Acton M electrical problem

I'm working on am Acton M for a lady with a disability. It has worn out the two batteries she had. I offered to replace the battery with a larger capacity LiFePh4 48v 25amp hour. Her's were 48v 12amp hour. I did a test run with the new battery mounted in a new battery box mounted between the feet on the frame. Worked great. When I installed a new power connector, with the polarity verified, I got an arc when plugging in the power from the battery. The key was not in the schooter. I'm looking for a schematic for the Acton M to help get her schooter going again.