Hello Phil and Esteemed Wrenches, Sorry for the delay in my reply and thank you for pointing that out... My answer was incomplete as I did not address low charge currents. You won't believe how many times I have seen end users trying to charge batteries with a 10W PV panel!
You're right. The use of low charge currents can also lead to problems. In addition to longer charge times and possibly undercharging the battery, low charge currents often lead to stratification, and worse, they may fail to polarize the electrodes, which will lead to other problems as well. It is generally not recommend to use an initial charge current that is lower than the C/100 rate. As for the (higher) charge currents as a function of hour rates, using the C/10, C/15, and C/20 will yield the best cycle life. The demarcation point is usually the C/5 rate. I've seen/designed lots of systems that do go beyond the C/5 rate, but I usually recommend that they be professionally maintained to avoid the problems that are usually accompanied by higher currents. Because of the stochastic nature of the charge systems in RE applications, it is difficult if not impossible to have/maintain a constant current source, so the higher charge currents may or may not be a problem. The reality is that it is better to charge the batteries than to undercharge them even if the currents are not ideal. Temperature should also be taken into consideration, but we'll leave that topic aside for now (unless anyone is interested). As for the L16RE-2V battery, (as you correctly pointed out) it is essentially three paralleled 370Ah cells. We are currently making it a true 2V cell and should have it available very soon. The L16RE-2V was initially designed that way because we were getting lots of requests for a high capacity and light weight 2V battery in the L16 footprint or 903 BCI size and that was the quickest way to design, test, and validate it. Strings in parallel are an interesting topic that still needs further studies/tests. Most academic/industry papers place the limit at 6 to 9 strings. Other papers state that there is no theoretical limit as long as the proper instrumentation, monitoring equipment, and the proper maintenance practices are in place. Utility-level battery banks often exceed these limits and are very successful at it. There is a lot to be said about this topic (it is one of my favorites ones and I have many others), but we'll leave it aside for now (unless anyone is interested). As for the watering, there are a lot of excellent watering systems available today. We have one ourselves that we call the Hydrolink Watering System, and I love using it in the lab/field because it saves a lot of time. I do feel your pain with the specific gravity measurements as I've spent countless hours and have ruined countless pants doing it myself!! Here's a general formula that may help: OCV = SG + .845 I will go ahead and send you the graph on your personal account. If anyone else would like a copy of the graph please let me know. Thank you for your reply Phil. I know that you are all very busy, but is everyone aware that the RE Series batteries operate at lower specific gravities? If anyone would like the SG table please feel free to contact me. I'm here to help you. Best Regards, Ronald Paredes Technical Product Manager - Renewable Energy Trojan Battery Company 12380 Clark Street Santa Fe Springs, CA 90670 Tel: (562)236-3000 Ext. 3066 Fax: (562)236-3279 [email protected]<mailto:[email protected]> www.trojanbattery.com<http://www.trojanbattery.com/> Trojan Battery Company - Clean Energy for Life(tm) From: [email protected] [mailto:[email protected]] On Behalf Of Phil Undercuffler Sent: Thursday, July 15, 2010 10:15 PM To: RE-wrenches Subject: Re: [RE-wrenches] Trojan L16 2v vs 6v Wow, this has been a wonderful and fruitful thread. First, Ron let me thank you for some impressive responses, and great information. I have some nits, but those are small and inconsequential, so I'll leave them to later. However, I want to chime in a bit just to set a frame of reference, if everyone will be patient with me. I'm concerned that one person's interpretation of "grey" will be misconstrued and quoted as gospel "gray" from now until eternity. Specifically, Ron you answered Darryl that "Your recommendation to customers is 100% accurate... It is better to use low charge currents." I want to be clear, however, that your definition of "low" is extremely different that that of most solar designers. Darryl was suggesting "...never charge faster than C/10 and C/15 or C/20 is better" whereas you were saying "It is usually recommended to use a range of 10% to 13% of the battery's 20-hour rate." I know that might not sound like much of a difference, but let's take a quick survey -- who among you wrenches normally install greater than 6kW of PV per 1,000 AH of battery at 48 volts? Whoa, not a lot of hands..... Do the math, but that's roughly what it takes to obtain a C/10 charge rate with straight PV. In all my years of working the wholesale trade, the systems I've seen which could theoretically achieve a C/10 charge rate are statistically insignificant. Systems which could perhaps achieve C/20 (except for those pesky oft-ignored daytime loads)...lot more hands in the air now. Systems which which hope and pray to someday hit C/100 (if the owners go off on vacation)....Bing Bing Bing -- you win the prize! Climbing off my soapbox, my point is that Ron, your definition as a battery manufacturer of "low" should be used with a clear reference to what "high" would be. Just want to clarify that before we all start hearing "but Trojan says I should charge at low current levels, so my 500W PV array will be just fine for that 1,000 AH battery!!!" The one question I have about the new Trojan 2V L16s would be what, exactly, is the benefit they provide for the designer and installer? When I look at the design, it looks like it's just using an existing case with three 350AH cells connected in parallel (2V) inside, rather than the standard series connection (6V) or better yet using a single 1,100AH cell. If good design practice calls for no more than three strings (cells) in parallel, then good practice would call for no more than a single string of these specific batteries, since any more than that would mean 6, 9 or more cells in parallel. Count the caps, folks. How about those watering chores. Measuring specific gravities? Shoot me first, please! I think Blair was right about looking at true industrial cells. So on to the nit -- can you send me a copy of the Excel file of the IEC 61427 graph? Gmail doesn't show the Excel graph, and the .png file shows up as little more than a thumbnail -- I can grok the essential info from the other graphs and tables, but that 13 years worth of data just doesn't come through in a legible manner, least not to my aging eyes. This discussion is much appreciated Phil Undercuffler This e-mail message and any attachments that accompany it may contain information that is confidential, privileged, or protected from disclosure. It is intended solely for the use of the individual(s) to whom it was intended to be addressed. If you have received this e-mail by mistake, or you are not the intended recipient, any reading, disclosure, copying or other use of this communication is strictly prohibited. If you have received this communication in error, please immediately advise the sender at their phone number listed above, or by electronic mail, and also permanently delete the original and all copies of this e-mail and any attachments from all locations. Thank you.
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