I use the DC VOLTAGE verses BATTERY STATE of Charge to get an idea of my battery state of charge. This is no where absolute but gets me in the ball park. The absolute test of course is to use a hydrometer and read a specific gravity of 1.225 or greater in each battery cell.
When camping off the power grid If I read my battery bank DC voltage being around 12.0VDC will tell me my approximate battery state of charge is 50% charge state. This is where I want to start re-charging my battery to get it back up to its 90% charge state or more to be able to use it again for the next day/night run off the batteries with almost full performance of the battery.
Like said this is not absolute but gets me in the ballpark. I keep a close watch of my battery DC Voltage reading when camping off the power grid and have planned out my battery drain to not go below the 12.0VDC before 8AM the next morning when I am allowed to run my generator to recharge my batteries.
I have this homemade meter panel inside my trailer monitoring my two battery banks for 12VDC and also how much current is being drawn from my batteries to run my Inverter and 12VDC connected items.
Roy Ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS