Miserable battery life after installing a new iOS update is normal, and iOS 12.4 is likely not to be any different. Installing updates seems to throw off the battery calibration, as well as putting the additional workload on the battery.
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So, what's the first thing you should do?
Yes, you read that right. Nothing. Just give your iPhone a few days – and a few recharge cycles – to settle down. Just use your iPhone as normally as possible, but recharge it a little more regularly if you feel that it won't make it through the day. It's possible the battery recalibration won't have much of a negative effect on battery life, and it just seems like the battery life is dropping faster than usual.
If after a couple of days you still feel like battery life is poorer, then it might be worth recharging the iPhone to 100 percent, then discharging it down to below 10 percent (try for 5 percent if you can) before recharging it again.
This simple trick can work wonders for the battery.
Still got battery problems? There may be a bug in iOS 12.4 causing a rapid battery drain that will require Apple to push out another update (although I do not see any reports of such a bug yet, it is early days). Another issue may be a buggy app that you have running.
The good news is that iOS 12 contains several tools and features to help you diagnose and solve battery issues.
Below you will find information on how to use them to help solve problems.
iOS 12.4 draining your iPhone battery fast? Here's what you should do