iPhone 6s battery only lasts 3-4 hours

It was a local third party who changed the battery. this third party has usually been a trusted with other people’s phones and i was recommended by a friend to have them change it after Apple told me it would take up to 2 months for them to get battery replacements in. The phone was also doing much worse than this before i had the third party change the battery twice and I’ve only had this phone since Xmas of 2017. I have never rebooted the phone completely and I try and always use powersave mode, i keep the screen brightness below 50, I have never let the phone die completely without either turning it off or being able to charge it with about 5-15% left. I really think it might be something with the actual phone but just not sure what else to do besides get a whole new phone. I was going to try rebooting it first to see if that helped but not sure.

Yeah I knew that. I just needed to get the battery changed sooner rather than later because I’m going on a study abroad trip in September. I’ll go back into that place and ask them. I did just buy a pretty good portable charger from Anker which will suffice until I get back and probably will have to get a new phone. Still just annoying though. Thankyou for the help kind stranger!

And also like i said if I go into the battery page of settings it says that my battery is running at 100% efficiency so I’m just not really sure what the deal is.