Battery Cycle Life Ratings [RE-wrenches]

Thu Nov 4 07:29:52 PST 2004

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Hi All,

In regards to this thread on battery life etc, I thought that knowing what
sort of voltages are being used might be informative.

I'm curious about bulk charge voltages that you all are using?

EQ settings?

Are you using different set points for CC or inverter?

Thanks

jay

Peltz Power

> Allan,
>
> I have an interesting "lab" experiment going on presently with a 24 volt
> L16G battery bank, that speaks to some of your questions.  The customer
set
> up a 3-string, 12 battery system when he installed his original system
> (himself) 5-1/2 years ago.  The original system had an SW4024, DC well
> pump, and 720 watts of SX panels.  Started with a C40, upgraded to an SB50
> three years ago.
>
> Last year, he added a 4th string to increase his battery capacity, and an
> additional 240 watts of PV.  The system wasn't keeping up and the
generator
> was coming on way too often.  He does maintain the batteries faithfully,
> thought.  The increased capacity took care of things until this past
month,
> when L16's from his original bank began to fail.  He asked me what to do
> (he definitely wanted to replace as few batteries as possible).  I gave
him
> the standard recommendation that all 12 older cells should be replaced,
but
> at the very least, make sure to only replace an entire string at a
> time.  Since he had 3 dead cells at that point, he started by replacing
one
> string.
>
> Anyway, here's some interesting data: He wanted to equalize the bank after
> we inserted the new string.   I told him that sounded good, but that I
> wanted to make measurements at the start and end of equalization.  He uses
> a 10KW generator for equalization, so we were getting a very constant
> equalization charge.  Here are the individual string charging amperes
measured:
>
>                    15 min.  1hr 45 min.
> Brand new string   4.7          4.3
> 1yr old string     6.3          6.1
> 1st orig. string  10.4         10.6
> 2nd orig. string  33.1         44.0!
>
> In the string that had the excessively high currents, we identified a
> battery with a bad cell at the end of equalization - it must have failed
> when we shuffled the batteries around.   The other measured differences
are
> not likely attributable to cable connections, as all battery cables were
> completely cleaned and redressed when the batteries were replaced.
>
> I'm not completely sure what to make of this data, but it definitely seems
> to me from these limited observations that older strings will hog
available
> charge current, thereby potentially decreasing the number of days that the
> newer strings will be able to obtain a full 100% acceptance charge.  That
> can't be good.
>
> I'm also wondering if equalization on older cells near end of life should
> be done much less often, particularly if sustained undercharging is not an
> issue.  In the newer strings above, charge currents decayed slightly over
> the equalization cycle, as I'd expect.  On the other hand they increased
> somewhat in the older string that had no failed cells.  That shouldn't
> happen.  And in the string with the bad cell, it looks like the EQ cycle
> might actually have been the thing that took it over the edge!
>
> I'm certain (from some other observations that I've made on other systems)
> that measured differences in charge amperes are observed at lower voltages
> as well (acceptance/float potentials), they're just not as pronounced.  Is
> anyone making these kind of string charging current observations on their
> own (or their customer's) systems, and what are you finding?  We're going
> to keep much closer watch from this point on.  I'm beginning to think that
> final acceptance charge current measurements might be a very useful
> predictor of battery wear and expected remaining life.
>
> BTW, he's now replacing the entire bank.  Once the cells start going....
>
> John Raynes
> RE Solar
> Torrey UT
> 435-325-2110
> www.re-solar.com
>
>
> At 10:49 AM 11/3/04 -0700, you wrote:
> >
> >
> >Wrenches,
> >As I follow this battery life thread, and we learn that some of our
> >long-held assumptions may be myths (such as that L16s inherently last
longer
> >than golf carts) a couple of other questions come to mind:
> >
> >1. It has been commonly understood that new flooded batteries are never
to
> >be mixed with old batteries; the limit is 6 months to about a year,
> >depending on use. Why? Is it, as I have believed, that the new batteries
> >would only perform as well as the old--that is, remaining cycle life,
> >charging characteristics, etc. would match the older cells. Is it worse
than
> >this? Will the mixed bank perform worse than the old cells, or cause the
> >entire bank to fail prematurely? What if eight 6V batteries are doubled
to
> >16, and the new and old units are evenly split among each parallel string
> >(in either 24V or 48V configuration)?
> >
> >2. I have also designed systems with smaller banks relative to charging
> >capacities, based on manufacturers' recommendations for best performance.
> >But a C/15 to C/20 rate is still hard to achieve, and can lead to
excessive
> >discharges if the owner isn't attentive. In the early days of offgrid
> >system, battery banks were way oversized relative to arrays (I still get
old
> >timers thinking in terms of one "battery" (a golf cart) to one module
(which
> >back then was a 35 watt Arco 16-2000 or the like). That's about a C/100
> >charge rate.
> >
> >What I have noticed is that the grossly oversized battery banks batteries
in
> >these old systems still tended to last about 5-8 years; that is, about as
> >long as well-sized banks. So I could use some real-world discussion of
how
> >others size batteries to arrays and loads, outside of all the textbook
> >numbers.
> >
> >Allan at Positive Energy
>
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