It's an interesting idea, but it woud add a lot of weight and bulk to the phone, which you would only need let's say for the second 1/2 of the day
So would you want to carry that around with you all day?
The alternative, which I use, is to have a battery pack (mine's a 5000mAh) which I plug into the charging socket of the phone via USB-microUSB cable when I run low.
The battery pack has the same dimensions and weight as the phone, but I keep it in a bag until needed, so it doesn't affect the phone's bulk until I decide to charge.
And they are cheap too, I got mine off Amazon for €22 I think it was. NewTrent is the brand, I can recommend it. The price fluctuates quite a lot. There are also some cheaper alternatives, but I wouldn't bother with a 2000mAh pack, since it wouldn't pack much of a punch. Also the NewTrent one has 2 output sockets and can provide a 1A output, giving a fast charge.
And you can keep your favourite case too, no worries about GPS signal either