Hi Jeffrey -- you can sorta determine depth of discharge based on voltage,
or you can get a more accurate handle on it by checking the specific
gravity of the electrolyte with a hydrometer.

The wiki has some possibly useful stuff, at

http://vanagonwiki.net/index.php/Aux_Battery#Determining_Battery_State_of_Charge

and from there you'll find links to other possibly useful sites which
write about how to determine battery state of charge.

--
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

On 6/9/2008 10:32 AM Jeffrey Vickers wrote:

> Speaking of not draining your battery past a certain point---how do
> you determine what that point is? I recently charged up my batteries
> and after a few hours, they both settled down to 12.75. Now that I
> know the fully charged voltage, how do I determine the fully
> discharged voltage so that I can find the voltage number that
> represents 50 - 80% of discharge? I've seen a few charts but the
> numbers don't match my numbers...
>
> Jeff / San Anselmo
> On Jun 9, 2008, at 9:58 AM, Automatic digest processor wrote:
>
>> Date:    Mon, 9 Jun 2008 07:38:27 EDT
>> From:    Frank Condelli <RAlanen@AOL.COM>
>> Subject: Re: Battery Woes/Choice
>>
>> In a message dated 08/06/2008 1:31:31 P.M. Central Standard Time,
>> LISTSERV@GERRY.VANAGON.COM writes:
>>
>> The less  a battery is drained/discharged the longer it will last.
>> Strictly
>> speaking,  flooded batteries are not designed for repeated deep
>> discharges,
>> so when  used in this manner, they fail prematurely. Frank was
>> acutely aware
>> of this  fact, so chose to buy from a place that had a great warranty
>> service, as  did I.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> You  do know that there is a good way to extend the life of the
>> standard
>> automotive  starting flooded cell battery ?  Koolatron and others
>> sell a limiter
>> that will shut off the drain on a battery once it reaches critical
>> levels
>> preset  by the manufacturer of the gadget.  I forget all the details
>> of it but I
>> got mine from Kootatron and it was made for their fridges to keep
>> owners from
>> wrecking the batteries in their vehicles.  See the Koolatron website
>> for
>> details.
>>
>> Cheers,
>>
>> Frank  Condelli
>