Simple Discrete Battery Charger with Status LEDs

[nathanpc]

Today I had the idea of creating a very simple discrete battery charger based on this circuit bigclive showed in his video since it already had that really cleaver "cell in" indicator trick, but I wanted to add a "charged indicator", so I just added another transistor to indicate just that and light a green LED:



I've done some simulations and also breadboarded the circuit. It works quite well, but as you might have already guessed the LED starts to turn on before the battery is completely charged, but that's fine all you need to do is check if the LED is bright and not dimly lit.

Another idea that I had was to add a another transistor to switch off the charger when the battery is full, but I still haven't tried it out yet:



I just thought it was a nice little circuit to share if anyone is interested. Feel free to let me know if you liked it, suggest any improvements, or point out any mistakes I've made.

[Paul Price]

You have presented a design with unknown battery voltage, charge requirements, resistor values, etc.
I hope you aren't doing this because you have hopes to patent your cleaver design.
You have left out the cleaver "battery-in" indicator.

That being said, using a LM339 comparator instead of discrete transistors to detect voltage levels would make your end of charge point and charge operation LED indicators quite precise in operation, all with just a small increase in complexity.

[nathanpc]

Paul, it's intended to be used with NiMH. I left out the battery voltage because you can tweak R3 and R4 to any battery voltage you want, also the "battery-in" indicator is part of the constant current regulator. I've done this just as a fun experiment to play around with transistors and learn more, I would never patent this or use it in any serious charging circuit, it was just a project to learn more and create a extremely simple charger entirely with discrete components.