ZeroLemon makes these pretty nifty portable battery packs that you can recharge (albeit very slowly) using the large solar panel on top. The company also produces some rather ridiculous-looking extended battery cases that, in addition to providing your device with a metric ton of power, turns it into a plastic brick. No, they don't make the thing stop working—they make your phone the size of a brick.
The case attaches a 2,800mAh battery to the back of your Galaxy S6 to more than double how long your phone lasts throughout a day. The material may also make the phone's glass back easier to hold.
The regular price is listed as $59.99, but you can currently get the ZeroLemon battery case on Amazon for 50% off.
As a commenter has pointed out, you can get the case for $19.99 by using the coupon code NHASAPTH.
Just note, it's not compatible with the Galaxy S6 Edge. But if you wanted to cover the phone with a case, why did you pay extra for those attractive curves in the first place?