Disclaimer: Electricity is dangerous. High voltage and high current can kill you! Do not try to build this circuitry if you don’t have enough knowledge in electricity. If you still want to build this circuit, do it at your own risk! I don’t take any responsibility of what you do and its consequence. This article is intended for informational purpose only.

Not long ago I built a battery charger that uses capacitor as current limiter and voltage devider. Despite of simple schematic, this circuit has very good performance. It charged my 6 Volt 235AH dead battery from ~1 Volt to over 6 Volt within 15 minutes! But the charging process is kind of scary because of the (hydrogen and oxygen) bubbles crackling sound created during the process. Do not charge a battery in a closed (non-ventilated) room. Do it outside your house.

Here is the picture with the box opened:

Here is a picture of my test circuit:

Here is the capacitor that I use (AC capacitor must be used):

This is my schematic:

The function of the switch is to change the total capacitance value of capacitor. The higher the capacitance value, the higher the charging current, which also means faster charging rate but with more heat generation.

And here is my explanation of how this circuit works:

And here is the simplified explanation: