[RE-wrenches] Five L16s in a 24V string?

[Starlight Solar Power Systems]

Allan,

The battery "pair" is operating in parallel so the current flowing through that string will be divided between the two. This means that each battery in the parallel set is only being discharged half of what the others in the string are. So, of course, those batteries will be lasting longer since they have half the DoD each day. A very strange setup, I’ve never seen this. Since I rarely see a true battery failure (most are damaged by the owner) I can’t say that I’ve noticed any pattern.

Larry Crutcher
Starlight Solar Power Systems


On Sep 30, 2015, at 8:43 PM, Allan Sindelar <allan at sindelarsolar.com> wrote:

Wrenches, especially the grizzled ones,
I was referred to a couple for off grid service about 75 miles from here. They have an older homeowner-built 24V system (that I haven't seen yet) and have lived with it for decades. They have ten L16s in two parallel strings. I questioned this when I first heard it, but the owner told me that the most positive cell is the one that always fails first, and he hates the effort of periodically rotating his batteries, so the battery nearest the positive cable has another L16 in parallel with only this battery. He explained that he has lived this way for thirty years. I don't yet know what life they have gotten from their battery sets, but until recently they lived on 750W of PV, and now have around 1kW - not a large array.

I respectfully replied that in nearly thirty years in the field, this is the first time I had heard of this approach. My own experience is that while sometimes I had sometimes seen marginally more water consumption in the lead positive cells as they aged, when I had tested for failed cells in old strings, the failed cells tended to be randomly located, and not frequently enough at the positive end to suggest a pattern. I had also long ago read (Surrette's manual?) that rotating the lead cell is good practice, but unless a system has a single series string, wired to make this possible, nobody ever did this rotation, and it didn't appear to make a noticeable difference in long-term performance.

Has anyone else heard of this approach? Is there validity to the logic? What can I learn here that I didn't know?
Thank you,
Allan
-- 
Allan Sindelar
 <mailto:allan at sindelarsolar.com>allan at sindelarsolar.com <mailto:allan at sindelarsolar.com>
NABCEP Certified PV Installation Professional
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
505 780-2738 cell


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