Power for my H3DII50

[DBF]

I am looking for a Power-Supply that lasts real long.
This is my Idea:
Take the CR123 Battery Grip, solder a flex Cable into it and buy some "fat" rechargeble Batteries (7,2 Volt) and connect them (taking care of Isolation of all Contacts and of course + und - ).
I would take the connected Battery in my Pocket.

Is there a Reason, why I should not do this? Can I damage the Camera/Back by too low Voltage?

I have used an Image-Bank2 for longer lasting Battery on my last Trip, but the FW-Connector at the I-Bank broke - is not rugged enough.

I have the GPS-Module atteched at the Camera. But this searches for 2 - 5 Minutes after switching on the Camera the Position.
I like to have it working any Time, when I want to take a Photo.