In my case I don't need to know exactly what my SOC is. I just know it is close to being 50% SOC when my DC VOLTMETERs read around 12.0VDC and that is good enough for me to go ahead and start re-charging procedures. I probably can re-charge from 40% SOC to 60% SOC and not do any harm to the batteries...
I have been using this logic for several years now and many camping nights. Batteries remain strong and are just now starting to show some off performance since being installed in late 2008.
This is what works for me but if you really need to know exactly what charge state your batteries are at then the only real way is to measure the SPECIFIC GRAVITY of the battery acid in each cell of your batteries.
The approximate state of charge is good enough for me to know so that is what I look for with a quick reading of the Battery DC Voltage.
I'm sure others may do it differently... Just passing on what we do here and what works for us...
Roy Ken
