How long does the battery last? Is replacing it easy? My 2009 A1342 lasts anywhere from 2.5-5 hours on a normal day, with what I'm pretty sure is the original battery, which is surprising, but I've considered whether or not I should replace it one day. Also, where did you find the battery?
Ifixit battery swap instructions.
Looks like they’re all over eBay for about $20 shipped USD. Essentially you pop the back off and you’ll have access to the battery, hdd and ram, so could upgrade/max out all three in one swoop.
Good luck