Run the laptop off the power adapter with the bad battery inside.
Download and run battery eater (http://batteryeater.com/download.html
- extract and run brpro.exe) off it
Go into info -> power supply -> battery 1 -> check the max current battery capacity mah (idk if thats exactly what its called, on a desktop so cant verify)
Report back numbers (or if its too low, then that means the battery cant hold a charge anymore cause of age)