I replaced the battery in my mom's old iPhone 4s. Battery is direct from iFixit. I've replaced iPhone batteries before and everything seemed to go right. Problem: I charge it fully before bed, and when I wake up it has like 30% power. My iPhone 6 will have battery percentage above 90% in the morning. I understand the 4s battery is 1430 mAh vs the 1810 mAh in the 6. But the drain difference seems too big. I did a DFU restore of OS 9.3.5 (the highest a 4s supports). Didn't help. Ran battery to zero once to condition it. Auto-lock is set at 1 minute. Mail accounts are set to fetch, most notifications turned off, etc. Seems like the iPhone just doesn't fully go to sleep to save battery. Any ideas on how to slow the battery drain? Is that just how it's supposed to work? Should I give up and get her a newer iPhone?
The only thing I could recommend for an older Apple device is to call Apple Support and have them run diagnostics on the device (or you can visit a store).
Worst case scenario, the only thing that may be the culprit is the logic board. Apple no longer has those so you may need to contact an Apple Authorized Service Provider and see if they can order the parts or find someone that could be selling the parts.
Before giving up, I did a little more research and found that iCloud stuff is a common battery killer. I turned everything iCloud off (Photo stream, Backup, Contacts, Reminders, etc.) except Find My Phone. Did a hard reset, then gave it a full charge. This morning it had 12% after 17 hours of standby. Not great, but usable. I think that was the big remaining culprit.
p.s. I think previously I was getting like 9 hours of standby power after a full charge. So turning off all that iCloud stuff appears to have doubled the battery life. I'm guessing either the hardware in the 4s can't practically handle those features, or the OS isn't really designed for a 4s, or the logic board is shot somehow.