Mac Rechargeable Battery - 13-inch Serial Number
The battery serial number is recorded in 4 places I have seen:


Physically on the battery (and optionally on the box if you purchased the battery in a box as a separate item)
In system profiler when the battery is installed in a running mac.
I have asked at the genius bar from time to time for them to look and see if there are records from when the mac was built in the factory. From time to time a client needs to know what hard drive or what battery came with a specific mac and that information is sometimes available.
Saved to the filesystem - use spotlight to look for files that have the mac's serial number. System profiler will save all hardware as a text/rtf/xml file if asked to. Some crash reports squirrel away a copy of that information so if you look, it might be there even if you didn't save it explicitly.