Laptop battery only charges to 20%

Hi! I have a Compaq Presario 1220 laptop, and even after leaving it running on AC power for over 2 days, the battery meter only shows the battery being charged to 20%. The laptop is about 8 years old, but was always run on AC power. The battery is a 9.6V Ni-MH battery, part # 293768-001. How can I get the battery to charge to 100%? Thanx! 🙂