Users are complaining their iPhone battery life is worse after updating to iOS 12.
After releasing new iPhones, and then the iOS 12 update, users of current iPhones are complaining that after updating to the new version of iOS, their battery life has got much worse.
It seems a coincidence that, with the new iPhones releasing now, your old iPhone's battery starts to slow again...right? Well, no. You are correct that your battery life may be worse, but Apple isn't causing it.
The reason is this: after updating your device, lots of system code and elements that help the phone run get switched and moved around, and updated. Your iPhone has to re-index those pieces of code, plus apps and data, putting a heavier demand on the processor - creating the drain on your battery.
There is good news though - this doesn't last forever. After updating, your may find your battery life is worse until about 24-48 hours later, when the battery life should improve and be better than before. This is because the iPhone has finished re-indexing data that the update moved around or added.
Turn off automatic app updates.
There is also one more reason why your battery may be bad. After major software releases, developers start updating their apps to support the new release. If you have Automatic Updates turned on, this means your iPhone will be updating a lot more apps than usual, in the background, also draining battery life as well.
To turn off Automatic Updates for your apps, go into Settings, iTunes and App Store, and tap the toggle next to Updates. Every so often, go into the App Store and manually update your apps, as you will miss out on new features and fixes for apps you use.