It's true: 11.7V pretty much means that the battery is at 25% state of
charge. It's a good place to stay above: "Occasional dips into [this
range] are not harmful, but continual discharges to those levels will
shorten battery life considerably." ["State of Charge,"
http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Voltages]

10.5V at 77 degrees F is considered fully-discharged.

Determining a battery's state of charge by reading its voltage is not
real accurate; measuring each cell's specific gravity with a hygrometer
is the most accurate method, but it's messier business than I care to
deal with inside our Westy. Given the inaccuracy of the voltage method,
maybe 11.7 volts as a trigger point ain't bad. But one wonders how
accurate that cut-off gizmo's voltage measurement circuit is.

It comes down to how deeply you are willing to discharge the battery. I
try to keep our house battery at no less than 80% state of charge (per
the XBM battery monitor, no less than 12.4V on the volt-o-meter).

--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR

TC typed:
>> Yes, but it cuts out at 11.7 volts.... too high to be of any use for RV's.
>>                                         David (dsl82westy)
> please explain  I thought that was about when a 12v battery has 25% charge
> left
> 12.8-13.2v is ~ full charge, & fully discharged is about 11v
> so 11.7v is really low discharge pt even on deep cycle batteries if you want
> optimal life of the battery
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~~~~~
>>> Would this sort of low voltage cut-off gizmo do the trick ??
>>>
>>> Tom
>>>
>> http://canadiantire.ca/browse/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524
> 443294641&FOLDER%3C%3Efolder_id=1408474396672454&bmUID=1182208017428
>