Let me talk about the first thing first. The Sealed 12v Battery is not a 12-volt battery. 12 volts is just a nominal convenience term used to distinguish one battery from another. A fully charged Sealed 12v Battery, allowing "rest" for several hours (or days), without load (or charging), will balance its charge and measure approximately 12.6 volts between the terminals.
When the battery reads only 12 volts under the above conditions, it is almost completely depleted. In fact, if the quiescent voltage of the battery is only 12.0 to 12.1, it means that only 20% to 25% of its usable energy remains. It is either dead or has undergone a deep cycle, and the battery can only go through a limited number of deep cycles before it is truly dead.
A sealed 12v Battery can only provide useful energy in a limited range-from more than 14 volts (when fully charged and uncharged) to 10.5 volts in use/under load (when the lights dim, the water pump groans and the TV screen becomes smaller). Unless it is being charged, the voltage of the Sealed 12v Battery will not exceed 14 volts for more than a few seconds. The minimum limit is 10.5 volts (used during testing), which is obviously not satisfactory in actual use. Experienced RVers try to use no more than 20% to 50% of the battery's available energy before charging. This means that they will never let the quiescent voltage drop below 12.5. Except in an emergency, they will never be more than 50% charged before being charged (at a quiescent voltage of 12.3 volts).
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