The battery was charging completely but only holding a charge for about an hour on my mid-2009 MacBook Pro. At the battery icon pull-down menu there was an alert to "Service Battery." I researched the battery on iFixit.com and ordered an "A Stock" unit. I followed the easy on-screen guides and within an hour of receiving the battery my MacBook Pro was operating normally. The replacement battery works great!
Easy. Took less than 15 minutes to do.
I would do as I did, research the battery, review the installation guides and make sure you have the correct tools to do the job.
If you do the above, you are virtually guaranteed a positive outcome. Beware, I suggest ordering your parts from iFixit.com, there are a lot of websites/vendor