[A/S] Re: Boondocking Battery specs, which is most important?

["Maurice Riggins" <maurice.riggins@xxxxxxxxxxxxxxxx>]

For Optima battery specs, go to:

http://www.1st-optima-batteries.com/

This place was the first to point 
out to buyers that the blue tops 
were the same as the yellow tops 
except for having no side terminals
AND have a year longer warranty.

For a fair comparison with Group 31
flooded cell, lets look at the D31M
which has SAE posts.  It has a 20
hour rating of 75AH at a discharge 
rate of 3.75A, and a reserve of 155 
minutes or 2.58 hours (rounded), at 
a discharge rate of 25A.

So the Peukert exponent is:

log(20 hours)-log(2.58 hours)/log(25A) - log(3.75A) = 1.08

The overly simplistic Time= AH Rating/Amps doesn't account for 
discharge rate, where the needed formula is:

Time = Capacity Number/Current^Peukert exponent

so the Capacity Number can be derived by

Time X Current^Peukert exponent=Capacity Number.

Using the published 20 hour Amp-Hour rating:

20 hours X 3.75A^1.08 = 83 is the Capacity Number

Going back to the formula that considers discharge rate:

83/3.75^1.08 = 20 hours (19.9 due to rounding) and that's where the 
amp-hour rating comes from

83/25^1.08 = 2.57 hours (due to rounding) = 154 minutes and that's 
where the reserve capacity rating comes from

Since the exponent is in the denominator, the smaller it is the less 
capacity it loses at HIGH, not low, amp draw. 

If we look at the 20 hour rating of the flooded cell Trojan 31XHS at 
130AH, it totally devastates (watching too much war coverage) the 
75AH rating of the best Optima at low current levels! 73% better!  
But because the big Optima has a lower Peukert exponent, the 225AH RC 
of the 31XHS is only 45% better at the higher 25A discharge.

--
Maurice
WBCCI 5446