> A fire extinguisher large enough for a 500wh battery weighs 12 kilos - most people can barely lift that!

Almost everyone can lift that overhead. 90% of men and more than 50% of women can lift a bit more than twice that overhead. It's a common job requirement.

There would have to be something really wrong for an adult to struggle lifting 12kg. That's a large watermelon, and unlike the melon, the fire extinguisher got a handle.

Handheld fire extinguishers top out at that weight - because the vast majority of people will be able to use them and it's better to have two than make them any bigger.