The 650CCA has nothing to do with the long term discharge capacity. It is a rating of how much current can be delivered for 30 seconds at 0F and still maintain 10.5 volts. Typically, this battery is rated at ~65 A/H total capacity. As the voltage of the battery starts falling, it is no longer delivering 8 amps into the heating element. Also, since this battery is not designed for this, after a few cycles, damage has been done to the plates and capacity is permanentley lost. At some point the deformed plates will short out in a cell causing you to have a 10 volt battery.  Then you will get that great acid boil over as the alternator pumps current into that 10 volt battery and overcharges the remaining cells. For the event of a charging system failure, these batteries do have a "reserve capacity" rating. This is a rating of how long the battery can support a 25 amp load so you can drive home or to get help.

Dennis

----- Original Message -----
From: Dave Vickery <davevickery@HOTMAIL.COM>
Date: Tuesday, June 20, 2006 12:58 pm
Subject: Re: Understanding Amp Hours

> A real world example.  A new Sears diehard 650 CCA aux battery
> will last 4
> hours until completely dead running the 8 AMP fridge.  On 12 V the
> fridgeruns continously pulling 8 amps.  So assume 50% depletion in
> 2 hours = 16
> Amps available.
>
>
> >From: Loren Busch <starwagen@GMAIL.COM>
> >Reply-To: Loren Busch <starwagen@GMAIL.COM>
> >To: vanagon@GERRY.VANAGON.COM
> >Subject: Understanding Amp Hours
> >Date: Tue, 20 Jun 2006 08:43:35 -0700
> >
> >The answer to another question here on the list raised this question.
> >Given
> >that we have the amp hour rating for a battery and also given
> that we don't
> >want to draw it down below about 10.5 volts, about 80%.  Does
> that amp hour
> >rating assume draw down to the 80% level, or draw down to 'dead',
> zero?>I've always assumed that if I have a 50 amp hour battery
> that I can draw 1
> >amp for 50 hours before reaching the point that will require
> charging, or
> >will I only get 20% of that, 1 amp for 10 hours, before reaching
> the point
> >where I need to recharge?
>