He said he fully discharged them before disassembly; I think he is quite aware of the danger if you skip that step.
Big Clive discharged those AAs before disassembly also!!
That's part of the reason why he was so surprised when the flames erupted... Big Clive isn't often at a loss for words, but he was absolutely flustered at about 18m into the video.
Just because a cell is in a discharged state doesn't mean there isn't still the potential for chemical reactions and energy release from the materials composing said cell!
What's inside Eneloop and LIDL NiMh cells (fire, apparently)
...
I thought that discharging the cells completely would avoid little unexpected combustion incidents.... I was wrong.
https://youtu.be/tBg4ximDrsk?t=980I was rather disappointed that Dave didn't even need an Explosion Containment Pie Dish...
No flames even? I suppose we should all unsubscribe!
Edit: I did get a good chuckle when at 12m Dave said "I won't pretend to know the exact chemistry inside..." because it was
very reminiscent of Big Clive's "Ok... Right... Yeah... Not, not sure the science of this, but this is all going on fire quite rapidly. So, that's quite an exciting chemical. Oh, this isn't necessarily quite good. Where is my Explosion Containment Pie Dish?"
But then there were no flames....
Of course, I suppose to fit the 2500 mAh into the AA size case instead of C, they have to do things a tad bit more "
energetically"?