My iPhone is not showing battery health what’s the behind that ?

If you had a 3rd party battery replacement done, the iPhone cannot read that battery for health. It will usually report that you do not have a genuine Apple Battery installed too.

Peter, their profile shows an iPhone 6s Plus with iOS 14.

Thanks ChrisJ4203

So it should show it. Non genuine replacement or damaged battery seems most likely then.

Even the very last 6S produced around 2018 will surely be below 80% by now and need or had a replacement.

I just want also to make sure we are not talking about the % at the top of the screen!

Yes, I was referring to the Battery Health in Settings. As far as I know, it may or may not display with a non-Apple battery, but most of the time I've seen it won't, and should display the warning that it is not an OEM battery.

Has the battery been replaced by a non Apple repair shop?

yes that is clear.

Consensus is you need a new battery and strongly recommend Apple does it for US$49 or your equivalent

Battery Repair st Apple

iPhone Battery & Power Repair

see if any locations near you are open:

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You will probably need to send it away.

I am on my phone browser so I cannot see what phone model or iOS you have.

What are they?

Did you have this phone from new?

Using another charger will not affect whether you have health % shown in Settings, Battery, Battery health.

I say that in case you are talking about battery charge % at top of screen

Some information here. But you must need a battery by now.

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

Battery Repair st Apple

iPhone Battery & Power Repair

see if any locations near you are open:

Find Locations

Thanks for clarifying that. Still learning!

I wanted to make sure that iqra132 is not talking about % at top of screen as I have been caught that way before.