I installed the ML Toys 775 http://www.mltoys.com/product-p/stage-iv-quad.htm
I have three grandsons, three PW vehicles, New to upgrades, needed to have the 775 run at 12v as I waited for a matched set of batteries to arrive. As soon as he started riding in the grass the extra friction tripped the thermal fuse. On the blacktop, no issues. I installed a second battery in parallel. The second battery was 12ah, as you probably know the stock is 9.5ah. This cured the tripping problem, and no ill effects such as over heating. I know for series they should be matched, do not know about parallel.
I did not tighten the axle nuts enough, worried about damaging plastic parts. There was slop in the outer drive mechanism, that caused issues when I did go to 24v. This wore down a plastic collar on the housing. I cannibalize the original gear housing, and replaced those parts. This time I snugged down the axle nuts.
As soon as I got the vehicles I wired in battery tender pig tails for a better charger. I just tapped in right before the PW battery plug. With two parallel batteries I made another pig tail, battery tender to F1 connectors . Plugged that into what was my charging port. The F1 pig tail connectors are color coded for charging. For running, I need to reverse polarity, or it runs backwards. With parallel batteries it will blow a fuse.
I am having an issue running 24v with the battery tender connector over heating. Not the other wiring, just the battery tender plug. I am going to change it, beef up the jumper wire battery + to battery - , see if that helps. It is the oldest of all the pig tails.
I had a bad case of siblings rivalry going on. The 24v is for the sensible, rule following, eight year old. His hell raising, rules be darned, younger brother, lost interest in riding when his older brother stared riding rings around him, literally. They have both been involved with the work in progress upgrade. I told the younger brother that if we get the problems all fixed, I will upgrade his ATV too. He decided grandpa is not so bad after all.
Grandson number three, the youngest, is autistic, rides a Wild Thing, and uses those joy stick controls like he was born with them. He just has fun, no rivalry, but I am going to get him more speed too.
It's fun being a grandpa.