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

[Dan Fink]

Thanks again to everyone for the comments and suggestions both on and off
list.

I ended up having my class isolate and load test the suspect battery that
had specific gravity so low on one cell that none of our instruments could
even register it. Because this is a 24v series parallel battery bank of 12v
Trojan J185s, we had to isolate the other battery in that string also and
performed the load test on it simultaniously with the suspect one.

We used 12v car headlights for the load test, which put us at about a c/20
rate for 2 hours. The results? The bad battery showed a c/20 capacity of
only 12 amp-hours by both voltage and SG. The other tested battery was
fine. So we are convinced that this was a manufacturing defect in one cell
that only manifested itself after 2 years of service.

The remaining 2 batteries in the bank are, I'm sure, very happy not to be
supporting their slacker friend anymore. The orphan battery in the string
will get  a new buddy shortly, and even though this is outside the time
window I recommend for my clients, these batteries don't get worked very
hard as they only run a solar thermal system that draws about 120 watts
when everything is running....which is only when the sun is shining.

Thanks again to the Wrenches here for all your advice.

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



On Tue, Nov 24, 2015 at 10:12 AM, Dan Fink <danbob88 at gmail.com> wrote:

> Thanks everyone for the replies and ideas both on and off list.
>
> Based on your input, next week when the system is in Float I plan to have
> the class isolate the faulty battery and measure voltage, let it rest for a
> couple days, then load test at about C/10. Predicted result is that it will
> show extremely low amp-hour capacity. Since the other battery in the string
> will then be isolated also, we will load test it at the same time to
> compare.
> Best regards and thanks again;
>
>
>>
>> At 09:49 AM 11/22/2015, you wrote:
>>
>> 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?
>>
>>
>>
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