yes, there is a battery length limit
the H10 is not too long
I have had the same issue as the OP, that sometimes when I assemble the light, it goes into Automatic ramp up, and the only way to turn it off is to remove the battery, and reassemble in the correct order, tighten the tail over the clip first. Tighten the head last.
Im dont think the issue the OP had is battery related. As I said, Ive had the same issue, and imo it is an assembly problem…
imo, the FWAA is entertaining, finnicky, and dangerous. I dont recommend it as an edc due to the button being too sensitive, and Im not a fan of 4 clicks every time I want to use a light… works fine unlocked, on my desk.
for EDC, I still vastly prefer the Jetbeam RRT-01. It has none of the dangers of the FWAA, wont turn on in pocket, no need to lock out, always starts at a safe level, and dead simple to dial in the right amount of light, intuitively.
The FWAA otoh, requires ramping up and down to find the sweet spot, and ramping down is not a consistent commant, sometimes it just requires 1H, other times it requires 2H…
the mental overhead to use a FWxx is quite demanding… having to remember what memory mode was set to, whether to turn on with a click or with a hold… reading the manual to change settings… worrying about burns… none of that comes into play with an RRT-01
oh and the clip on the FWAA is imo, not very secure… the RRT-01 pocket clip is far superior…