Bypassing DSLR battery to power camera from power supply cheaply with bidirectional optocouplers
On my Panasonic, when I need to do long time lapse, and before the special adaptor could be bought, I hacked open an existing after-market battery, and attached two fine wires (strips of foil actually) directly to the battery tabs (the flex coming out of the cells), where they attach to the little protection pcb at the front of the battery.

They brought the battery connection around to the back of the battery, and out a hole at the back, and through the (provided) hole in the battery cover. Tape the battery case up with Kapton tape, as it is very thin.

Then float the battery with a simple lm317 regulator. The float only has to meet the average long run current drain, as the battery powers the camera during shots.







simulate this circuit &ndash; Schematic created using CircuitLab

The arrangement works fine - well it did once I thought to put the diode in... 

You need to ensure you do not overcharge. If the cell voltage is 3.8V/cell (Panasonic has a 2 cell battery = 7.6V) when floating on the external power you are well away from overcharging (>4.2V/cell). This voltage keeps the battery at ~3/4 charge, which is fine for me. 

Give some consideration to how you could cock things up. Note D2 to protect against reverse polarity. If you are doing this in a car, consider if any exposed metal parts of the camera are connected to one or other of the battery terminals. (if the metal was connected to B+, touching camera to car would be destructive)