Anyone got a Battery SOC Algorithm ?

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 I'm sure the smart battery chargers and solar controllers have remaining capacity in a battery figured out.

I'm more sure they don't! 

For starters they don't know where the power is going, I suspect nearly all solar regulators go to float too soon as they think power going into a fridge is going into a battery. Victron might be one of the very few exceptions as their smartshunt will tell a smart regulator soc & amps in/out so the regulator knows what's happening at the batteries. I recently got a smartguage which from testing came out as a very accurate gauge of battery state of charge, very interesting running that alongside a victron smartshunt with all the data going into influxdb. with some tweaking of battery capacity on the smart shunt you can get the 2 to display fairly close output but temperature and current draw makes a big difference. Warm batteries have quite a lot more capacity than cold batteries. IMHO, with lead acid it's much more important getting them back to really fully charged (not what the regulator or bms says) often than it is know the exact soc at any given moment. 
Interesting project but so many variables with peukerts and charge efficiency all varying with temperature it is a very complex calculation to guess soc...

Yes I'm happy that I can now accurately monitor (and record) Battery voltage and amps In/Out. 
I already have a NASA BM1 on the boat which has a SOC function. The SOC function does appear to count Amps In/Out but its clearly not as sophisticated as the Victron.
In practise I ignore the SOC function and just use the BM1 as a Volt/Ammeter.
The idea of trying to emulate a Victron BM is a just lockdown brain teaser but I prefer it to Sudoku.