Dan et al,
Thanks for all the input. The 1 mA per A-hr capacity (20 hr) for AGMS was very important to know. Our AGMs are 250 A-hr, so that equates to 0.25 A. This is exactly what we are getting right now in float mode. The problem that I was having arose from the Blue Sky 3024 DiL CC only staying in acceptance mode for 2 hours each day (this is the factory default). So it took quite a few days to really top off the battery bank. In order to program the scenario I want, I will have shell out a few hundred bucks for an "IPN-ProRemote" to access and adjust the acceptance mode parameters on the 3024. - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 _____ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Exeltech Sent: Monday, February 01, 2010 6:11 AM To: RE-wrenches Subject: Re: [RE-wrenches] Couple of Qustions about AGM batteries AGM batteries in good condition and fully charged will consume approximately 1 milliamp per rated amp-hour in "float" mode at 25C. Thus, your 250 amp-hour battery will require 250 milliamps at the manufacturer's rated float voltage (including temperature compensation). Discussion now turns to whether or not it's preferable to leave AGM batteries connected to a source of current in float mode. Continuous float current flow with the battery at 100% state of charge, and supplied over a long period of time (typically months to years), leads to a condition called "positive plate corrosion" (PPC). In this process, material in each positive plate is gradually diminished, being leached into the electrolyte and/or deposited on the negative plate. Once PPC has occurred, the lost material cannot be recovered and re-deposited back. Reduction of positive plate material leads to a decreased ampere-hour rating for the battery suffering from PPC, which then tends to aggravate the corrosion situation over time. The rate of PPC can in of itself be reduced by a *slight* reduction in float voltage at 100% SOC, and of a value to reduce the float current from 1 mA per rated battery A-H to 0.5 mA per A-H. The problem in doing so: most equipment on the market isn't designed to nor is capable of monitoring battery float voltage and current sufficiently accurately to be of benefit. You didn't say at what temperature the batteries are kept. Cold batteries have a self-discharge rate lower than when hot. AGM batteries lose less than 1% per month at 25C (77F) through self-discharge. Thus if your batteries are new/healthy, you'll be down less than 2% in six weeks if charged to 100%, then disconnected. If your batteries are consuming 2 amps at 26.4 Vdc and 21C, either they're not 100% fully charged .. or something else is amiss. Incidentally, battery "float" current increases with age from the 1 m-A per A-H stated above. That being the case, I'd agree with Warren. At 2 amps, either you have unknown load(s) consuming power .. or something else is wrong (could be as simple as the batteries not being at 100% SOC). My recommendation: First determine why the batteries are consuming 2 amps. Once you resolve that issue .. six weeks of current under proper float conditions (as defined by the manufacturer) won't significantly help or hurt the batteries. If the batteries are truly drawing 2 A at 26.4 V (presuming that's the proper temperature-compensated voltage for those batteries), it's a troubling sign - and I'd leave the batteries connected to your current source, as it indicates a high rate of self-discharge. If you're able to reduce the float current to the 250 mA range at the manufacturer recommended float voltage .. once you reach that current level, I'd disconnect them for the six week period. Finally .. I also agree with Warren. Sealed AGM are not intended in the sense of the word for "equalization". The manufacturers often call their equivalent to this a "freshening" charge. It's not as rigorous as a flooded-cell equalization due to the sealed nature of AGM batteries. A true "equalize" would likely over-gas the cells, potentially opening the safety vent on the batteries. When this happens, if it's a self-sealing vent, you'll have lost some of the available H2O. If it's not a self-sealing vent .. you'll have destroyed the battery. Batteries are a chemical and mechanical complexity that often defy easy answers, particularly in a forum such as this. Hope this helps. (By the way - I'm on the road, and if you post any reply/questions, I won't be able to answer for at least a week or more.) Dan --- On Mon, 2/1/10, Warren Lauzon <wind...@wind-sun.com> wrote: From: Warren Lauzon <wind...@wind-sun.com> Subject: Re: [RE-wrenches] Couple of Qustions about AGM batteries To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Date: Monday, February 1, 2010, 5:05 AM The answer to systems not being used is nearly always leave them on float. However, 2 amps seems a bit high for AGM batteries unless you have some kind of phantom load. Equalization for AGM's is different and for a different purpose than with flooded. The current should be quite a bit less than you would need for a flooded, since the purpose is not to stir the liquid up but simply to get all cells in the series up to the same state of charge. AGM's don't usually need much equalization. ............................................................................ ...................... Northern Arizona Wind & Sun - Electricity From The Sun Since 1979 Solar Discussion Forum: http://www.wind-sun.com/ForumVB/ ............................................................................ ...................... ----- Original Message ----- From: "Peter Parrish" <peter.parr...@calsolareng.com <about://mc/compose?to=peter.parr...@calsolareng.com> > To: <re-wrenc...@re-wrenches.org <about://mc/compose?to=re-wrenc...@re-wrenches.org> > Sent: Sunday, January 31, 2010 4:19 PM Subject: [RE-wrenches] Couple of Qustions about AGM batteries > We have put together a 850 Wdc portable stand-alone PV system. It uses > a Blue Sky 3024DiL charge controller, a pair of SunXtender 250 A-hr AGM > batteries (24 volt configuration), and a Xantrex Prosine 1000 inverter. > > We have been using this system for powering "events" and getting some > good marketing from the activities. Everything has worked out well, and > the system is not being used now and probably won't until March or April > sometime. > > We have let the system fully charge the batteries through bulk/absorption > and now float. The float conditions are 26.4 V and 2.0 A at a battery > temperature of 21 deg-C. > > My first question: is it alright to let the batteries float every day for > the next 6 to 8 weeks, or should I disconnect them from the rest of the > system until we are ready to use the system again? > > My second question: since the 3024iL can perform an equalization, > is there a safe equalization procedure for these AGM batteries? > > - Peter > > Peter T. Parrish, Ph.D., President > California Solar Engineering, Inc. > 820 Cynthia Ave., Los Angeles, CA 90065 > CA Lic. 854779, NABCEP Cert. 031806-26 > peter.parr...@calsolareng.com <about://mc/compose?to=peter.parr...@calsolareng.com> > Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
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