Battery Cycle Life Ratings [RE-wrenches]
John Raynes
john at raynes.com
Wed Nov 3 12:13:37 PST 2004
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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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