Negative Ids using PersistenceManager.ImportEntities

I have 2 PM’s setup.  On PM1 I create a Batch entity that contains no Batch.Bills and has a BatchID of -1.  I then import the BatchEntity into PM2 which has just been created and has no entities in its cache.  The Batch entity still has an ID of -1.  I do a bunch of work and now Batch.Bills is not empty.  I then build an ArrayList containing Batch, Batch.Bills and import that ArrayList into PM1 with MergeStrategy.OverwriteChanges.  When Batch goes into PM1 its BatchID changes from -1 to -2 and all Batch.Bills now reference -2. 

1)      Why does it switch the ID on me?

2)      Can I stop it from switching the ID?

3)      How do I get a reference to Batch?  I can’t get it by ID because it’s changed.  I can’t get it by RowState because PM1 could have any number of new Batch entities.