Converting a 9.6 volt Makita Drill from Nicad to LIPO

I have had an old 9.6 volt Makita drill since around 1989 or 1990. I bought it when I was an A&P. Since then I have replaced the batteries about 4 times. I recently bought an 18v Lithium ion Makita. I wanted to continue to use the old Makita but did not want to buy any more batteries as they are kind of pricey and do not last long. I have seen several videos on the internet on connecting RC LIPO batteries to the Makita and they work fine.... EXCEPT that none of the people who do this take any precautions to prevent over discharging the LIPO batteries. LIPO cells do not like to be discharged below 2.7 volts per cell. If you do that you kill them. The good thing about them is that they are relatively cheap. You can pick them up for $15 to $20 for an 1800mah or 2200mah battery respectively. this is 200 to 500 mah more than the replacement batteries from Makita which are 1300mah to 2000mah.

My choices for not over discharging them were to make a circuit the cut off the voltage at around 9 volts or set off an alarm when they got down to 9 volts. I went with the latter as it was easier to build and some of the parts can be scrounged from an old computer power supply. The parts list is as follows:
1 - Makita drill
1 - 4.7k resistor
1 - 47k resistor
1 - 1k resistor
1 - 18k resistor
1 - LED any color
1 - LM393 (pulled from the old Power supply)
1 - TL431 (or a KA431 pulled from the old PS, I got 3)
1 - Piezo buzzer (not a speaker a BUZZER it will have all the buzzer circuitry built-in)
1 set of Deans connectors (male for the drill and female for the battery)
at least 1 - 3 cell LIPO battery around 1800 to 2200 mah (any bigger will not fit). Do not use any more or less celled batteries as this circuit will not work right for them.

Optional:
1 - Piezo buzzer (not a speaker a BUZZER it will have all the buzzer circuitry built-in) 
1 - variable voltage dc power supply or a 9 volt battery that has been discharge to 9v or less, and 1 good 9v battery.

This Schematic:

 I started by building the circuit on a breadboard. I used the variable DC PS to make sure it worked. then put the circuit together by just soldering the components to each other. It's ugly but it works.

Next I encased the circuit in epoxy so that it can take abuse.

After each step I made sure it still worked.Then I mounted everything in the Makita.

I drilled a hole in the top center for the LED carefully routed the wires around the motor adding extra heat shrink or hot glue in places where the motor might rub its connectors on the new wiring (nothing really prevents the motor from turning slightly and it would eventually cut through the wires. The piezo buzzer fits but does not work as it is pinched too tightly when the case is closed (it worked outside the case). I will have to find a smaller one. The pizeo buzzer is attached to the positive side of the LED and to pin 1 of the LM393. The one I have handles something like 3 to 18 volts so the 7+ volts there works fine. I had some trouble getting the case closed again as the gears and such moved, but I did get it closed. So now when the drill runs the battery down to around 9.1 volts the LED starts to light up and gets brightest when it is almost exactly 9v. The buzzer will sound too. I added the buzzer since, if the drill were left unattended the under current circuit would slowly run the battery down and I wanted an audible warning to remind me to unplug the battery. As an added bonus when the drill is started the LED flashes once, this is nice as it lets you know the circuit works. It works fine as you can see in the video.

If your a wuss you could just order one of these and plug it in to the charging port of the battery. WUSS!