strange issue with Iphone battery, please help!

Over the last 24 hours, I've noticed a rather strange issue with my phone's battery.Yesterday, I took the phone off of charge before going to school. It was idle for most of the day, accept for a few minutes in the morning when I checked in using swarm.When I got home, I noticed that my battery had gone from 100% to 88%, and that was with it being in standby for most of the day. I made two phone calls, which dropped the phone from 88% to 68% within the space of maybe 45 minutes.I put my phone on charge at around eight pm, US eastern time. Nearly 12 hours later, voiceover is saying that my phone is at 100%, but it is still charging. I have an IPhone SE that is just over a year old. It is running the latest version of IOS. I have things like background app refresh turned off. Any ideas?

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#1 To me, it sounds pretty

To me, it sounds pretty reasonable. Even if your phone is in stand by, it does a lot of things. Search for cell towers, wifi networks. If 4g or wifi is turned on, it occasionally might communicate with various servers to update info / check for updates ETC: From there the initial 12%.
Now, the phone calls might have dropped the rest 20%, that sounds quite all right to me.
Also, it is normal to see "Charging" alert even if the battery is at 100%. What iPhone does when charging is to charge the battery up to 100%, then stop charging it, until battery drops to 100%. Then it starts charging it again. Thats to keep the battery safe and healthy.
You must've seen the indicator on the charging up phase.

#2 Hmm,

Hmm,
Sounds like it might actually be a battery issue. I would recommend getting a battery app like battery life. It'll give you loads more info than just 100%, you can look at the capacity of the battery and see how much it deviated from the full capacity of the battery from when it was new, check the voltage of the cell, and look at what it charging with. I'd really recommend this, for instance you can check the actual capacity of the battery in miliamps, and determine how much your battery has decreased over time. Normally, these can last for about 2 years or so before you notice a significant drop in performance. Also, do you close all apps once you're done using them? It might just be that the battery has a diminished capacity and the best way to fix it is to go and get it replaced.

#3 Something you could try

a possible test you could try is to charge up the battery fully and lead it on the charger until you intend to go to bed. then remove the phone from the charger and leave it idle on the lock screen overnight. if you see a drop in of more than a few percent, say 10%or more then you may have an issue. however the downside to this method is that it will only indicate that you have a problem, not what is causing that problem. For that Justin's advice is better.
Hope this helps.
Lou

#4 Do you have any apps using

Do you have any apps using your gps location? this could also drain battery, even when the apps are closed. I too have some times where the battery drains faster then expected or I think it should have but I just checked my level with the battery life app and the wear is very low at 1 percent which is very good.