On Mon, Nov 22, 2010 at 05:37:12PM -0800, Eric T. wrote:
> A hydrometer test can not reveal a shorted cell. An intermittent short
> certainly not.
Thanks for your input; I appreciate it.
Just as a thought experiment, consider this: if a cell is shorted 100%
of the time, then its hydrometer reading will never come up. If it's
shorted part of the time, then it'll only come up part way.
If I'm only running the engine long enough to charge the batteries -
i.e., long enough to charge each 2V cell to 2.2V - then the
faulty/partially shorted cell will always be undercharged, since it's
"exposed" to the charging current for less time than the other cells
(the cells around it will be overcharged, since they're getting 6V
across two cells of 2V capacity.) Based on that reasoning, a hydrometer
test certainly should reveal an intermittently-shorted cell, as should
measuring the voltage across that battery (which would be lower than
nominal; if one cell is out, then the 6V battery should show somewhere
around 4V.) If it's shorted while charging but not while measuring, it
would still show a difference in voltage - since only two of the three
cells would be charged - i.e., a theoretically "fully charged" 6V
battery with one cell that's only 50% charged should show around 2.2V +
2.2V + 2V, or 6.4V instead of 6.6. And yes, I did pay attention to
differences that small when measuring, and did disconnect the battery
under test from the rest of the circuit. The differences were all in the
.01-.03V range.
Again, I very much doubt that the problem was anything as grossly
obvious as a shorted cell; I'm certain that I would have discovered it
long ago if it was. What troubles me is that I can't come up with a
mental model of what it actually *was* - and thus don't have anything
but a shotgun method for troubleshooting it in the future (i.e.,
disconnecting one 12V battery or pair of 6V batteries, charging the
rest, and watching for absence of "bad behavior" such as cycling.) I'm
not happy with that.
> A cell shortind affects the state of charge of the entire connected battery
> bank by drawing current from the other batteries, and the alternator.' If
> you install a DC circuit breaker between each positive battery post and the
> point where they all connect together, and size theese breakers at the
> supposed maximum current draw of each battery under your intended max load,
> then when a cell shorts the shorted battery's breaker will trip, indicating
> both that there is a problem and which battery has the problem.
Clamping a DC ammeter on the individual batteries' cables while charging
and comparing the readings will do the same thing, and require a bit
less work. I did that for a fellow cruiser last winter, and discovered a
30A difference between the two banks; the problem was a tight-but-not-
tight-enough nut on the positive post (most people don't realize just
*how* tight those need to be.) A little lube (Norm's recipe :), a bit of
muscle, and his engine running time was cut from 8 hours a day (!) to
less than 2.
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