A 18v lipo battery (5 cells) nominal voltage, cell voltage must not drop below 3v (3v x 5 = 15v)or damage can occur. The switch simulates voltage drop to danger level and operates a relay to switch off the main power to the relay which will power the motor. (Replace the green LED with a 12 to 20 volt relay).
The variable resistor is set to 33% wiper position.
The comparitor is used in conjunction with a zener diode voltage reference point. The diode is to stop back EMF caused by a relay.
Can be used with 3 cells which would make a 11.1 volt battery (near enough 12 volts) with a cut off set at 3 x 3 cells = 9 volts.
I used a 36v Bosch cordless lawnmower lithium battery to convert a nickel metal hydride battery in a good quality drill.
I used a multi charger bought on ebay for £15 used in radio controlled models to charge it. The critical thing is not to over-discharge it.
