There’s really not much to it. The ebike battery just sits in an empty space in the housing and is the same voltage (36v-40v) as the original packs (just bigger). I padded it with some upcycled packing styrofoam to keep it in place and cushioned.

I’d have to take it apart to get pictures, but in a nutshell:

I cut the positive lead off of the original battery socket and spliced it to a XT-90 connector. I left the original ground connected to the socket and also spliced it to the XT-90. That left the ground and the yellow “data” wire going to the original socket. I drilled a hole in the mower housing to bring out the barrel socket for the charging input.

I only disconnected the positive from the original battery socket for 3 reasons:

1. The mower will only stay running if it detects an OEM pack, and the data wire and ground are needed to trick the motor controller. I have to leave my old/dead OEM pack in the battery bay. I was going to pull the board out of the dead pack, but this works well enough so haven’t bothered.
2. I don’t want to parallel the OEM pack and the ebike battery for safety reasons. In theory, I could have extra capacity that way but the batteries aren’t the same chemistry, capacity, or charge level.
3. If I ever want to return it to a stock config, I just have to solder one wire back.

And this is the mower itself (stock photo). The batteries are interchangeable with other tools, and are far too small for the mower. They draw about 2C (twice the capacity) so that’s 10 amps from a 5 amp-hour pack and that’s pretty rough on them. With the 10 amp-hour ebike battery, that’s only 1C so I get both better runtime and less wear and tear.