[RE-wrenches] Battery Specific Gravity versus Electrolyte Level

[William Miller]

Friends:



Here is a conundrum I have often pondered.  Maybe someone can solve this
for me:



Flooded cell O&M manuals instruct not to top off battery electrolyte levels
before equalizing.  They also instruct to check SG before adding water.



Imagine two identical cells, one low on water, but above the moss-guard,
and one at the recommended level.  You equalize the array and then check
the SG on both.



I am imagining that if the cells were equivalent in capacity, they have the
same amount of acid.  However if one has less water, it has a higher ratio
of acid to water, therefore it will read higher in SG.  If the above is
true, it is not accurate to compare the SG in cells unless the water level
is equal, or close to.



If you were to add distilled water to the cell that was low, the water
would not immediately mix thoroughly.  Then to accurately compare SG, you
need to wait some time after watering and you would need to be somewhat
accurate in keeping all cells at the same water level.  How long would this
be?



Checking SG is but one part of flooded cell maintenance.  My monthly
procedure is this:



1.   Check to see if water is above moss-guard on all cells.  (I sometime
skip this if I am confident the cells historically do not dry out in one
month.)

2.   Initiate an EQ charge cycle.

3.   Shortly after the EQ cycle, check and record SG in all cells.

4.   Re-water batteries to correct level.



The above takes long enough.  If I have to add water and let it mix before
I test SG, the process takes even longer.  Nonetheless, I want accurate
results.  My advice to clients and actions taken are dependent on these
readings.



Anyone have any wisdom to pour in?  I am going to experiment to see what I
can learn.



William Miller



Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com

CA Lic. 773985
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