Why not buy one of those easy-to-find extra battery USB-charger things and carry that with you instead?
This is a much better solution than having to give all the contacts another number to try you at if your phone's dead.
If you're going to carry an extra device, might has well make it 99% battery/device ratio.
Also -- should have been a better shopper when picking your phone. I recommend the phone finder at GSMArena to narrow down requirements (including talk/standby time for the battery).