>>31693288
Leave it off and charging for hours.
A battery at very low voltage will sag when a current draw is placed on it. This means when you turn your phone on and demand power from the battery, it cannot meet the demand, the voltage drops rapidly and the phone switches itself off to prevent a deep discharge.
This usually happens as a result of some combination of the charger/cable combo providing insufficient current and the battery being old and worn-out.
Leave it off and plugged in for a few hours.

>>31693371
>All 'fixes' to this say to let the phone charge for a while, but I can't, it won't stop turning on then dying.

>>31693433
Use the 30 seconds in which it is on, to switch it off.
User-commanded 'off', and low-voltage 'off' are 2 different concept in idevice firmware. In the latter, it will switch on at 3.7V, in the former, it will only switch on from the power button.

>>31693371
do this. probably worth getting the battery replaced for the long term, it’s seven years old and batteries degrade with use. should feel like a new phone afterwards.

>>31693288
1000 euros for that
you deserve it faggot

>>31693433
The fact that your charger can't simultaneously power the phone and keep the battery's voltage up would suggest that you're using a 1A charger, like the small wallwarts that came with older iPhones.
Try an iPad charger or any third-party that supports fast charging over Lightning, and you shouldn't have this problem.

>>31693371
>>31693288
You can't charge an iphone while it is switched off. It has to be on. It's time to get a new phone, man. Get a second hand one so it's a little easier on the wallet.

>>31694520
>You can't charge an iphone while it is switched off.
No this can't be right
Right?

Does your phone have wireless charging capabilities? There might be something wrong with your charging port, but wireless charging should still work.