I just bought a Pram battery off eBay which the seller assures me is new. I installed it without the main battey in, did a few restarts and it never holds the date correctly. I told the seller it was DOA, but he says you have to leave it in as it needs at least 24 hours to charge, and must have the main battery in when installed. I've never heard of this before - I thought Pram wasn't rechargeable? Is the seller's advice correct?