[RE-wrenches] intermittent battery problem; ...Battery Sulfation

Larry Crutcher, Starlight Solar Power Systems larry at starlightsolar.com
Sun Oct 23 09:19:22 PDT 2011


Thank you Darryl.
The important thing is internal temperature. Any current up to the manufacturers specification can be used but C/20 means I usually don't need to baby sit the battery temp during long term recovery attempts. Adjust accordingly.

One thing I didn't mention is that it's not harmful to deficit charge a battery for a few days or perhaps a couple weeks as long as the battery gets a 100% charge within that time frame. This may require more frequent equalization. 

Larry 



On Oct 23, 2011, at 6:22 AM, Darryl Thayer wrote:

> Thanks Larry, this is one of the best explanations I have seen.  I am saving it for my customers and students.  I would emphasize the C/20 (as you said) or a little less for perhaps C/30 for this long term equalize or desulfation. Too fast an equaliziation can loosen (blow the plate material off the plates) .  (I built a 60 hZ desulfator and have used it twice in 40 years, it seems to work) 
>  
> Thanks again great piece of writing and explanation, everyone should read
> Darryl
> From: "Larry Crutcher, Starlight Solar Power Systems" <larry at starlightsolar.com>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Sent: Saturday, October 22, 2011 2:06 PM
> Subject: Re: [RE-wrenches] intermittent battery problem; ...Battery Sulfation
> 
> 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
> 

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