What caused my iPhone's battery health to decline overnight?

“What caused my iPhone's battery health to decline overnight?: When I got my phone out of the Power and I checked the battery health and it was at first 87 but today when I got up and checked the battery life it went to 87-86 i could of had not charge it over night”

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Determining Why the Battery Health Decreased:

You Can’t. Unfortunately, there is no way to determine why, nor when, the Battery Health decreased. Apple recommends you get the battery replaced, when the Battery Health reaches below 80%

Hello,

If it won't charge:

Check the charging port for lint or other debris (use a bright light and a toothpick).

If your iPhone, iPad, or iPod touch won‘t charge

Also see:

About Charge Limit and Optimized Battery Charging on iPhone - Apple Support

If your battery health is low, consider a replacement:

iPhone Battery Service

Good luck.