Thanks Ed for unraveling some of the misunderstanding of solar cells and batteries. It is pure folly to think that you can apply faulty math to highly optimistic and sometimes politicized ratings while totally ignoring system efficiencies and come to any meaningful conclusion. The standard way of rating audio power was to measure the RMS power at say 3% distortion. When marketing became all about watts the advertising folks took over and the manufacturers started using peak to peak watts and other wind at your back measuring systems. The same is true of solar cells where rebates, based upon power rating and other government intervention, have made a joke out of rating systems. Solar rating problems begin with the sun. It's always moving and therefore the angle of incidence is always changing. This immensely effects efficiency of lattice structures. The sun and Ohmic losses cause heating which further reduces efficiency. The result of all loses is a small fraction of rated values. Let me illustrate with some real test data. For a 12V/5W rated (approximately 200 square inches) panel with the American manufacturer's recommended controller I was able to charge a gel cell at just over 400ma (13.5v @ 402ma = 5.427W). That's slightly over the manufacture's rating. To obtain these values an ammeter was used to adjust the tilt and pan of the array for maximum. Current was continuously monitored and the time measured until the value halved (201ma). The controller maintained the 13.5v charging voltage so the power was halved as well. Sounds great but here is the rub. The time to half power averaged 15 minutes and then only between 10am and 2pm. If the array was laid flat on the ground and not oriented toward the sun I averaged less than 10 watt hours per day in June in Southern California high desert. That wouldn't run one K3/10 very long.
73, Fred, AE6QL -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Edward R. Cole Sent: Sunday, July 01, 2012 12:36 AM To: [email protected] Subject: Re: [Elecraft] Field Day experience with K3 Dave, ab9ca/4 wrote: This isn't right. The capacity of the battery in watts is 110x12 or about 1320WHr. If the load is 122w/hr the battery should be totally exhausted after about 11 hours. ---------- yep, I confused AH with WH, so that does not explain the problems the original writer had. Another way to analyze would be using just the current load: 2x17a = 34a x 30% = 10.2amp Then 110 A-H/10.2 A = 10.8 hours (theoretically) But battery discharge curves display the actual battery operating voltage one can expect to see. so one would probably reach an unworkable voltage sooner than 10.8 hours. The only good analysis would be if you have the discharge curve for the battery. These usually assume a <10% load (e.g. < 11 A). Using the discharge curve you can predict battery life under load. I used a 30% duty cycle which provides the time weighting factor (integration factor). You can argue that in FD operations the duty cycle could be something other than 30%. I assumed that one does a lot more calling in FD than normal assumed Tx/Rx ratios that commercial radio industry uses (10%). In my professional observations of radio using backup battery power the useful life never approaches the theoretical expected battery life. Most of the time the system becomes unusable in about half the time expected from a battery bank. This is on a properly floated battery bank with periodic equalization (hams normally do not do this). Theoreticals are only a good starting point as they assume fully charged new batteries (rarely is either true). The 120AH solar charging system "theoretically" provides 10A at 12V which would lead one to believe it would supply the operating load of two radios. I rarely have seen solar panels output their full rated power. In the brightest sun it is normally running about 80% of label ratings. So there was something else occurring to cause the voltage to sag to 10v in the original posters operation. 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-? DUBUS Magazine USA Rep [email protected] "Kits made by KL7UW" http://www.kl7uw.com/kits.htm ====================================== ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
