I‘ve just rebuilt mine.
Take care to find the three chrome coloured screws. 2 underneath the leatherette - yes it is leatherette even though it doesn’t look like, and one on top besides the stick to connect the battery pack to the body. I didn’t find them all and ripped the screw holders.
That didn’t matter anyway, since the replacement cells I used were too large and I had to hold the cover in place with electric tape.
I use the original charger with no problems so far, but it doesn’t change to green, even after charging for 24 hours.
You need to unscrew the metal plate at the top which sits under the ergonomically shaped handle and reflects the handle‘s shape. 2 black screws.
Underneath there are some cables. Take a picture of their positions and desolder the red. There are two black cables, desolder the one located closer to the Center of the board.
No worries about pulling them out together with the batteries, you should replace them with new and longer ones so that you can install the cables first, solder them to the battery pack and pull them gently while inserting the new battery pack. Cut them to length before soldering.
The only problem about voltage differences should be, that the charger doesn’t detect fully charged voltage. But since the internal electronics of the camera are normally supplied through another battery behind the handle, the only drawback might be reduced torque from the electric motors.
Maybe some day I might write a step by step instructions including pictures on my blog. If so, I will link it here.
Edit: I just noticed, that using the power winder now doesn’t support shutter priority AE any more. But I can‘t recall, if it worked before I replaced the cells.
What I can say for sure is, that using my Motor Drive shutter priority does work as intended, so it is not a fault inside the camera itself.
But maybe I have contact problems after desoldering the power pack’s mainboard.
