I could only find one reference to this subject .

If I'm not mistaken, it had been calculated using the Reissner–Nordström metric.
Try this :
ReissnerNordstrøm solution
Once the limit is exceeded, the BH turns into a naked singularity, which is possibly nonphysical.
Naked Singularities are VERY DODGY indeed  only detectable with a Mass Detector, and then only if you have a toujgh idea what to look for.
I seem to remember that a Naked Singularity has NO Event Horizon which is detectable  VEREEEY DODGY INDEED.
One HELL of a Danger to Interstellar Travel.
I know next to nothing about this, but wouldn't a Black Hole with this much charge be generating Hawking radiation all over the place, mostly with the opposite charge going into the hole? I would expect that the hole's the electric field would even be contributing to the production of Hawking radiation. and in the resulting pair the charge unlike that on the hole would be pulled in and the other member of the pair pushed out. Conceivably that emission could put a practical limit on the amount of charge that you could actually get into a Black Hole, perhaps even a fairly low limit.
Thank you, Markus
I already used this info in post 5. Can you help me figure out where the more stringent limit, shown here,on page 182, comes from?
It seems pulled out of nowhere....
That's an interesting thought, mvb.
In my opinion the entire scenario is purely academic anyway  I would not expect to find any BHs with massive amounts of charge in nature. After all, where is that charge supposed to be coming from ? Given that BHs are the results of collapsing stars ( let's forget about primordial ones for the moment ), I don't see any way how such charge could be picked up; after all, we are not seeing any stars with huge amounts of net electric charge.
I don't know if anyone has worked through the math for Hawking radiation with anything other than a Schwarzschild black hole. Does a charged and/or spinning black hole produce the same result, I wonder? And what if you throw EinsteinCartan theory into the mix ...
I don't know about Hawking radiation, but a charged black hole in EinsteinCartan spacetime would not have a singularity at its center; furthermore, it would have not one but two event horizons, and, to top it all of, you could not fall into its centre.
Isn't geometrodynamics fun !
P.S. You could also go and embedd your rotating, charged black hole into a 5dimensional Kaluza Klein spacetime with torsion, where one spatial dimension is compactified. Might as well go the whole way
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