*sigh*
>> That's absolutely preposterous. The voltage vs. capacity curve always falls
>> more gradually when draining at a lower current - that is, the usable battery
>> capacity will fall at higher discharge rates. Undoubtedly, a car battery would
>> last a hellava lot longer than the builtin unit in this situation, so....if you
>> do not know what you're talking about . .
No...
sit down. Re-read what I wrote. Pay particular attention to the definition of the word "lifetime." Note that I did not say "capacity" or "reserve time." Though after a few deep discharges using a car battery, you might well be able to substitute either of these terms into my statement.
Next, examine the physical differences between a typical high-current car battery and a deep-cycle battery. I refer you to the "Deep Cycle Battery FAQ." There are a number of copies floating around the net but you can find it
here.. In particular, I refer you to section 4.2:
<<
Car batteries are especially designed for high initial cranking amps (usually 200 to 400 amps for five to 15 seconds) to start a car and not for deep cycle discharges. Deep cycle (and marine) batteries are designed for prolonged discharges at lower current. [The plates in car battery are more porous and thinner than in deep cycle batteries and use sponges or expanded metal grid instead of solid lead.] A deep cycle battery will typically outlast two to ten car batteries when used in deep cycle applications. Starting an engine will typically consume less that 5% of a car battery's capacity. Whereas, deep cycle (or marine) batteries are used for applications that will consume between 20 and 80% of the battery's capacity.
>>
-- arcas