If you have some knowledge of electronics and batteries and testing equipment and soldering capabilities, you could try tearing the battery unit part. It's possible not all of the cells are damaged, and only a few that are not able to hold a charge. At 25.8v, it looks maybe 1/2 of the cell clusters (cells in parallel) are not holding charge or dead, but also possible only a cell or two in that cluster may be totally dead dragging down that cluster.

It's a bit of technical detective work, but considering the price of brand new battery pack might be worth the time if you only need to replace a few cells.

... a note of caution ... messing around with and charging batteries can be very dangerous - overheating and exploding batteries are a possibility, so do it only if you know what you are doing ...