I don't really know when this happens and what causes it, but while my device is charging, VoiceOver announces that it is charging when I navigate to the battery icon in the status bar, but the percentage just remains the same when I check back a few minutes later. There have also been cases where VoiceOver would just stop talking, making me find out in a few seconds that the device had just shut down. It appears to actually begin charging when I unplug and replug the charger. Note that the battery health percentage is displayed to be at 87 percent, and the battery doesn't drain so fast to make me think I should replace it with a new one.
Comments
Calibrate the battery gauge
Not sure what to suggest here, but have you tried calibrating the battery gauge by letting the battery drain until the device turns off, leaving it a few hours, then charging to 100%?
Will try it.
Thanks for the suggestion, I'll inform you of the result.
Battery calibration.
Battery calibration might actually damage your phone draining it to 0 percent, then back up again. That will probably just put strain on your battery and not actually fix anything. Modern lithium ion batteries don't need to be calibrated, and in fact it's usually harmful. I'd recommend trying a different charging cable, or power brick. It may be an issue with the cable.
Right but
The device keeps on shutting down anyway, so the battery must already have suffered more than enough damage.
At 87 percent battery health.
You're right on the edge of a healthy battery and needing to replace it. An old phone I had started acting weird with the battery around 85 percent, random shut downs and freezes. I'd still recommend a new charging cable. Discharging to 0 percent is just wasting a charge cycle.
Apple says it is at 80% you…
Apple says it is at 80% you should think of replacing your iPhone battery. It might be the brick, the cable or the internals of the device getting loose. Experiment with the bricks and cable to see if there is any improvement.
I'm sure there's nothing wrong with the brick.
I've been charging other devices there, and my iPhone elsewhere. I may be able to experiment with another cable.