That's crazy. I've had three iphones, all of which I jb'ed multiple times and I never noticed a sliver of difference between stock and jb in terms of battery life. I'd go as far as to say that 8.4 does better than 8.1.2 on my 5S in that regard.
Check general>usage>battery to see what uses the most power. Install batterylife from cydia to check if your battery isn't dying.