[RE-wrenches] Odd battery problem - very low SG in only one cell

[Dan Fink]

Esteemed Wrenches;

I'm looking for input on what could be going on with a battery bank at the
college I teach for. It's a stand-alone system with 4 Trojan 12v T-1275 in
series / parallel for 24v, 660w of PV, running only a drainback solar hot
water system for space heating. So, 3 pumps and 3 DTCs, max load 100 watts
and doesn't run all that often, outback CC is usually in float. The
batteries are only 2.5 years old and are regularly maintained.

My lab class tested and maintained the battery bank last week, and on one
battery there was one cell, in the middle of the battery, with a SG so low
it wouldn't even register on our refractometer or midnite hydrometer. That
battery reads about 0.2v lower than the others. All the other cells in that
battery read reasonable SG, with 2 cells on the downstream side reading SG
slightly low, but still in reasonable range. This was of course all before
topping up the electrolyte with distilled water.

Our troubleshooting exercise looked at:
~ Measurement error? Nope, I was right there supervising and 2 different
lab classes got the same results with 2 different instruments;
~ Stratification? A suspicion especially since the system is usually in
float. We equalized twice, no change in SG. I DID hear that likely this was
the first time the batteries had ever been equalized, but no way to know
for sure.
~ Spilled electrolyte from tipped battery replaced with water by a previous
instructor's class? Can't see this, all the other cells would have lost
electrolyte too;
~ A student accidentally discharged electrolyte into the bucket instead of
back in the cell, then was replaced by distilled water when topping up? I
can't see that either, the midnite hydrometer needs only a small sample.

So, I'm left with "likely we have a cell that is failing for some reason"
and students secretly smug that the professor is also stumped.

Any ideas?


Dan Fink
Adjunct Professor, Ecotech Institute
IREC Certified Instructor™ for:
~ PV Installation Professional
~ Small Wind Installer
Executive Director, Buckville Energy
NABCEP Accredited Continuing Education Providers™
970.672.4342
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