Why does my controller keep dying after a while?

I was fixing a controller with a broken charging port, everything else works fine. After replacing the port and testing it i see the yellow light which is good so the charging port is working. However after leaving the controller for a day it won’t seem to turn on. Connecting a charging cable dosen’t help as i see no light. I assume its a battery problem so i replace the battery with a fully charged one it turns on and works fine, only to come back a day later and it dosen’t turn on again. Whilst the controller is on it charges fine , the battery is also fine so what could be the problem? It seems as if the controller drains the battery very fast how can this be fixed? It seems the only way to get the controller working is to swap the battery or never let it turn off.