Automotive starting batteries are not intended to be discharged except for the
amount required to actually start the engine.  The excess capacity they have is
for emergencies and use of more than a few percent of the charge seriously
impacts the life of the batteriy.  Full discharges of one will cause it to fail
earlier than you would expect, and lowers the cranking amps available.  You
will probably not notice this under normal use, but it may fail when you need
it most.

I speak from more than one bad experience with DC coolers.  Yes, I should have
learned the first time.

I personally would never draw more than about 10 or 15 amp hours from a starter
battery, and that only on a very occasional basis.

You would be much better off with a separate deep cycle.  They are made for
that type of service, and you won't strand yourself if it goes dead.

tom

On 23 Sep 2003 at 9:31, Sam Williams wrote:

> I bought a 12 V electric cooler box on Sunday in preparation for our upcoming
> trip & left it on to check it out.
> It is a small kind that draws 3 amps.
> Well, the vanagon started on Monday and was moved in the driveway in the
> morning.
> Last night the battery was dead as a dodo.
> So it is on the charger and boy that is not trivial in a vanagon, You have to
> move the passenger seat forward and rotate it 45 degrees, to get to the
> screws holding down the battery cover!
> Well, it is on charge and we will leave on Wednesday night!
> Sam W
> 63 singlecab
> 70 bay Westy
> 87 Vanagon Westy
>

------
Tom Ring K0TAR, ex-WA2PHW  EN34hx
85 Westphalia GL Albert
96 Jetta GL The Intimidator
taring@taring.org

"It is better to go into a turn slow, and come out fast, than to go into a turn fast
and come out dead." Stirling Moss