Batteries can be rebuilt. Most of the Lithium ones use 18650 cells just replace the cells with like ones and solder together. The main issue is opening the battery pack up. But if you can get that done you can usually make a better battery anyway using a higher quality cell. Did it with a 12V battery 15 years or so ago, and oddly enough it was only a 10v battery pack so I added the cells to bump it up to a true 12V pack.