Wouldnt the ascent decompression kill the occupants anyways? Getting them up to surface isn't the saving grace to something like a battery failing. Maybe I'm missing something?

No. Subs are different from scuba. They are kept at atmospheric pressure.

For scuba, you are breathing pressurized gasses much higher than 1atm at depth, which causes decompression issues.

There's no decompression sickness on those subs. The inside is kept at a similar pressure as the surface.

the whole point of the pressure chamber is that it remained at 1 atmosphere. until it failed.

That's not a backup, that's a contingency. I'm saying why not put another fully charged battery in the sub so it would be less likely to be completely unpowered at some point.

I don't know anything about submarines, but it doesn't seem crazy. Maybe they figure the battery has a 1 in 100 chance of failure and the sub can only make 50 trips in it's lifetime and it wasn't worth it considering the other systems like the weights and the hydrolic something or other that they actually used to get to the surface. Everything is a trade off and the extra battery would have cost money, take up space, could catch on fire, etc.