The capacity of the gell electrolyte is fairly small
It was designed to power LOW CURRENT DRAW instruments for a long time and to be recharged by a trickle charger
It was not designed to power high current demand devices like starter motors.
High currents can & does cause Hydrogen gas to be generated
These batteries are totally sealed so they don't leak & damage expensive equipment
So too much gas = bang , the cases split & acidic ( very corrosive ) gell gets tossed everywhere.
They were popular for kick start motorcycles till they started to pop
The gell is still sulphuric acid just the same as every lead acid wet cell battery
Al they did was daa some rosin to the acid so it forms a paste so the battery can be installed upside down & the electrolyte stays in place.
Now remember your high school physics & chemistry
To work ions ( S04 ) have to MOVE from one plate to another
And the thicker the medium those sulphate ions have to travel through, the slower they move
To provide a few milli amps you only need few ions to move
To provide a lot of amps you need a lot of them to move, all at the same time which they can not do so the gell breaks down & hydrogen gas forms.
A molecule of hydrogen gas occupies 25 times the space of a hydrogen atom.
This is why overcharged sealed batteries swell .
Way in the back of my mind is Gell cells were a by-product of the space race designed to power stuff in outer space where there is no gravity but were not particularly successful
Ni-Cd & Ni-hydride batteries were also by products of the space race and found to be more successful and if you dig back through battery history pretty sure space was the origin of the Li batteries as well
