Galaxy Core battery problems

[AC Question]

I've had a Galaxy Core I8260 since 2014 (yes its a pretty old phone). Some months ago, the battery started to drain very fast, thinking this was a software problem I did a hard reset, but then it kept happening and I did one more hard reset. It kept happening.
So, I bought a new battery and the problem resolved for a time. But it started to happen again. Fast forward to today: I bought YET another battery and the thing drained from 23% to off in a minute.
So, now I'm thinking this is a hardware problem, because of the age of the phone too. I am willing to do the repair on my own, in Argentina good techinicians are a rare encounter. But the phone is old and finding replacement parts is already a pain here. So, if you think this is a hardware problem, do you think it would be worth it? Or, please tell me if you have any other idea.
Thanks!

 

[jasonandrews25788]

are you sure that you have an original battery?

 

[LifesGood5]

are you sure that you have an original battery?

Yup, when it started to happen it had the original battery and the other batteries I bought, I compared them with that one and the guidelines that Samsung specifies here Use Samsung-approved batteries, chargers, accessories, and supplies

 

[LifesGood5]

Sooo, I just swapped my charger with my mother's charger. Hers didn't charge my S5 at all, and mine charged her Core and the battery worked fine. I mean, how can the charger affect the battery AFTER it has been disconnected? There must be something I'm ignoring here.