Motorola Moto G — Moto G switches off by itself, either overheating or battery calibration, cannot figure

My Moto G is switching off by itself pretty much whenever I use it a little more, or sometimes just as I start using it. Even if I have 45% battery it suddenly shows 0% battery and switches off. After it has switched itself off, it behaves as if it had no battery, meaning that I can only switch it on and have it on afterwards, if putting it on charge/with powerbank. Means for majority of the time, to use it, I need to connect it to powerbank, which is crazy. Even when connected to powerbank, it is often living its own life, lagging, responding inappropriately, or suddenly closing apps. Yesterday it switched off 7 times, and since I also have internal SD card that works as internal memory, I also see that it reacts on this constant switching off, and for a short while, when powering on, shows as not being read, which means I'm afraid even the SD is gonna be damaged in the end.

I was thinking it could be that battery isnt calibrated properly, but I have had the phone for almost a year and had no problems before. This whole switching off started a couple of days ago out of thin air.

The phone gets warm, so I'm wondering can it be overheating? Lagging and app closing could indicate that. We have summer here and 20C, but a phone should be able to survive 20 degrees Celsius one would think. I dont even play games on it.

I have had batteries dying on two Samsung Galaxies, and there I see battery going rapidly down, not just jumping from 40% to 0%.

Would factory reset help? The same problem persists in Safe mode as well, I tried. Dont really know how to proceed. Would charging to full 100% and then going down to 0% (dunno how as I cant even start it without powerbank) and again to 100% help? Could it be battery calibration problem?? Any advice much appreciated.

Yes, the inactive apps, of course.. I tend to switch them off when I have too many running. I guess my problem is a worse one here, unless there has been an update I havent paid attention to, it could be battery due to overheating. And as we know, MotoG has inbuilt battery that I cant change myself. I bought it from abroads, so saving the phone is all up to me, no guarantee here any more. Im wondering if charging to 100%-emptying to 0%-charging to 100% might help, or doing a factory reset can help.

Can't switch the battery, Moto G3 is welded together, so I can't get to the battery. Otherwise it would be the first thing to try.. I also observe that it does work if i barely use it, and especially if I dont use it, lets say, in warm environment such as a bus. If I would try to chat for 20 minutes in a bus, it would switch off for sure!

Yeah it is really true, the back on Moto G3 is welded and I dont get in. It is really weird, I have been observing it for a while and it seems that some days it is ok and some days it is useless. I think it might be reacting on heat, especially if I use it much. Right now will try to get it down to 0 and then recharge full, see if it helps