What should I do if I buy someone else's old battery car and it hasn't been smooth?

You can negotiate with the original owner to return the old battery car that has been dissatisfied.

When buying and selling second-hand battery cars, you must communicate with the seller in advance. If there is a problem with the goods, no matter whether it is solved or not, if it is not smooth due to its own reasons, it can only be repaired by itself.