Hello everyone.
First of all, I'm a 'long time listener, first time caller' and want to say this is a great site and I appreciate all the info here.
When I originally saw this I was impressed with the simplicity. I should probably say now that this is NOT my idea and I don't even have a battery bank to use this on to see how useful it might be or whether you would have to recalibrate it as the battery bank ages. With that said here's the link to the guy's main page: (click 'ev gas gauge' at the bottom of the page to go to the meter set-up)
http://www.qsl.net/k5lxp/ev/evmain.html
This setup has limitations and is obviously for the bank as a whole not individual batteries but I think it is interesting for what it is. I think his explanation is sufficient and I hope that someone may find it useful. I did a search of the board & didn't find an equivalent circuit and I don't know if an easier/cheaper digital circuit exists or even if the modern inverters already provide an 'instantaneous' SOC reading or not.
Until I can contribute something of my own I thought I would post this as a way to say hello with something more than just my hat in my hand.
Cheers!