OnStar Backup Battery

Hello,

My 06 Impala battery went dead and oce dead the OnStar backup battery kicked in to keep the OnStar system alive. The battery was replaced in the car and then the OnStar light was red. I called OnStar and it was reported that the battery was dead for the above reason and I have to replace it.

Does anyone know how much this battery is and is it user replaceable, or do I need to bring it to the dealer for this?

Thanks, Tom