Flooded batteries with chronically low SG (was More on battery management) [R

[Windy Dankoff]

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Windy,

Your mention of Kelly's comments/questions prompted me to take time  
to look at the board this AM.  Several new postings in addition to  
her previous one, including from you - which again you are right on  
the $.  ...  Will be fun to see the feedback your HP article  
generates.  As far as specifics for the group (and Kelly), I'll see  
what I can put together ....  Some is below .. with more to follow as  
time permits.

Regarding chronically low SG... As many of you may already know, it's  
often a bad omen, indicative of a failing cell.  Low SG can be  
attributable one or more of several causes.  The most common: 1)  
Serious sulfation, which permanently extracts sulfur from the acid  
leaving a weakened solution; or 2) Weak acid from the onset caused by  
either dilute acid from the supplier, or too little acid added in the  
beginning, with the deficiency made up by adding distilled water.  In  
rare instances, I've seen batteries overfilled, then overcharged,  
causing considerable acid to boil out the top of the cell.  Adding  
distilled water to compensate for the loss then only serves to weaken  
the remaining acid.

Corrective action depends on the cause.  In case #1 .. the best would  
be replacement.  In the alternative, *possibly* a chelating agent  
such as EDTA (ethylene-diamine-tetra-acetic acid), followed by  
addition of *some* fresh acid and a restorative/equalizing charge.   
This is a trial-and-error process with the manufacturer's spec'd SG  
the goal.  Bear in mind when such steps are taken, lead is lost (it  
precipitates to the bottom of the cells), hence A-H capacity is  
reduced.  Since this process will not strip lead equally from all  
cells (it's dependent on the degree of sulfation), there will be a  
permanent A-H imbalance in the battery thereafter.  The imbalance may  
be minor .. or not.  SG cannot be used to measure cell capacity.   
Only load testing can do that.  EDTA was reportedly used for removal  
of lead and other toxic heavy metals from humans beginning in the  
1940s (or so).  Reports on results vary.  The same success  
variability is true for flooded lead-acid batteries.  I do not  
advocate use of EDTA.  It's tantamount to injecting "self-sealing"  
goo into tires when you have a leak. Very simply, proceed at your own  
risk.

In case #2, a hydrometer can be used to carefully extract the weak  
acid from each cell, which is then replaced with concentrated acid to  
bring the specific gravity up to the manufacturer's specifications  
(typically 1.275 +/- 0.005 @ 77F), followed by an equalizing charge.

As an instructive note, I've seen "hydrometer" referred to as  
"hygrometer".  These are NOT the same instrument.  The former  
measures specific gravity.  The latter, relative humidity.

Sometimes .. though not often .. low SG can be caused by errors in  
the measurement methodology.  Are you sure the hydrometer is  
correct?  Can it be read as accurately as needed?  How are you  
measuring?  Acid/water solutions will stratify over time if not  
mixed, with the water slowly rising to the top where it's read with  
the hydrometer.  The proper method for taking a sample is to draw a  
full measure of acid into the hydrometer, squeeze it carefully and  
steadily back into the cell, and repeat three or four times to "stir"  
the solution.  Then .. draw in a full measure into the hydrometer and  
take a reading.  This must be done when the cell/battery is at room  
temperature (77F nominal).  If above or below this temperature,  
tables and charts exist that provide SG correction factors.  These  
can be found on battery manufacturer websites.

When are you measuring?  Batteries must be allowed to sit idle (no  
charge or discharge) for 6-24 hours before readings are taken (longer  
being preferred).  Gases (primarily H2 and O) dissolved in the acid  
as a result of the charge process can give false SG readings, always  
to the low side.  Since batteries in RE systems are invariably in  
constant use, this is an almost impossible goal to achieve.

In rare instances, an acid neutralizing substance such as baking soda  
(used for cleaning battery tops, and is often also used in an attempt  
to prevent corrosion to connections), is inadvertently introduced  
into cells.  This too reduces SG and can only be correct by first  
fully charging a battery, removing the old acid, and replacing it  
with new - a difficult and messy process at best.

In all instances, take proper precautions for handling and/or  
disposal of acid.

[ top-secret mystery maestro ]


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