Talking about backup batteries, I sometimes carry a portable USB "power bank" as a backup power source for my flight computer (Nook), in case of problems with the main glider battery, 12v-5V converter, or the associated cabling. I found one of the rare models with a real (slide) on-off switch - the far more common ones with a pushbutton seem to turn themselves off whenever the Nook isn't drawing enough current.

So, unlike the LiFePO4 batteries discussed in this thread, this is a lithium-polymer battery, a less stable chemistry, albeit a lot smaller (about 2-3AH at 5V). Is it safe enough to carry in the cockpit? Before you reflexively say "no", remember that you are (probably) carrying at least one such battery, in your cellphone. There's also one inside the Nook, and any other tablets people use in the cockpit. And inside some varios, Portable powerFlarm, who knows what else - almost anything that is rechargeable these days.