My iPhone battery dropped from 100% to 0% while I was sleeping and on airplane mode

I have serious issues with my battery for a long time. My iPhone is 1.5 years old, second hand, and in the last months the percentage are dropped even when I do nothing with the iPhone for hours. I can’t survive more than 4 hours without low battery mode. I’ve tried to reset it and leave it without any iCloud account to see if it’s a matter of hardware or software. For one day and a half the battery was normal again, and than, without any of my data or accounts on it, when it’s completely blank, the battery become crazy again. One month is passed and I was going to buy new iPhone, but today when I woke up I discovered that my iPhone is on 0%. No guess what, before I’ve go to sleep I’ve charged it to 100%, and every night I turn on the airplane mode, and this night too. My battery was supposed to stay with 100%, maybe 98%, but zero??? I don’t care about the warranty, I don’t care about the Apple Store repairs payments rules, my iPhone is gone crazy and it’s Apple battery fault. I won’t pay to fix it and than lose the water resistant because of it, I’m refusing to buy new expensive iPhone, I don’t have the money for it. It is Apple responsibility to help me now when their iPhone battery or software is goes bananas even beyond the warranty period of time, because I can use the device anymore while I’m not sure when it will dropped down the percentage again. Please be fair, Apple, and gave me a decent solution. I’ve always bought only Apple original chargers, I’ve never done anything beyond the instructions. Take care of my iPhone.