"I worked for 27 years in the power industry where the generators are filled with hydrogen (as a cooling medium), explosive limits for hydrogen are 4-74% in air, much more in a oxygen rich environment. Leaving the caps in place ensures a hydrogen rich mixture in the battery, should an ignition source ignite venting gas then the flame should stay external to the battery."
For the sake of completeness it is important to understand that the mixture in the cell has more oxidant in it than hydrogen.... simply because of the lightness of the H2 gas.
However, in an oxygen rich environment you only need 4% to be hydrogen, and it will detonate... not just burn.
So it is not true that a leak will/may have flame on the outside, and not progress into the cell from venting gas..... it will.... have no doubt at all..
It is not like hydrogen in the power station, where the gas has no oxidant mixed with it, and has to wait until it gets to the air around it........ Hydrogen mixtures with oxidant have exceptionally low energy requirements to ignite, in the range of only milli joules.... so any kind of even "so fine you cant see them " static sparks can cause total detonation.....it does not need to be contained to detonate, same with acetylene... they burn so fast that any mixture of gas and oxidant will explode rather than burn.... hence the bang you hear igniting the acetylene in the gas torch. ( yes still technically burning......., but more like an explosion than a rapid burn )...
Those cells exploded... either poor leads, internal break, or static spark.... but that was an explosion. I've been too close to too many of them to not recognise it when I see it. It is fortunate that the explosion did not spread to the whole lot. I have not seen that happen, usually only a few at a time.
Although energy to ignite is in the tenths of a millijoule range, the temp required is quite high comparatively. (>1000F)... so red ( dull not bright) hot wires may not cause ignition, but the melted plastic..... should it actually ignite and burn and / cause a plasma ( flame) of any kind.... then away she goes.
My 675AH battery box
It has 24 T105RE batteries in three strings. Separate leads of all the same length to each string with 100amp enclosed fuses inside the shed for each string (48v).
It is big on ventilation, and isolation from other flame sources.... never again I hope.
Edit fixed up typing fault in AH figures... not 750ah... thats the 100 hr rate