I've been looking for a (used) second battery for my TC for ages now, to improve it range. I find €1000+ for a new one way too expensive. I finally found one on classifieds site but instead of it being 60V30Ah as my battery is, it's 60V26Ah. Will that work? Can I use them in parallel or will that cause all sorts of problems (like one battery charging the other causing the TC to play dead).?
I think you will have problems as they will discharge at different rates due to the different capacity and get to a point as youve mentioned of one trying to charge the other one as the voltages will be different.
There is a post somewhere on here where someone tried it using a battery from a different model and they got that exact issue.
You might be ok running them separately and just swapping the cable when one is flat, but not sure if the 26Ah battery is from the early TC and may have a BMS with a lower max amperage on it that your TC controller may exceed depending on how old your TC is, I have one of the early TCs and it has a lower power rating on the controller that the later TCs.
