Hello Ken,

At 7:34 AM -0700 8/23/99, Ken Starcher wrote:

>Also the cyclic loads on the battery bank need to be considered, for
>long life, allowing a battery to be discharged more than 20% of its full
>charge reduces its life by 10% for each 10% under this level.
>discharging to 50% each time gives 70% of life instead of 100%.

Can you give me your rationale for these numbers? I've been developing a
theory on this subject for the last couple of years and I'd like to have
someone disprove it.

If you trust the numbers from the manufacturers' cycle charts, you find
that the old "your batteries will last longer if you shallow cycle them"
line appearrs to be a myth. In fact, you get the longest life--measured in
AH cycled through the battery-- by cycling in the 40-60% DOD range.

It's true that shallow cylcling makes them "last longer" in years, since
you are using them less. But that's about like saying your car will last
longer if you don't drive it.

I think a better measure is how much usefulness you get out of the batts
for the dollars you spend on them. This leads me to seeing AH cycled
through as a better measure than years. I also think if you buy a battery
bank that never gets cycled more than 20% DOD, you've spent twice as much
on batteries as you need to.

Here's something I wrote to the list in February 1998 ("I love quoting
myself, it adds spice to the conversation"):

>OK, I just dug up the chart we were looking at. It's from the Navy/Sandia
>Manual on PV systems (fig 2-45 on page 47). It shows daily average depth
>of discharge compared to number of cycles. The chart seems to be for deep
>cycle, lead-acid batteries. Here are some selected figures (not exact as
>it's a small chart):
>
>80% DOD, 1850 cycles
>70% DOD, 2100 cycles
>60% DOD, 2400 cycles
>50% DOD, 2800 cycles
>40% DOD, 3100 cycles
>30% DOD, 3700 cycles
>20% DOD, 4300 cycles
>10% DOD, 4900 cycles
>
>So if you took a hypothetical 100 AH battery...
>
>80 X 1850 = 148,000 AH
>70 X 2100 = 147,000 AH
>60 X 2400 = 144,000 AH
>50 X 2800 = 140,000 AH
>40 X 3100 = 124,000 AH
>30 X 3700 = 111,000 AH
>20 X 4300 = 86,000 AH
>10 X 4900 = 49,000 AH
>
>So _IF_ the chart's data is accurate, we'll get longer battery life (total
>AH cycled through the batteries) if we deep cycle them than if we shallow
>cycle them. This goes against conventional wisdom which says we should
>take it easy on our batteries and they will last longer.
>
>Reactions please!

Other cycle life charts I've seen since then end up with the peak ot 50-60%
DOD.

Ian

Ian Woofenden <ian.woofenden@...>, Fax: 360-293-7034
Associate Editor, Home Power magazine
The Hands-On Journal Of Home-Made Power
Editors are Professional Idiots - We misunderstand text so our readers won't.

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