Does my W540 need a new battery?

[CharlieDog]

I recently got a five-year old W540 off eBay. It's working great overall, and so far I'm very happy with it. I did a battery gauge reset test, and after several tries it completed the test successfully.. It says the battery only retains 32% of its full charge. Also it's been cycled 66 times. Maybe the previous owner kept it plugged in and charged at 100% all the time. I set the full-charge limit at 95%, because I usually keep it plugged in. It has the 9-cell battery. Should I be concerned about the battery? If I replace it, is there any problem going with a 6-cell, since I hardly ever would operate it on battery power? TIA