IOS 15.8.6 on Iphone 6S Plus. Incredibly unexpected battery drain.

Was the battery replaced 4 months ago by Apple? If so, take it back for free diagnostics. The update did very little, except to extend the certificate to use iMessage and FaceTime after 2027 where the previous certificate was set to expire. There were no security patches or features added in the update that would have changed any operations.

If the battery was not replaced by Apple or Service Provider designated by Apple, then you got an aftermarket battery and are going to have very little options.

Was not replaced by Apple, it’s a 3rd party battery replacement

This is not an Apple issue. Yes, the shop probably told you that the battery was "just as good" as an Apple battery. It isn't.

Take the phone back to shop and ask them fix the problem.

Can you please answer the question from Mac Jim ID?

Was the battery replaced by an authorized Apple service shop in your area? If it was, the name of the shop will appear on the list when you tap to open this link:

Find Locations

yellow amarillo wrote:

Was not replaced by Apple, it’s a 3rd party battery replacement

Bummer. Genuine Apple parts are not available to unauthorized service providers. There's no telling what you got.

Take it back to them and see if they can help you. Apple won't touch it. Honestly, it's time for a new phone. That one is obsolete.

This may help --> Prolong and Maximise Battery Performance ... - Apple Community

Most likely third party, I don't think Apple will replace an iPhone 6s Plus battery any more, Getting a service for your Apple product after its warranty has expired – Apple Support (UK)

Third party battery is either defective or of sub par quality or both.

We are blaming the guy, yes we usually do that, but Apple would not have replaced the battery on an obsolete phone. It's either "Mr Fixit" on the high street or recycle the phone.

yellow amarillo wrote:

Was not replaced by Apple, it’s a 3rd party battery replacement

As I said, not your bad, Apple will not replace batteries in obsolete phones.

But then the phone is still worth less than the battery, and becoming useless as well as officially obsolete

I don't think Apple will replace an iPhone 6s Plus battery any more

The current support status of the iPhone 6 and 6s is "Mixed", according to Mactracker, meaning that some services may be available and some not. Battery wise, things will depend on whether the Apple Store / service department has the correct battery in stock. The same would be true for authorized Apple Service shops.

I asked the authorized service shop near my home about this, and they still have batteries in stock for the iPhone 6 and 7. The other authorized service shop across town did not have the correct battery in stock and said that they could no longer get that battery from Apple.

Bottom line......check around.

Re: “well was a 2.4gb update”

My guess is that Apple’s systems are set up so that it was easier to distribute a large image, in which only the updated security certificate had really changed, than to distribute a patch to the certificate by itself,

I doubt if Apple made any changes that would have any long-term effects on the amount of battery power the phone requires to operate.