Several years ago I bought a Gyro Wheel for my daughter, who probably couldn't even walk at the time. The idea seemed really good; a heavy cast-iron wheel is spun up and the angular momentum keeps the new-rider upright while they learn to ride.
I don't suppose it revolutionised bike-riding, as it seems that the company has disappeared. Now my daughter can reach the pedals, when I finally dug it out the other day it was sad to find it wouldn't charge. Googling seems to show that it's a fairly common problem.
After pulling it apart, I found that it was fairly straight-forward to replace the battery-pack. Getting in requires a torx screwdriver but is otherwise straight-forward.
The battery looks like a bit of a custom job, with two-halves of a 9.6V battery-pack split into two. You won't be able to buy anything that directly replaces this, but you should be able to find individual AA-sized NiMH rechargable batteries with welded-tabs that you can use to create a new battery-pack (don't buy batteries without tabs and try to solder directly onto the battery terminals, it won't work). Unwrap the plastic from the existing packs and just follow the polarity; if you carefully de-solder the existing wires you can save yourself a lot of work making new ones. Wrap it up securely and put everything back together.
Below are some photos just to give you an idea of what to expect.