Samsung Galaxy III Neo has been acting weird the past year.

[Android Central Question]

I have this phone for, I think, 3 years. This year I noticed that my phone has been getting very hot while using it. Sometimes it would get at temperatures that would make my hands sweaty or my desk warm if I put it on it. I often checked if the battery was the problem by removing it and touching it, but its temperature would be normal. Then I would notice that the high temperature was coming from the bottom of my phone. Could it have been the CPU? Anyway, this stopped happening after I lowered the brightness to as low as possible and activated power saving mode (without limiting the cpu's capabilities). It was after a while that I noticed my battery kept decreasing either fast or drop suddenly. For example, it was at 30% and suddenly got a warning that it's at 15%. Plugging it to the charger would either drop the battery lower or even turn off the phone. Turning it on would show that my battery is either at 0% or 1%. Sometimes it could drop from above 15% to 5% or even turn off without a warning. I'll add that I've never had this problem before downloading heavy games. Games that requested from the cpu to do heavy tasks and this phone was at the minimum to run the game. Examples: Mobius FF, The Alchemist Code. Since that day I haven't increased the brightness unless there was sunlight and I was outside. Not even 10 times did that happen. Also have never deactivated power saving mode since then. I've even activated flight mode for a month but the situation only got worse since then.

I'll also add that my charger's cable was damaged (just a small cut to the cable, not the wires) and for a short time I was charging my phone with the wires being exposed to outside of the cable. I replaced my charger but the problem remained.
This week have been even weirder. Phone can charge from 1% to 100% in either 4 hours or 2 hours. It used to take a long time to even charge it 1% while using it, yet I noticed that yesterday it went from 70% to 85% within 5 minutes. While playing a heavy tasked game.
Any ideas what caused the battery's behaviour or the temperatures? Is it just the battery being faulty or the phone's age?

I can provide more information if needed.

 

[themadscientist]

Sounds like old age is catching up,The battery is more than likely beginning to degrade,As well if you have not factory reset this device in that long It is well loaded prob full of apps and cache which can put a strin on it too

 

[RandomUser42]

(Created an account after posting the question). I have actually done a factory reset recently (around two weeks ago). It helped a lot with the storage space. But it doesn't seem like the apps are opening any faster. I forgot to mention that if I turn my phone off and charge it from 0% it will take at least 3 hours to fully charge. In general, I've noticed that it takes longer to be charged to 100% when it's turned off. Should I replace the battery or is it not worth it with the phone's age?

 

[Rukbat]

From the symptoms, I'd say the battery is growing a dendrite. (Lithium Battery Dendrite links) The only solution to that problem is replacing the battery.

As for the phone running slow, that's how power-saving works - it throttles the CPU clock speed. You won't notice it with things like Phone or Contacts (or email), but you will with some games.

 

[RandomUser42]

Yeah, that's why I unchecked the box which says that it will slow down the CPU. If it's just the battery, that's good. It's about time I replaced it. Should I just buy a battery with the same mAh or is there a specific battery for this phone model?

 

[Rukbat]

There's a sp[specific battery for every phone model. See this page for some.

 

[RandomUser42]

Thanks. Also something to add, the phone is no longer getting hot. Even if I have it on from 100% to 0% and playing games, the phone is normal. It just loses its battery very quickly. Which is probably the reason why it doesn't have time to increase the temperature. It used to get hot even when I was charging it, but no more.