If you read through the link @texases posted and don’t understand what they are talking about don’t mess with the battery pack yourself. There are a couple of things going on in that post; 1) Finding any bad cells in the chain of batteries that make up the pack and 2) Equalizing the charge by discharging each cell and recharging each cell so that all the cells in the pack are fully charged to the same level - and not reverse charged!!
A multi-cell battery is a bit like drinking from 6, 10, 20 glasses all at the same time. Re-fill with a sprayer that covers the whole group of glasses and hoping each glass re-fills to the same level, 5000 times. Once you take a few thousand drinks, the level of liquid in each glass will no longer be the same. Some may be broken and hold no liquid at all. You job is to find and replace the broken ones and re-fill each glass so it is exactly the same to start all over. And a few of those glasses are weak and ready to break soooo you’ll have to do it all over again in 1000 drinks.
Did I paint a picture that scares you? If so, pay the $3600. If not, have at it!