If you want to recycle one of your old camera batteries (i.e. Sony / Canon), here's a way to take old cheap generic battery chargers as a good mounting plate. If you were to search for 'LP-E6 Battery Plate', you'd find some options may be bulky and much more expensive over one of these chargers. You can use this method to use Canon LP-E6 batteries to power your Sony A7s, or even Sony NPF L-Series batteries to power your Sony A7s.
The cleaner way to do this would be to just wire the dummy battery directly to the battery plate, but I still wanted the option to use my AC Wall adapter and so I added a Male plug to my battery plate. Remember although this video shows my Canon LP-E6, you can certainly use a much larger high capacity Sony L-Series NPF-970 Battery instead for super extended run times.
You can find these battery chargers sometimes on auction for just 0.99 Cents via eBay (Found Here).
Canon LP-E6 Battery Chargers
And if you plan to use Sony NPFx Style Batteries, you can find those Sony NPF Battery plates on eBay too (click here).
Sony L Series NPF Style Battery Chargers
Oh and you'll need the Sony Dummy Battery which comes as part of the Sony AC-PW20 AC/DC Adapter Kit. Here's a few links below:
Sony AC-PW20 AC-DC Adapter Charger - via eBay
Sony AC-PW20 Adapter - via Amazon
Ok and so if you're a bit weary about figuring out the Negative and Positive wires on a Barrel connector (male plug), these little adapters are PERFECT. Just run your wires directly into the properly labeled areas and tighten down the screw. No soldering required for this 5.5mm OD | 2.1mm ID Barrel Connector (find them here).
5.5mm OD | 2.1mm ID Barrel Connector