Tim Hutchisen wrote :
> Amps/hour is an older method to rate a battery. It is not
> used much any
> more. It meant that it would take 20 hours at 57 amps until the battery
> would drop to 1.75 volts per cell, or 10.5 volts total.
Ok, I'm being picky, but this isn't quite right. Amphours are amps times
hours. So 57 amphours (20hr rate) means that if you discharge the battery
at 2.85 amps, it will hit 10.5 volts in 20 hours; giving up a total of 2.85
* 20 = 57 amphours.
The time is important here, because the efficiency of car batteries goes
down dramatically as the current goes up. If, for example, you discharge
the same battery at 25 amps, you won't get anywhere near 57 AH from it.
Modern battery makers do sometimes make the 25 amp rating available, it's
called the "Reserve Capacity" and is measured as the number of minutes at 25
amps before the voltage drops to 10.5 volts. This is also a good number for
automotive use, it gives a reasonable indication of how long you can drive
after a complete alternator failure (or leave your lights on and still start
the car).
> Beware of the cranking amps (CA) number however. Those
> sneaky marketing
> types will often put the CA number in much bigger print than the
> CCA number.
> The CA is rated at 32 F and is a bit tainted, especially for us
> northerners.
But is entirely relevant for those of us who live where it never freezes, or
never drive our LBCs in freezing weather. Some battery makers call it MCA
(but it's the same thing).
> The weight of the
> battery is another good way to judge the battery's power, the
> heavier it is, the better it will be.
Not necessarily true these days, there are actually several slightly
different chemistries running around and some are lighter than others. As
an extreme example, an Optima "Red top" weighs 38 pounds, is rated at 800
CCA, 104 RC. Interstate's "Green top" group 24 also weighs 38 pounds, but
is only rated at 460 CCA, 90 RC.
Randall - feeling picky today <g>
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