> I replaced the battery on it ~6 months ago.

Where did you purchase the battery from, and how is the performance, if you do not mind me asking?

I also use a mid 2009 Core 2 duo, but lately I have struggled to find a source for a battery that will last more that 1.5 hours in typical usage. I assume the issue is that any authentic Apple battery must have been manufactured so long ago that its capacity is severely degraded after having been stored (at non-ideal temperature) for so long, and it seems like even many third party batteries available are old and had inferior capacity to begin with.