A hydrometer test can not reveal a shorted cell. An intermittent short certainly not. 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.
Eric Thompson S/V Procrastinator South San Francisco [email protected] ----- Original Message ----- From: "Ben Okopnik" <[email protected]> To: "LiveAboard List" <[email protected]> Sent: Monday, November 22, 2010 3:48 PM Subject: [Liveaboard] Batteries > I've just come off a passage, and am sitting here in harbor repairing > stuff (that's what happens when you let things get away from you... > [sigh] In process of catching up now.) Interestingly, all the failures > are "good" ones, in the sense that I'm actually happy about them. Why? > Because every one of them, except for a stupid mistake that I made > (don't ask), is a _diagnostic_ failure: it finally and unmistakeably > tells me the answer to a problem that I've been chasing and couldn't > quite solve. > > One of them, however, troubles me in its implications, so I'd like to > throw it to you folks on the list (perhaps I'm spurred by Norm's great > example of diesel troubleshootery... thank you, Norm! All the years I've > been working on diesels, and still learned something new.) > > PROBLEMS (some seemingly unrelated until now): > > 1) The engine used to rev up and down, about 750RPM, on a ~10 minute > cycle while charging batteries. The start of this coincided with my > installing a new alternator, however, so after making some measurements > here and there, I gave up, thinking that perhaps it was normal. On the > ammeter, it would go to -12A or so, depending on what I had running at > the time, stay there for about 30 seconds to a minute, and then jump up > to over 100A and slowly taper off to about 60A - then repeat a few > minutes later. > > 2) The battery capacity had, in my perception, decreased significantly - > but hydrometer testing showed the batteries to be fine (I'll return to > this later), and even a 100A load tester, which I bought for this exact > purpose, showed all the batteries - tested individually - to be fine. I > even doubled the test period; yep, batteries still held up. However, two > things still bothered me: the time between full charge and the 50% point > had gone way down, to roughly half, although my usage patterns hadn't > changed - and firing up my little generator + charger (max 35A, 18-20A > nominal) when the batteries were low immediately made the system voltage > go above 13V. In my experience, that voltage should come up relatively > slowly, then essentially stop at 12-some volts, then follow the charge > curve up to 14.4V. A poor connection to the battery might have accounted > for that, but I tightened the heck out of it and checked for voltage > drops across the cables when charging at 100A or so - and they were all > low and reasonable. > > 3) Starting the engine, for the past year, has been the bane of my life. > OK, so I didn't have a starting battery (that's one of the changes I'm > making while I'm here), but - cripes, 4 Trojan T-125s have always been > more than enough to spin that starter like a mad spinning thing! I was > almost down to pulling the starter (a big job on this boat) and taking > it to a shop to be tested - but then, firing the engine up right after > the batteries had been charged worked just fine. 10% below top, and, > well, out came the can of ether. Grrrr... > > SOLUTION (a.k.a. "diagnostic failure"): > > At about 4 a.m., while in the middle of an offshore passage, I heard the > engine note change - maybe a 1500RPM drop - and a slight screeching > noise. This stopped after about 10 seconds. By that time, I had the > engine hatch open and was looking around... and it suddenly happened > again, at which point I saw a couple of sparks (!) shoot out of the > alternator. At the same moment, I noticed that the two starboard > batteries were somewhat swollen and their plasic cases had turned dull; > one of them actually looked like it had started to melt near one of the > terminals. I also noticed that the ammeter shot up to 100A+ during that > time, and back to -18A when it wound down. I immediately put the > throttle to idle (didn't quite want to leave the boat without way on - > there was no wind) dove down into the compartment and disconnected the > negative terminal to that battery pair. Then, I tried upping the engine > speed again. > > "And suddenly, there was peace." > > All of a sudden, the entire system was working just as it should have > been all along. The engine no longer cycled; the ammetter showed a > steady 6.7A (the batteries were charged to the max, so it was just float > current.) I was actually annoyed/bothered enough by this unexpected > change that I turned off the engine and immediately restarted it - and > it cranked right up, just as it always did on a full charge (but now it > was with only two of the Trojans in the circuit.) That is, taking those > two batteries out did not seem to reduce the system capacity. > > And *that* is what I mean by a diagnostic failure. Overall, quite the > learning experience. > > At this point, I've been sitting in this harbor for a day, working on > repairing things, making lists to make sure I don't miss anything, and > so on. Seems that the use capacity of the system has indeed been > reduced - I've left the computer on all day while going about my tasks, > and the system voltage is lower than it would have been when all four > batteries were in the circuit. > > While I'm here, I'm going to replace all four batteries and add a > starting battery (and weld up some new battery boxes while I'm at it), > which should take care of all that. The frustrating, unanswered part of > this that remains is, why didn't any of the tests - either the regular > equalization and hydrometer testing or load testing - reveal a problem? > What was happening with that set of batteries that caused the engine to > cycle? Was there a cell or two that shorted as it warmed up, and > "unshorted" as it cooled off? Hard to believe - a hydro test would have > revealed that, I'd think. > > Any suggestions, comments, or even pertinent thoughts to toss into the > pile would be highly welcome. > > > -- > 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
