Can a Custom ROM affect the performance of my battery? My Device is Samsung Galaxy S4 I9500. A week ago, I flashed a custom ROM on my Device, now my phone's battery drains out even if the phone is on stand by. Previously, it degraded but not so drastically. Sometimes the battery percentage increases automatically.

Is it a battery problem or Is it due to the Custom ROM?
It's the custom ROM but it may not be the ROM itself. Most of the times it is the kernel that comes with the ROM that is most influential. But, it is possible that the ROM has a feature or two that is draining the battery.

Also, since it is an S4 I have to ask, is it a Samsung ROM or AOSP? Because AOSP is so much different that usually because it is not the most compatible with the hardware it will take a toll on the battery. Samsung knows how to customize it's software the best so generally Samsung ROMs (doesn't have to be stock) are better than AOSP ROMs on Samsung phones or any OEM phones for that matter.