Far as I know the batches information is stored as PI Points. If you look at your points you will see a bunch of points named with a GUID. Some of these will be you batches; as far as I remember the PIBatch does add a descriptor to the points indicating that they are batch points. One thing to bear in mind is that a number of OSIsoft modules use a similar method.
The batch structure is stored in the module database dat file.
To restore a batch you would need to restore the archive which has the batch data; therefore you would need to know the approximate date range.
Last daft question. Why are you think of archiving the "old" data? Harddisks are cheap and the old data doesn't impact on the performance of PI.