We bought the “Fisher-Price Thomas & Friends Ride-On Train” for our toddler, and after some time, the battery stopped charging.
It uses a Power Wheels 6V 2400 mAh NiMH battery (Model No. 1102822518 / Part No. 3900-8218).
Apparently, this battery was fairly unique (and problematic) to this toy and has since been discontinued.
I, of course, did not want to scrap an entire Power Wheels toy that worked just fine except for the battery.
One thing I will say about it, though, is that it’s surprisingly repairable. I was able to open it up just by removing four Phillips screws.
From what I gathered, the circuit board is what makes this battery unique. It allows charging the NiMH battery via micro-USB instead of the typical Power Wheels charger.
All of the wires seemed fairly self-explanatory, except for two small additional wires coming from the battery pack.
Well, it turns out these 6V battery packs are just five rechargeable AA batteries strapped together. The wires were just going to a small thermistor to measure the temperature of the batteries as a way to gauge how much it’s charged.
After that, all I had to do was desolder the battery pack from the board, buy a replacement, solder it to the board, and tape the thermistor to the new pack. Then I sealed it all back up and I was good to go!
I did come across a service that seems to do this for you, but it turned out to be fairly straightforward to do it myself.