adamfromwalidrogi wrote: "This is original battery with capacity 82%."
82% is close to Apple's borderline health mark of 80% – so the original battery is weak, just as expected after years of use and charging cycles.
"This happens also while phone is being connected to the charger. There is also no relation between current battery level and crashes - doesnt matter if its fully charged or alsmost empty."
Exactly. A weak battery is still weak regardless of whether it's connected to the charger and regardless of its current battery level – and so the weak battery will cause crashes.
Also, the long process of applying an iOS update to an old iPhone with a weak, original battery can be the proverbial "straw that broke the camel's back."
...So have the battery replaced and all will (probably) be well:
iPhone Battery & Power Repair
About genuine iPhone batteries
Repair Provider Verification
(Articles found by searching Apple's Knowledge Base.)
õ¿õ¬