Science... data... standards... You'd think those applied to something like the state of charge of a 12 Volt deep cycle battery. Because it's not subjective, like politics, fashion or the world's best recipe for chicken fried steak.
A 100% charge is ________.
A 50% charge (which one should avoid going below) is ________.
But here's what you get when you search the interwebs. About the only thing they agree upon is that green is good and red is bad.
The last chart seems most credible to me, because it accounts for temperature. It at least offers up, "It depends," to the question of full and half charge. Serious, accurate science is big on qualifiers. Other qualifiers might be the type of battery and the length of time without a load being applied. However, that might give us charts beyond the comprehension of mere mortals.
The second chart uses the term leisure batteries. I first encountered that yesterday, on a nomad forum. What? Batteries for relaxing? Batteries for non-work purposes? Batteries for which accurate data is unimportant?
"Hey, don't sweat it, pal. They're just leisure batteries. Come here, take a load off your feet, loosen your tie, have some of the world's best chicken fried steak."
Oh, they mean batteries that power the systems in your leisure vehicle, as opposed to the battery that starts and runs the engine.
I could be wrong, but leisure battery has sort of an old midwestern feel about it, like calling a casserole a hot dish, or a water fountain a bubbler.
In the name of caution and long battery life, I'll avoid anything lower than 12.4 Volts. Even if it means I can't reheat hot dish leftovers.