Yeah, it's a laptop from 2006 - the battery is just dying from age (and possibly heat).
The voltage should remain the same no matter what - it's the Wh (wattage-hours), or capacity that will change.
Run HWMonitor: HWMONITOR | Softwares | CPUID
It should show you what the designed/current capacities of the battery are that the sensor is reading.
Also, check here for some useful Lithium battery info: Useful Lithium Battery Information
(specifically the first post will pertain to Lithium-Ion batteries in general - the 2nd post is geared more towards phones).