RoyB is right on my target. We mostly boondock and battery management is therefore critical. I use the same chart he displayed, my inverter shows the input voltage and I carry a voltmeter. Also, I have a digital voltmeter mounted in the battery area along with my solar controller. And RoyB's chart laminated and stuck in there as well.
12 volts is the bottom. Done that on 3 Bs and 2 Cs. Works for me and I don't have to fuss with all the techno stuff. Also, like RoyB, I get many years of service from my batteries.
Paul