help on hacking a powertool to run on off-brand battery

Bene wrote 03/01/2023 at 21:34 0 points

Hi all,

I got my hands on a Scheppach branded weed wacker without battery and charger. Its a 40v lithium system. I already own a 40v lawn mower so I thought I'd just use this battery, similat to how I use Makita batterieys on Parkside tools (with a 3d printed adapter).

Here comes the problem: Running the tool off the Güde-branded battery with only the power connected works only for three seconds after starting the machine. There is a third contact in the battery bay which I hoped would be for the thermistor of the original battery (which I never had in my hands).

Anyways, connecting a 12k resistor between negative and this pin did not work, nor did any value that I dialed in with a 8k potentiometer. Opening the housing shows that there is only a 25A 36V switch on the handle and a controller bard for the low voltage cutoff between Motor - and battery -. This board also has the third battery connector connected to a point labelled "COM".

So I suspect there is some communication between battery and tool that I cannot replicate because I don't have the original battery and also no oscilloscope.

I dont want to simply bridge the board as this would also disable the low voltage cutoff which is not included in the battery.

the board mainly has a shunt, a mosfet and a MCU (Holtek HT66F018).

So do you have any good advise in which direction to proceed?