Why is my phone not telling me the correct percent of the battery?

This phone (Samsung Galaxy J7,2015) was given to me in 2018 from my dad, just a year later it stopped showing me the correct percent of the battery so it just randomly dies. Even after a battery replacement it still wasnt showing the correct percent and dies at a random percent. And when i accidently dropped my phone and the battery flew out after charging it and it showed 80% to 100%. I got an idea maybe removing the battery after chraging it will fix the issue and kept doing this and maybe itll fix it, but no. So i stopped doing it and just kept it there untouched. And when i charge it from 0% itll show 20%+ and wont go higher than that, so i have to use the phone and let it decrease, itll stay at that percent for many mins until it decreases slowly and ones its low ill charge it and it will charge up to 30%+. This issue is so stressful that i needed some advice on how to fix it. If this is just a software glitch and i need to software update, well i cant since the device just says “The latest updates have already been installed”. Pls help me as i dont want to deal with this again. If i need to plug my phone into a computer well i cant since it died. One more thing this device runs android 5.1.1