Any owner of the Sony A7s will immediately tell you that battery life sucks big time. Some guys may opt in for the $300 Dollar Sony VGC1EM Battery Grip (found here), which allows you to run (2) Sony batteries at the same time.
$300 Dollars is a bit expensive, and there are Generic VGC1EM Battery Grips for $65 Dollars (found here), but because of the poor life of each Sony OEM battery, even two most likely still wont be enough for a solid day of production.
Recently I posted up a short little article about how to make your own Sony A7s external power source using cheap battery trays (here). Since both the Canon LP-E6 batteries and the Sony NPF style batteries run similar voltage, you can use either one to power up your Sony A7s for cheap and not to mention several times longer than the A7s battery.
An external power source has been very handy, but this means always having to mount the external battery with the camera at all times. You can see how SatoStudios has been mounting one to either a cage or shoulder rig (below).
After testing out the Canon LP-E6 batteries (which I have more than a few dozen of), I thought about modifying a generic Canon Battery Grip I had laying around.
This would mount under the camera, clean up any loose wiring, and still be able to tripod mount my Sony A7s. So I basically gutted the generic Canon Battery grip, rewired the battery terminals inside directly to a Sony Dummy Battery adapter.
Now because the grip isn't smart enough to switch over once a battery is depleted, and it may not be wise to run these batteries in Parallel, I decided to add a simple toggle switch. The good thing is that the Sony A7s seems to read the battery levels fine, and the toggle switch allows me to use either Battery A(1) or Battery B(2).
Although she doesn't look pretty, I didn't have to spend any extra money outside of a $13 dollar Sony Dummy Battery (found here), and EACH Canon LP-E6 battery runs several times longer than the Sony A7s. I haven't put any run times tests together but theoretically it might do 4-5 hours.
While it may not be a worthwhile project just anyone should tackle, it's a neat way to recycle my old gear. Not to mention improve shooting times on the Sony A7s by several times over while keeping cords and mounting to a minimum. It's a nice way to balance out the small body when using larger lenses too.
I don't suggest just anyone tackle this project unless you have a bit of electrical know-how. Of course it's fairly easy if you understand basic electrical wiring. I have another spare grip from a Canon T2i, and now attempting to build a battery grip for my BlackMagic Pocket Cinema Camera. I believe I can fit a 3s Li-on RC Battery into the slide out tray.