It's lithium-ion batteries where there is a particular problem.

I couldn't quite work out what sort of batteries you are talking about here. If you mean 10-12 AA or AAA batteries, albeit it rechargeable, then that is fine. If you are talking about a dozen specialist lithium batteries then you would potentially have problems. If the former, I'd just pack them carefully and not mention them at check-in. If the latter you really must declare them.

By careful packaging: either in their original selling pack, in a plastic box without risk of contamination from metal or liquids (I have a small Tupperware box for my hand luggage batteries). I'd further wrap them in a plastic bag. It's more to protect your other possessions really. If a battery doesn't look to be in tip top condition, I would leave it behind. Don't leave any batteries in the recharger, that tends to get security worried.