I have a Galaxy S5. Build number MMB29M.G900FXXU1CRH1. Knox version 2.6 Kernel version 3.4.0-14131106.
At night I turn on ultra power saving with just the phone and messaging activated in case any emergency calls come. Until two nights ago I had no problems. Each morning when I deactivated ultra power saving the phone resumed normal service and there was a good reserve of battery power remaining.
Yesterday morning, when I deactivated ultra power saving, I immediately got a message that the battery was critically low. It showed 1% remaining and it powered off. I recharged immediately and the phone behaved perfectly normally during the day. Last night I reactivated ultra power saving as usual. At that stage, the battery level was 63%. Eight hours later when I deactivated ultra power saving exactly the same sequence occurred as yesterday and I had to recharge immediately.