That could be a battery problem.
Have you installed iOS 11.3 or later?
If yes, what can you read going to Settings>Battery>Battery Health (beta) and tapping on it?
If you don't have iOS 11.3 or later, have your device/battery inspected / replaced at your most convenient Apple Store or Authorized Service Provider: Apple - Find Locations - Location Selector
Regards
Giulio
That could be a battery problem.
Have you installed iOS 11.3 or later?
If yes, what can you read going to Settings>Battery>Battery Health (beta) and tapping on it?
If you don't have iOS 11.3 or later, have your device/battery inspected / replaced at your most convenient Apple Store or Authorized Service Provider: Apple - Find Locations - Location Selector
Regards
Giulio
1st it's updated to 11,4,1
2nd battery maximum capacity is ( _)
Are you saying that you can't read the battery maximum capacity percentage?
If that value doesn't show up, have your device physically checked, as I suggested in my previous reply.
Regards
Giulio
I have an iPhone 6s , suddenly it turned off while its battery was 70% , when I turned it on it becomes 1% then become 70% in 5 sec , after that every 3 minutes I restart by it self , and the same when I plugged it into charger , it turn on by itsel
