Restoring the battery pack using standard "AA" size rechargeable batteries.
Notice that the top half of the battery pack goes inside the bottom half. Knowing this, you should be able to break open the glued-together pack a little nicer.
The original battery pack holds 2x9 special size NiCad cells in parallel and the open wire at the right is not in use any longer, but is left hanging. The + wires has a non-destructive fuse. If the battery pack is short-circuited or overloaded it will open the fuse and reconnect automatically again after a second.
Close-up of the capacitor(?) connecting minus and ground.
You must never, ever, solder directly on the terminals of a Ni-MH rechargeable battery!