic below the circuit is adjusted to charge 12V batteries, but it can easily be adjusted to
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