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Battery Volts and State of Charge
Batteries that are in good condition and fully charged will have a resting voltage of around 12.6 volts. As they are used to power items the voltage will drop as the battery gives up some of it's energy. Eventually, the battery will be "dead" and will need recharging. Note that a fully dead battery has 10.5 volts of resting charge. However, the minute you apply a load to it, the voltage plummets and nothing happens. Just remember that 12 volt batteries are not full at 12 volts and dead at zero. They are dead at 10.5 and fully charged at 12.6.
If you want to make your batteries last longer never discharge them below 40%. If you run your batteries to 40% you will get twice the lifetime out of them than if you were to run them down to 20%. Running them down below 20% is very detrimental to battery life and you really do gain very little usable power from them at that level.
Of course this assumes that the battery has been resting. If the battery was recently charged the voltage will be much higher, quite likely into the mid-13 volt range. This is not a true Resting Charge and is referred to as a Surface Charge. In this case you should let the battery rest for 2-3 hours before testing.
If you want to test your batteries on an RV that is not being used please refer to the charts on the Battery Testing Webpage in this category. However, most RVers just want to know how much "juice" is left in there batteries when camping and they do that by looking at a voltmeter. In that case use use the following chart, which compensates for batteries that are in use.
Submitted by Mark Quasius - 2/07/06
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