Alright, I got it working!
First step was cleaning the old stuff out. I removed everything including those contact posts that connect it to the camera, which were just press fit into plastic holes and could be twisted a bit to loosen them and then pulled out.
Cracked the case unfortunately trying to get the metal insert out. I thought it was held on with double sided tape but it was just the foam tape under the cells overlapping, but I flexed it a little to try peel it off the imaginary adhesive.
Half assed attempt to hide the glue repair. I might come back at this.
Spot welding the cells. I bought the battery spot welder a while back and the thing makes me nervous. Just a big old lithium battery you directly short out. They're vape batteries and designed for it I suppose, but I still don't like it.
Once everything was soldered back in place and tested working, I applied a few dollops of hot glue to hopefully prevent it sliding around. The new cells are a bit narrower and looser, and while the foam holds them in place, some sharp bumps or dropping it could have the packs slide and possibly short something. Also the removed and reinserted contact posts were glued in to hopefully prevent them rotating since their tabs could touch and short.
If you can get the top metal cover off there's little reason to dissemble it as much as I did.
The charging setup is quite crude, using an old RC hobby charger and the connector from a particularly busted motor drive. I'd hoped i'd be able to charge it all neat and properly through the DC jack but the hobby charger didn't like that, complained about over-voltage. Probably because there's a diode in the way or something... I think I forgot to take a photo of the top side of the pack's PCB, or i'd try trace it out. But yeah the two power pins are just directly connected to both end of the battery. I'll probably try make something a bit neater at another time.
But yeah, glad that's working now. The rechargable battery looks alot less silly than the huge tall AA one, although I think the AA one looks good with the control grip. Since they're quick detachable from the motor drive it might make sense to just keep the control grip always attached to the AA pack. You could use battery packs as a weird tripod quick release.
Something i've read occasionally is that the motor drive can only hit its top speed with the NP-90M, which didn't make alot of sense to me. The manual has the line
"for maximum film advance speed of 5fps, fully charged NP-90M must be used" which could be read as saying the BP-90M can't hit 5fps, but it could also just mean you've gotta use a freshly charged NP-90M to hit the advertised top speed. But maybe mid 80s alkalines couldn't provide as much current as contemporary NiCd?
Either way it's hard to compare them now since 12 fresh alkalines is about the same voltage as 13 charged NiMH cells, then there's different voltage fall-offs and stuff. But i'd say there's no significant difference in performance speed-wise between the two battery packs. I also suspect the motor drive is regulating the framerate electronically anyways.
