United changed their rules sometime in the last year. Now they only allow two spare lithium batteries per device; they must be in carry on and none are allowed in checked baggage. I followed the rules and had my spare video batteries in my carry on when I left Cozumel a few weeks ago, and I had no trouble in the security line. I don't know that it made any difference, but I had the batteries in a transparent baggie with black electrical tape over the contacts.