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I recently made an extra battery pack for my Nexto PSD. It uses Canon BP-511 batteries, but you could easily make one that used a Nikon EN-EL3 instead.
And if you have a different PSD that happens to take an external 7.4V battery pack you could do something similar for it.

I should mention that moz came up with the idea: this is just my implementation of it.

Thanks Dave

Moz kindly gave me the clues as to how he built his but it certainly helps to have the pictures in the link you posted.
I have built one that takes AA batteries (cost $3.50) but having the one that uses the camera battery is a better alternative given that I carry a couple of spares in my camera bag

Ray