How can I replace the battery?

My flosser is way out of warranty. It has worked very well but gradually lost battery capacity and recently quit completely.

The good news is that this cordless water flosser is ALMOST easy to open up. It is held together with just 2 sizes of ordinary Philips (+) screws. But after I have taken apart all that I can, the top and bottom halves of the inner case are still held together by something at the top. The battery compartment at the bottom opens a crack but not enough to be useful.

Water Pik support won’t help. They say the battery is soldered in, and even if I can deal with that there is a risk that the case will leak. I know how to solder circuit boards, I think I can put it back tight enough, and even if it does leak slightly the flosser runs on low voltage so there is no rick of shock. Also a local battery company sells rechargeable AA cells with wires already attached.

Hoping that someone has discovered how to open it up. Or if you have one that is out of warranty and has failed in some other way, maybe you would be willing to break it open and learn what hidden feature keeps it together. Together we can solve this.