Phone goes down to 3% battery after using it for 1 minute.

[Yakisoba]

Hello,
Since this week I have a big problem on my Samsung galaxy A5 (2016).
After I charge my phone to 100% and I disconnect it from the charger, I can use it for around 1 minute. Then it restarts automatically and the battery goes down to 3%. 
Also, sometimes after the restart, I write my pin code to unlock. But is says wrong pin. But it is the right pin, and I always have been able to unlock with this pin. I could unlock my screen from Find My Mobile site. And since it happend like 3 times I removed the pin. 

I have used this Samsung galaxy A5 for around 2 years now. 
Is this a battery problem or will resetting the phone will maybe work?

If someone knows the answer or a suggestion, pleas reply fast.

Thank you in advance!

[BandOfBrothers]

First put the phone in Safe Mode which disables your downloaded apps and leaves the stock ones working and then see if the issues remain. 

If they don't then an app(s) is causing this. 

If it remains then you could try backing up and factory reset.

Also a Samsung Experience Store or Samsung Service Centre can run a diagnostic test on the phone.