A 12-volt battery is not really a 12-volt battery. It’s just a convenient term. A fully-charged 12-volt battery in good condition, allowed to “rest” for a few hours (or days) with no load being drawn from it or charge going to it, will balance out its charge and measure about 12.6 volts between terminals.
When a battery reads only 12 volts under the above conditions, it’s almost fully depleted. Actually, if a battery’s resting voltage… after charging… is only 12.0 to 12.1 it means only 20 to 25% of its useful energy remains and it probably needs to be replaced.
12-volt batteries supply useful energy only through a limited range — from over 14 volts (when fully charged and unrested) down to 10.5 volts in use/under load (when lights dim, pumps groan and TV pictures get small).
No 12-volt battery will remain at over 14 volts for more than seconds unless it’s being charged. The lowest limit is 10.5 volts but this is too low and unsatisfactory in practical use.
Experienced RVers try to use no more than 20% to 50% of the energy available in a battery before recharging. That means they never let resting voltage get below 12.5. They never use more than 50% before recharging (resting volts of 12.3) except in an emergency. They know that, if resting voltage ever reaches 12.1, they have deep-discharged one cycle and that a battery is good for only so many cycles (from as low as 20 in an automotive battery to 180 in a golf cart battery, with the typical RV/marine battery good for no more than 30).
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