iPhone 6 - The battery charges up to 3% on the off phone and stops

Hey
I have the following problem, namely some time ago there were problems with charging the battery (fortunately, after a few days left, the phone was charging and I forgot the problem). I unloaded the phone to 0, after coming home it turned out that something was beginning to go wrong with him again.
At the moment the phone is charging up to 3%, the system gets up and ... stops charging. There is no charging icon on the phone at all (it surprised several times, in the sense it showed, but the phone did not load - it used to look like that before).
The charging socket was already cleaned, contacts also, so I exclude this problem. It was also tested on various chargers (iPhone, iPad, Chinese crap) and various cables (original and replacement with a braid - quite decent).
There was also a tested method of heating the battery with a hairdryer (I found somewhere on the web) - it also does not work.
Any ideas? Suggestions?
Thanks in advance for your help / advice.

Check the battery voltage and charging voltage.

IPad charger: 5.11V
IPhone charger 5.14V
Chinese 5.04V charger

I do not have a moment to measure the battery voltage in the iPhone because the pins from the meter are too wide to get inside the socket. As I will explain how to embrace it or something, I will let you know.

You must use something thinner. I would suggest that I check on the new battery.

Returning to the subject, I was unable to measure the battery voltage on the iPhone.
But the problem temporarily solved, probably until next time. It helped the same thing the previous time, namely, discharging the iPhone to zero completely by attempting to turn it on to a discharged battery (until the message about the payment to the charger stops) and leaving it alone for a few days (it seems to me that then some 'reserve' on the battery is completely discharged) in my case it was something like 48h, but I recommend (if you want to try it, leave it for longer than a shorter period - to be sure, because in the short time the process will have to be repeated )
What I noticed earlier, the iPhone was loading (turned off) to 3% and after rebooting the system stopped charging (and if there was a charging icon, nothing happened). After doing the above action, the iPhone started, it loaded, but it seems to me that the first 30% (on the battery) lasted longer than usual, which is puzzling for me because the last percent should go longer than the initial.
Anyway, the iPhone is currently charging up to 100%, I immediately disconnect it, start using it and see if it will be charging normally. It is true that I have no intention of unloading it to the end, so that the problem does not occur again, but I wonder what is the reason?
I can add that the iPhone did it a second time. The second time also helped the activity described above.
If anyone has an idea why this is happening, please answer. And what I have described, I hope that it will be useful to someone.

Update: After a while on the charger and charging up to 100% again, he stopped reacting to the charger, I will see how it will look later.

Update 2: At 99, 98 and 85% the phone did not charge when connected to the charger. After spending the night after connecting at 68%, he charged up to 100% again. I do not know.