iphone 6s useless after 14.4 update

My iPhone 6s installed the new iOS 14.4 update this morning and it seems to have wrecked it.

it was plugged in and at 100% when the update began installing. When it rebooted it was at 10% and it immediately restarted when the weather app was opened for a moment.

I plugged it in and it stayed sitting at 10% for more than an hour. I did a hard reboot and it suddenly decided it was charging again, but much too quickly.

it went from 10% to 100% in about 20mins. As soon as it was unplugged, it dropped to 10% again and rebooted.

this phone was working perfectly before this update installed and now it basically can’t be unplugged.

as you can see from this picture, the battery was fine and then plummets.