On (26 Nov 96) Chris Harper wrote to Ian Woofenden...
 CH> Is it better to discharge "deep" with deep-cycle batteries, as
 CH> it would seem to indicate? Or do they last longer with a mild
 CH> cycle.
This is a fascinating question that we discussed at length at Solar
Energy International this summer. 
For years I've had the impression that batteries would last longer
(more total amphours cycled through) if they were not cycled too
deeply. 
But the guy from Trojan Battery would not admit that that was true.
He said there is a certain number of amphours that will be cycled
through, regardless of cycle depth (barring destroying the battery
by totally discharging it). I had and have my doubts about the truth
of this.
A wonderful old guy from IBE (a maker of forklift batteries) pulled
out some charts with cycle curves, and we did some calculations.
According to the curves, and our calculations, their lead acid
batteries will actually cycle a LOT more amphours over their
lifetime if taken to a moderately deep discharge level.
I took a 220 AH battery and took the numbers of cycles from the
curve at 5% discharge, 20%, 60% and 80%.
At  5%, it's 5400 cycles, so 5400 X 11 (5% of 220) is 59,400 AH
At 20%, it's 4,300 cycles, so 4,300 X 44 is 189,200 AH
At 60%, it's 2,400 cycles, so 2,400 X 132 is 316,80 AH
At 80%, it's 1,500 cycles, so 1,500 X 176 is 264,00 AH
 
I suppose you could make a curve and find the optimum point, but in
the real world you'll never cycle them that accurately anyway.
Did we screw up the math, or the logic? Not that I can see. I would
very much like to hear any other information or opinion on this. If
Richard Perez is listening, his comments would be great!
Ciao,
                        Ian
... "I always travel by motorcycle," Tom said Triumphantly.                   
 
...  "If I owned both Texas and Hell, I'd rent out Texas and live in Hell."
--- PPoint 1.96
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