Usefullness of a mobile device diminishes greatly when its batteries cross the River Styx; passing from the land of the living into the realm of lost socks.
Add a battery case?
Sounds like a great idea, but you might have to give-up wireless charging (if your phone has it). Charging receivers need to be coaxially aligned and in close proximity to a power transmitter - a battery case will rain on your phones charging parade.
Here's an idea, why don't we integrate a universal Qi wireless receiver into the battery case.
Now we're talking!
If you don't want to void your warranty or risk letting the magic smoke of out of your phone or battery case... I would recommend using the Qi charging stickers that terminate in Micro / Mini USB connectors.
For this guide, we will use the Qi Universal Charging Module and integrate it into a Nexus 5 Limefuel battery case.