From owner-qrp-l@Lehigh.EDU Wed Jun 3 11:06:00 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id LAA11075 for ; Wed, 3 Jun 1998 11:05:59 -0400 (EDT) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12600-40490>; Wed, 3 Jun 1998 11:05:47 -0400 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12308-60710>; Wed, 3 Jun 1998 11:04:34 -0400 Received: from pantano.theriver.com (pantano.theriver.com [205.216.137.2]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id LAA36524 for ; Wed, 3 Jun 1998 11:04:27 -0400 Received: from theriver.com (a19.pm3-14.theriver.com [206.30.150.84]) by pantano.theriver.com (8.8.5/8.7.3) with ESMTP id IAA07754; Wed, 3 Jun 1998 08:04:18 -0700 (MST) Message-Id: <357565ED.4771EC0C@theriver.com> Date: Wed, 03 Jun 1998 08:04:13 -0700 Reply-To: foxes@theriver.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: Andy Fox To: "Low Power Amateur Radio Discussion" Subject: Re: How big a battery?.... (part 2, long) References: <99a2ccf5.357419e0@aol.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-To: AA2QO@aol.com X-Cc: Low Power Amateur Radio Discussion X-Mailer: Mozilla 4.04 [en] (Win95; I) X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Hello again, In the last episode of the "How big a battery?" saga, we had determined that our virtual rig required 3.8 AH from the battery between charges. This, in turn, meant that the battery required 4.5 AH, and that 9 AH is a good size for the battery itself. Remember, the battery will last longest if it is cycled between 100% and 50% charge. We need a way to bring the battery from 50% to 100% charge, or 4.5 AH. There are many variables to play with here. I will use averages to simplify the explanation. Keep in mind that in the Northern Hemisphere, there are fewer daylight hours in the Winter, and more in the Summer. Here in Tucson, AZ we get about 6 "peak sun hours" per day. We calculated that our battery requires 4.5 AH between charges. Does this mean "every day" or "each week?" It's up to you. For now, let's assume that our virtual rig is going to be used each day for the amount of time specified in Part 1. If we divide the number of Ampere Hours by the number of hours, we get Amperes. So, take 4.5 AH / 6 H = 0.75A. We need 750 mA charging current from our Photovoltaic (PV) module. Here is where you have to be careful reading specs. Here is some data for Solarex PV modules. Their website is www.solarex.com for those interested in their many other products. MSX-10 Lite 0.58A 17.1V MSX-5 Lite 0.27A 16.8V SA-5 290 mA 17.5V SA-2/12 145 mA 17.5V SA-1 80 mA 17.5V Note that the product of the Voltage and Current is Power. This is where the name of the module is derived. The PV module will put out the rated current up to the rated voltage. After that, it falls off to zero. This "maximum power point" is at the knee of the curve (sorry, no graphics!). The rated voltage is several volts higher than the batter voltage. This is necessary, or the module can't charge the battery. You will get a (nearly) constant Current from the module, NOT constant Power. Take the MSX-5 Lite, for example. Near the end of your charging cycle, your battery voltage may be about 14 Volts. Multiply this by the 270 mA current to get 3.8 Watts, not 5 Watts. These specifications are given for 25 degrees C = 77 degrees F. The module efficiency decreases with higher temperature. Since the module is sitting in the sun, the module temperature is typically higher than the standard temperature. I'll skip temperature derating for simplicity. Stick with modules in the high 16/low 17 Volt area and you should be OK. Beware of "self regulating" modules with Voltage ratings around 15 V. They might work in the Arctic, but they are a no go in the Desert. We have determined that we need 750 mA current. None of the modules above put out that much. Let's use the MSX-10 spec. to see how many hours would be required to charge our battery. 4.5 AH / 0.58A = 7.8 Hours. It will take almost a day and a half to charge the battery. This is where economics come in. This is where you trade off price, size, weight for speed. Most people determine that they can wait longer to charge the battery. Maybe they don't _really_ operate 20 hours _every_ day. Play with the numbers to find out where you're comfortable. Oh, there are limits to the rate at which you should charge the battery. "Gee, why can't I just charge the battery at 9 Amperes for half an hour?" You'll blow it up, that's why. A "good" charge rate is C/20, which means the battery capacity divided by 20. It translates to 5%. The absolute max charge rate (for Lead-Acid batteries) is C/10, or 10%. Our 9 AH battery has a "good" charge rate of 0.45 Amperes and a "max" charge rate of 0.9 Amperes. Let's see, .58 / 9 = about 6.5%. Not too bad - a C/15.5 rate. You could use a smaller module with less output, but it would require more time to charge the battery. Again, economic trade off time! There is a limit on "how low you can go" in the other direction, too. For example, if you have an 1100 AH battery (yes, eleven hundred!), a 5W module would be a complete waste of time and money. I don't think that this should be an issue for QRP operating; I just wanted to point out that there is a limit there. Now we've come full circle. We started with a rig with some operating requirements. We picked a battery, based upon those requirements. Then we found a decent compromise for charging the battery with a PV module. I have left out charge controllers on purpose. This is another economic decision. If you are at the rig/battery, it is possible to pay close attention to the battery voltage. For unattended charging or forgetful operators, a charge controller is a MUST. Plus, this post is quite huge already! Watson, WB4EXW pointed out that there were a couple of articles on this topic in QST fairly recently, one of which covered system sizing quite well. This is absolutely correct. Those of you that keep back isssues for more than a couple of months might want to find those articles and post which issues they can be found in. Have fun, and feel free to post questions and corrections. -- 72/73 de Andy, KK7HV - QRP-L #1286 - Tucson, AZ