Battery gauge stuck to 100 %

Hello

i have got an iphone 8 with broken screen and battery reported as « service ». I don’t know the history of the phone. I changed screen but before this i couldn’t turn off the phone since i couldn’t slide on top of damaged screen. I notice that the battery even in a poor condition (bad looking) was suite good enough to power the phone more than 5 days (the delay i got the new screen) without being exhausted . When i replace screen and battery, i observed that battery charge was quite slow. After one night it was charged at 100 %. And since it is stuck to 100% !!! Nothing changes , i found an infinite battery!!! This is quite painful to be blind on battery remaining capacity. When i charged it even with 100 % , some current is consumed (proportionnally to the previous usage of the phone).

with the old battery i can’t charge it more than 37 %.

do you thing it from the bsttery or from main board. (The main board doesn’t look to be a virgin board)

thank you