Linn Kairn ERR 02

Got a Linn Kairn pretty recently, and it has been sounding great. However, when I first powered it on after it had been sitting for sale for a while at the record shop I purchased it from second hand, it showed the ERR 02 code. After that first startup, it hadn’t reappeared at all, which made me think the backup battery just needed charging up. The unit has been off with the cord plugged in while I’ve been away at school for about a week, and it gave me the ERR 02 message upon powering it up tonight. Is this the start of the battery needing replaced, or does the Kairn battery discharge naturally over time?