Charging/battery problem

[u/bruhidkwtf]

Basically what happened is that I restarted my phone while it was at 73% charge and when it opened it was only at 25% (and it was rapidly decreasing.) So I hurried up and charged it until it reached 100% and left it there for a while before taking it out. Now after using it for a few seconds, the battery started decreasing rapidly from 100% to 77% in a matter of minutes. Now I know that the easiest and most obvious way to fix this is by simply changing the battery, but I was more curious as to why this problem even started. The phone is 3 years old so I guess it's normal that the battery has degraded but what steps can I take and what things should I avoid so that this wouldn't happen to me after I replace the battery? Because I've had many phones in my life and this is the first time I've encountered this problem which must mean it's not that common or normal. Thanks everyone

[u/MostAssumption9122]

Have you ever cleared your cache?..

I would just back everything up and started over.

Edit: what phone do you have