> although I don't think there's a limit on how many you can carry-on.

Correct. So long as it's for personal use.

> [1] Quantity limits: None for most batteries — but batteries must be for use by the passenger. Batteries carried for further sale or distribution (vendor samples, etc.) are prohibited. There is a limit of two spare batteries per person for the larger lithium ion batteries described above (101–160 watt hours per battery).

[1]: https://www.faa.gov/hazmat/packsafe/lithium-batteries

Unfortunately airlines often have more strict rules. Lufthansa says 20 max for example

https://www.lufthansa.com/us/en/prepare-for-your-trip/baggag...

Although 20 loose batteries is still plenty. The real challenge is the 15 devices per person limit

Do AirPods count as 3 devices because charging case + 2 ear pieces?

As a private person, international transport of devices with batteries is a pain.

I assume this is because the thermal containment bags they have are only rated to a certain limit, but distinct batteries hopefully won't ignite at the same time and can in any case go in different bags?

Heat a lithium secondary cell too much and it goes into runaway. When the cells are packed together in one device if one of them goes up it can take the others with it. The bigger the device the hotter it will be and harder to avoid it turning into a disaster.

For the 60V flex volt battery, which is actually 3x 20v batteries which the tool can configure in series, the advertised capacity in Ah is almost certainly measured at 20V. And 20V is also a marketing term, with nominal voltage being around 18.5. So the 6Ah battery is probably around 111Wh and the 15Ah battery is probably around 278Wh. So only one of them is oversized but you should find a data sheet that clearly shows capacity in Wh.

And this is reason why "Ah" unit is stupid and meaningless

IATA says a limit of 2 <160Wh batteries and a limit of 20 <100Wh batteries (which is a lot!), but also airlines are free to be more or less strict as they see fit, so basically check with the airline (it's rare that they'll be less strict as a policy, but they may be OK if you clear it with them beforehand, and some are more restrictive by default).