This is a cart that gets used once a month. It lives on the charger until it's used. Batteries are maintained.

When you start driving, it will run for a while, a short while, then it will start running slower, and slower, until it stops. If you park it and leave it, you can come back to it in an hour or two and it will start right back up, then repeat.

Is this common? Seems like something is heating up and causing resistance and that's killing it, or are the batteries just in need of replacing? How do you test batteries?

I'd like to get it fixed and sell it.