Recent FAA rules have restricted the carrying of Llithium /Li Ion batteries on aircraft.
Broadly it's NO spare batteries in hold baggage, and only a max of 2 or 4 (depending where you read) allowed in cabin baggage.
If they're in the appliance (like a phone) then they don't count as spare/loose so can go in the hold.
Even apart from the recent new Samsung phone there are plenty of videos online of "what can happen" (subtext - "especially with grey market copies which many of us use").
For me, with differing cameras, 2 spares isn't enough!
Has anyone had batteries hauled out of luggage?
It would be possible to argue that batteries in Camera Grips, which typically hold two camera batteries, are appliances therefore can go in hold baggage. Authorities/inspectors may not agree, of course.
Accepting that there is a risk even in cabin bags, I imagine that partly or fully discharged batteries would be safer, as the potential short curcuit current, gas pressure etc, would be lower. I would expect fully charged cheap batteries, subjected to low hold pressure, could be most at risk.
The only partial alternative I can think of is to use a ton of AA NiMH batteries in camera grips. They have less power density though, so I'd want a lot, which would also raise concerns. And not an option for compacts.