Car battery cannot be charged….

Hello dear motor-talker.. would have a question to you. I have a 45 Ah battery in my polo. The battery is now three years old. And lately she is a little tired when starting. So I decided to charge the battery with my regulated charger. The charger shows me that the battery is 65 percent charged. If I start the charging process then it is within 10 minutes at 100 percent and the charge is stopped. But then if I try to start she is as tired as before loading me happy about help… Lg Tom