I think I found the right battery relays


They make all kinds of relays and a lot of electric forklifts use them to switch their motors. I ordered a used one from Ebay for testing. 35€. The good thing is that they are made in the UK. Heavy duty and also heavy weight, but rated for high currents and voltages.

They don’t have Anti-Spark, but I think we could add a resistor to the contactor. If not, we could add an auxiliary relay just for antispark. The Arduino that controlls this will then connect the antispark before the main relay is connected and then disconnect it again.

Pacificmeister Build Info and CAD Sources

So you were the one bidding higher. Just kidding, those should work, I found a similar one on german ebay a few weeks ago, but I thought they don‘t come up regularly enough to be a permanent solution for me. I looked at the Albright SW180. Those are quite expensive new. Yours should be quite efficient, at 150A it‘s specifed for only 40mV voltage drop. That means you only have 6W conduction losses in the relay at full power. Thats more than the 2W an electronic switch would have, but still very small compared to the losses in the wires to the motor. Not as nice with your relay is the continous actuation power for keeping the relay on. The datasheet says 7 to 13 W. But again that should be at least ten times less than what your motor needs, so it will shorten your ride only a bit. Just remeber to switch it off after riding, otherwise it will drain your battery pretty quickly.