Here's an extremely simple way not to change your battery and keep experimenting with replacements as necessary -- well, I actually changed the almost exhaust original Varta battery of my battle DR
but that's not the point.
I thought this very simple wiring of a two way switch would let me keep trying external batteries of higher capacities without the need to dremel out the internal ribbing and doing other nasty stuff before being sure they'd work. I took the pictures months ago but I just remembered I had forgotten to put them here. Even if the pics are bad I hope everything is clear enough. If you have questions, just ask me.
The switch is fixed to the internal base with a minimal drop of thermal glue so it can be torn off quite easily. The plug that connects it to the board socket was taken from that original battery (now dead). The socket where the substitute internal battery is plugged was taken from a dead board to minimize soldiering, etc. The external battery plug and socket are normal JIT stuff I bought at eBay ($2 for 20 or something like that).
As you can see in the second picture, now I have my double capacity (and exactly double size) on the out side, and I have fixed it with a bit of duct tape while I conduct my experiments. If for whatever reason I need to get back to the original one, I just flip the switch with my thumb nail and I'm "back to normal" in a blink. I'll report about my experiment results (mostly a failure) in a few more days in
the other battery thread