With the battery case detached from the device, look at the side with the copper terminals. You should see several seams that can be exploited with a sufficiently thin shim. I carefully used a box cutter on the right hand inner seam of the battery case and separated the battery case into two pieces without actually cutting or breaking any of the small plastic tabs that allow reassembly.
Inside the battery case you will find a standard 4xAA NiMH pack wired to the copper terminals, with soldered connections. Cut the wires as close to the battery end as possible and strip a small amount for re-use or connection to a new pack or connector.
Here an Eneloop NiMH AA is shown for comparison.
- You can find 4 AA NiMH packs pre-made on eBay and Amazon for under $20.
4 eneloops will also fit in the compartment. I made a pack by taping and soldering 4 of them together in series.
But I think it would be better to buy a pre-made NiMH pack because (i) consumer NiMH batteries are not really made for direct soldering and could fail early or create a hazard, and (ii) if you try it anyway, you'll fight with some issues with melting plastic and clamping/geometry to get good connections.
- Reassembly is simply snapping the two pieces of the battery case back together.