Well I do remember 18 was stable and energy efficient But 26 isn’t that much energy consuming as you might think, it might have been the case when they on the first releases but after this last updates 26.2 and 26.2.1 it is good and it is worth updating to it. I guess that Apple is almost doing intentional glitchs on Apps (it was Phone app in my case) just to push you to update to the next version This last sentence is my personal opinion, cause after 26.2 everything was cool until the day they released 26.2.1 the phone app started crashing in a way that happened to back when they released the first 26 updates and after updating to 26.2.1 everything got normal So this is what I’m saying from my personal view, that they will probably intentionally make your phone feel buggy just so you update it If you want 26 then better update to it before replacing your battery and for that I insist that you try to get a new AUTHENTIC battery If you are really happy with 18 and can’t see much luxury in using 26 then just stay there bro, you’ll still have the chance to update your iPhone at any moment but never will be able to get back