Horrible battery life

Hello,
I have had my G3 D851 since it launched. The battery life was always on the short side but not so short I couldn't use the phone. I went to Lolipop and started noticing these issues. I recently went down to a usability of about 4 hours on my battery so I went and bought a new OEM battery to see if it helps. Well it did, I was able to get to 6 hours. Mind you this is 6 hours of standby mixed with maybe 1 hour of actual use. Today my phone shut itself down when it said it had 35% battery left. And I usually go from 100 to about 50 with just 40 minutes of youtube. Currently I am using SlimSaber 5.1.1. Im really having trouble using the phone because I have to carry around a 6000mah external battery pack and 2 batteries to make it through the day with moderate use. I think thats crazy since before I didn't have to do that. Should I just flash the full stock rom, radio, modem etc. would that work? Thanks

Horrible battery life?

I got an S4 i9505 replacement yesterday from my insurance company. It effectively seems to be new phone and battery, anyway when I turned it on it had 70% battery left, I logged into my various things and it sat about downloading and restoring and installing a 5.0.1 update, the process took about 30 mins and at the end of the 30 mins I had 9% battery left, I connected to the charger and it was full again in 30mins which I found odd.

Anyway let it sit on the charger all night, took it off this morning and with 2 mins of screen on time I am already at 94%, I can't see how this thing will last the day.

I am not a newbie, I have things like sync and GPS etc turned off and screen brightness lowered however in saying that even at the lowest setting it seems very bright.

Any ideas?