The views I’m about to express about flying with batteries are based on pre-pandemic experiences, and solely my own. And I’m definitely not recommending this as a legal option, or one I would have any liability for if you have a bad experience…
I’ve flown with ROV batteries on numerous occasions. As gcelec points out, airport screeners care about what they could see that would be dangerous - very dense and heavy things. If you treat the batteries and weights like your laptop, and take them out and place them uncovered in a tray, you will likely have no problems.
I’ve been asked several entertaining things about the batteries. The most common: “does this battery have a capacity of more than 100 amp hours?” I reply truthfully too - it does not - as I’m not responsible or inclined to explain the 100 watt-hour legalities, or the difference between watt and amp-hours to a nice person at the airport.
I’ve also witnessed the kind of profiling that exists in such scenarios (unfortunately.) As a tall, white, confident male I have never had the batteries taken, and only rarely questioned. However I’ve repeatedly seen that any friends I may travel with who don’t look like me are much more frequently stopped, questioned for longer and in more detail, and have a lower success rate (70-80%?) - even if I go through line immediately before or after them.
The rules are what they are for logical reasons - 100 watt hours is a lot of energy if expressed in minutes or seconds. Nothing compared to jet-fuel of course! That said, I’d recommend traveling with batteries in this manner that are ideally discharged - not dangerously low, but not freshly topped off either. The 18650 cells used in the BR battery are about as bullet-proof (in-safety terms) as lipo cells can be made- nothing short of violent and repeated punctures can cause catastrophe. The flat rectangular pack style batteries, while the same chemistry, lack the sturdy enclosure and inline thermal fusing - and are much easier to pack.