Hi Ron, As many on this list have suggested, it sounds like a sulfated battery condition. In your last message you revealed something to me that absolutely confirms this but perhaps you didn't recognize it.
Battery voltage readings are deceiving because they do not indicate capacity. 25.7 volts sounds like a fully charged 24 volt bank, but is it? Only if the battery was at rest for 5-6 hours could you have some confidence that the bank was full at this voltage. But this is not the case because the bank is in daily use, always charging or discharging. However, there is one useful indication that voltage can be used for: detecting a sulfated battery. You mentioned that the the battery drops to 24.5 in the early AM without any heavy loads on. For the 4KS25 battery this equates to about 800AH at the 72 hour rate. Then you said that the customer ran a 2500 watt generator for 5 minutes and drove the voltage up to 29 volts. Here's the Ah-Ha moment: That is EXACTLY the behavior of a heavily sulfated battery bank. A fast rise in voltage indicates sulfation. It is impossible for that tiny generator, or any charge source they own for that matter, to replace the hundreds of AH it would take to drive a healthy battery up to the absorb voltage of 29 volts. The bank is about 45,000 watt hours (72h rate). There would have to be over 20,000 Wh removed to be at that voltage. How many Wh's are replaced in 5 minutes by a 2500 watt genny? I'm sure you are getting the picture. Why did this happen to these expensive batteries? Glad you asked. Battery plates are not uniformly efficient in the electrochemical process leaving some portions with lead sulfate even after 8 hours of charging. Unless these portions are cleared off regularly by achieving 100% SoC and occasional, thorough equalization, the amorphous sulfate will convert to a crystalline form and grow. 99% charge, if not corrected in time, will always cause premature battery failure. Undersized RE charging systems, or perhaps oversized batteries, is the culprit that contributes to this all too frequent phenomenon of chronic undercharging. I say contribute because there are other factors. Fact: it can take 10-12 hours to fully charge a lead acid battery. Fact: The time element of battery charging is a highly misunderstood part. With only a few daily sun-hours to work with, how do we get a battery charged with PV solar? Properly sizing the PV array to the battery AND consumption is critical. One method I think is essential for nearly all PV systems is use a generator and charger, appropriately sized to the battery. By bulk charging early in the AM you can reduce the finish time to perhaps 5-6 hours of constant voltage charging, something easily done with a PV system. Ron, you might be able to recover some capacity in this bank if the sulfate has not formed hard crystals by now. You can try a very long charge time, up to 24 hours, at high voltage, about 31 volts. You will need a larger generator. Monitor the temperature and reduce current if they get up to 125F internal. In our shop I have recovered sulfated batteries with high voltage charging, as much as 3Vpc (do not do this with any loads connected), at reduced current, about C*.05, and a 1 kHz pulser that I built. My findings over the years is the current will begin to rise very slowly, peak then drop if the recovery is working. Sorry that this post is getting so long. There's just so much. OK, one last thing. In my last post to you I recommended a battery AH monitor. This problem could have been detected and perhaps prevented if they had one. Larry Crutcher Starlight Solar Power Systems On Oct 21, 2011, at 11:39 AM, Ron Young wrote: > Hi Maverick & everyone, > > I visited the site a couple of days ago and load tested the batteries, > checked individual voltages in the string of six Surrette 4KS25's (4.3v > each), checked all connections etc. The client told me that when they would > go to bed battery volts read 25.7. Through the night this would seem to stay > steady. About 4 a.m. as far as they could tell the voltage would drop to > about 24.5. This happened without a load present and with no charging present > (calm, no wind, no sun). They would start up the generator for five minutes > in the morning and see the voltage come up to just above 29v then turn off > the generator (a small Honda 2500) and the voltage would settly at 25.7 and > remain there most of the day even when using their light loads, some lights, > phone system, laptop and the Sunfrost. > > When checking the batteries I noticed they needed watering and mentioned this > to the customer. The electrolyte was just over the plastic screen above the > plates by about 1/4 inch. He said he had just watered them and always kept > them filled. I replied that they were low and when he looked he said no, > that's where I keep them! When I checked the specific gravity reading it was > very rich reading around 1.275 - 1.280. I topped them up properly to about > 1/4 inch below the bottom of the cell channel which took about 5 cups per > each battery. Of course as soon as I topped them up the s.g. dropped to > around 1.175. I told him to put the generator on for about ten hours and call > me in the morning. > > I heard from the client today and in the last two days the voltage has only > dropped from 25.6 to 25.4 overnight and s.g. reading is at 1.260 or better so > problem seems to be solved. > > So it appears the battery was under watered for several years. I'm still not > sure why this would result in a sudden voltage drop, especially in the middle > of the night with no loads present and no charging. Any additional thoughts > appreciated. > > Best Regards, > Ron Young > earthRight Products - Solareagle.com > >
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