You can go to Ebay and search this...
WE BUILD Dewalt 18 VOLT XRP DW9099 Battery Packs Dewalt BATTERIES
This will allow you to replace just the batteries.
That's it? Did you ever do it?
We personally have not, but have heard of others having good reports after changing the batteries out. You can go to youtube.com to watch how to replace the batteries if needed. Good luck!
I went there and the insides alone are $58 dollars. Might as well buy a brand new one. The cost is too high to rebuild rather than just replace.
I know they do have a ton to buy online on ebay. You can look here and buy new ones for a good price. They have used ones as well as new on there. We have purchased a ton of parts online, but I see this part may be not worth paying for. I wish you luck on your purchase.
Just found this item, its a really good deal. Hope this helps
or-ebay search 2 new dewalt 18v DC9099