weird battery behavior.not charging to 100%

[K-a-M-u-Z-u]

Hi.
I have thinkpad w500 with 6 cell battery.
suddenly, i have noticed a real drop in charge.the gouge says 100%, but it behaves like 20%.i can use it for about 15-20 minutes before a sudden drop to 7% with alarms and going to standby/hibrenation.
i have looked at battery information, battery condition is marked as "GOOD".(and green). cycle count is "45".
I have try to do "battery maintenance", and do the "battery gouge reset", but the battery says 100% and full battery WH capacity.
I have never dropped it, usually it stored in original package(anti static), fully charged in the closet.as i use the laptop connected to the AC usually.
any ideas what could be the problem?any way to solve it?
thanks.