I recently had the need to have a power supply for my Canon DSLR, an EOS 40D, to make some timelapse. For this pourpose I've bought an LM2596S step-down adjustable power supply module from eBay, it's very cheap, efficient, compact, and (I hope) safe... and it fits exactly inside my exhausted lithium battery case. And this instructable could be handy adapted for almost any digital camera battery, the only requirement is that the case contains the power supply module.
Step 1: Intro
Step 2: What's inside?
You have to cut/crack the plastic casing all around the perimeter, because it's glued verty sturdy. Inside you'll find two 18490 Li-ion batteries, which are very rare to find to replace.
Step 3: Keep the shells
Maybe you wish to keep the protection circuit as replacement...
Step 4: Drill the hole
Drill an hole in that position and file it or cut it untill it becomes of the same shape of the wire. I've also added some heat-shrinking tubes to toughen up the cable extremity.
Step 5: Glue the circuit
I've insulated the two unused contacts, and also added a piece of tape to not let the pcb touch them.
Then I've used a double-tape to lock the circuit in place. I've added another layer of tape over the one you see in the pictures.
You can then begin to solder the wires on input and output terminals.
Step 6: Solder it
Step 7: Label it
Step 8: The socket
[UPDATE] You can now see the detail of the cable coming out from the rubber cover near the battery door.
Step 9: Done!
Everything is ready, it's time to glue the shells pair, maybe with two elements glue, and try your new power adapter!