If a cell becomes 'shorted' when fully charged does the chemical reaction that normally produces voltage continue normally?
The symptoms you describe sound very much like an intermittently shorting cell. Thus the 100+ amps from the alternator and the resultant loading of the motor. If the cycle of the period was long enough to allow a near normal state of charge to be reached and then the motor was shut down the battery might only be getting shorted when the boat was underway. Maybe bow up a little moved some loose lead in the bottom of the battery. Any way you look at it, you were suffering from an intermittent cell (or 2) shorting condition. The circuit breaker concept would still have identified the bad battery/ battery pair. Eric Thompson S/V Procrastinator South San Francisco [email protected] ----- Original Message ----- From: "Ben Okopnik" <[email protected]> To: <[email protected]> Sent: Monday, November 22, 2010 6:16 PM Subject: Re: [Liveaboard] Batteries > 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. > > > -- > OKOPNIK CONSULTING > Custom Computing Solutions For Your Business > Expert-led Training | Dynamic, vital websites | Custom programming > 443-250-7895 http://okopnik.com http://twitter.com/okopnik > _______________________________________________ > Liveaboard mailing list > [email protected] > To adjust your membership settings over the web > http://liveaboardonline.com/mailman/listinfo/liveaboard > To subscribe send an email to [email protected] > > To unsubscribe send an email to [email protected] > The archives are at http://www.liveaboardonline.com/pipermail/liveaboard/ > > To search the archives > http://www.mail-archive.com/[email protected] > > The Mailman Users Guide can be found here > http://www.gnu.org/software/mailman/mailman-member/index.html _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://liveaboardonline.com/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardonline.com/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
