Archived from the Tools In Action forum (Bosch). This thread is read-only.
So I got my hands on an old but powerful Bosch GSR 12-2 drill set, which had bad Ni-Cd batteries. I decided to give it an upgrade with li-ion conversion as I have done with other tools before. I bought some reclaimed VTC5A 35A 18650 cells and a couple of 3S 25A balanced charging/discharging boards. After all the labor of assembling the battery packs when I finally went to test the battery, I was in for a surprise. The drill wouldn’t work as the board was going into protection as soon as I pressed the trigger switch.
My first thoughts were that I got bad boards. but upon some testing I found that the drill works fine when I very carefully depress the trigger switch, but shuts down on normal use. That gave me some hope and I began my search for answers. Soon I found that it is rather common occurrence on tools with brushed motors. Going through many foreign forums and hours of translation later, I had couple of options to try.
A few suggested putting a small 104 capacitor in parallel to the existing cap next to the transistor which triggers short circuit protection. This didn’t work in my case. Another option suggested was to put a 470uF cap at the output of the battery. This also didn’t work in my case. Another suggestion was to put an inductor coil in series at the output of the battery. This worked like a charm.
A small coil of about 1.5mm wire and 11-12 turns is sufficient. it absorbs the instantaneous inrush current while the motor starts spinning.
Suggestion was to put the coil in the battery pack. but as I had found only one such coil and two batteries, I decided to put the coil in the drill. Small coil fits nicely behind the motor in series. The air flow will certainly helps keep it cool anyway.
The procedure works on any similar boards such as 3S 20A, 3S 40A, 3S 60A, 4S 40A, 4S 60A etc.
If you find this information helpful, Donations are always welcome. 😇
By the way the converted batteries charge nicely on the original charger up to 12.43v at about 1 amp. 👍
