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#1
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2-Batteries
By all means a second battery should be installed in our electrically
driven modern sailplanes. After many years of quickly flipping my 3-position battery switch, and trying not to have my logger to momentarily dropout, I have concluded that is best to use 2 single-pole battery switches. That way one can have either or both batteries connected at the same time. I saw the light when Jim Hendrix brought his sailplane to Caddo Mills for Wing Deturbulator flight testing, and it was wired like that. You will hear much more about that amazing new invention at the coming SSA Convention. Thermally, Dick Johnson |
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2-Batteries
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#3
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2-Batteries
Better yet is to use diodes so that both batteries will always be "on" in parallel and you're always pulling from the best battery with no fiddling required from the pilot. Relatively low voltage drop diodes are available with 18 Amp forward capacity. For redundancy, I used two in parallel on each battery. If it is always drawing from the best battery, what is the time interval between switching. What controls the switching. Please advise. Udo |
#4
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2-Batteries
I'd like to see a full write-up with diagrams, photos,
and names/addresses of places to buy parts. This would make a good article for Soaring. At 19:48 25 December 2006, Udo wrote: Better yet is to use diodes so that both batteries will always be 'on' in parallel and you're always pulling from the best battery with no fiddling required from the pilot. Relatively low voltage drop diodes are available with 18 Amp forward capacity. For redundancy, I used two in parallel on each battery. If it is always drawing from the best battery, what is the time interval between switching. What controls the switching. Please advise. Udo |
#5
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2-Batteries
Better yet is to use diodes so that both batteries will always be "on"
in parallel and you're always pulling from the best battery with no fiddling required from the pilot. Relatively low voltage drop diodes are available with 18 Amp forward capacity. For redundancy, I used two in parallel on each battery. If it is always drawing from the best battery, what is the time interval between switching. What controls the switching. Please advise. Udo There is no switching. As the higher voltage battery drops in value, gradually a greater % will start drawing from the other. From that point, they will discharge equally. Assuming there is slightly different capacity, current will gradually increase from the battery with the greater capacity. Most diodes have about a .6 volt drop, which may be significant. Therefore, you will need diodes with a low threshold. Schottky diodes should be about ..3 volts drop. You can measure the actual drop using a digital volt-ohmmeter. There is a diode test range and it will show the voltage drop. Colin Lamb |
#7
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2-Batteries
"If both batteries are on-line all the time, how do you know when one is
getting weak and needs to be replaced? Or do you replace both batteries when voltage is marginal at the end of a flight?" Well, that is a problem. You do not know. Everything is automatic. If you have one battery that has lost most of it's capacity, the good battery will do all the work. The only way you will know is when the total capacity of both batteries has been reduced. And, then, you will not know whether one battery has done 50% of the work and they are both down in storage ability, or one battery is still 90% and the other one is 10%. So, you will have to determine the capacity of each battery separately. You could test the battery capacity by switching either off during discharge, or even by using 2 ammeters - but it is becoming more complex once we try to extract more information. You can also determine the capacity during charge. Colin |
#8
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2-Batteries
Nyal Williams wrote:
I'd like to see a full write-up with diagrams, photos, and names/addresses of places to buy parts. This would make a good article for Soaring. Here is a sketch http://www.mydatabus.com/public/emer...y/e/sketch.JPG and a pic of my install... http://www.mydatabus.com/public/emer...y/e/diodes.jpg There is a aluminum bracket which is lightly mounted to the bulkhead. The PC board just provides insulation and some support for the wires. Each diode has a heat sink, but I don't really think it was necessary. I don't have the part numbers handy, but I'll see if I can't find something. With 18Amps of forward current capability (each), there isn't really any "need" for the two diodes in parallel, but for an extra $5 and a few extra minutes of wiring, if any one diode ever did fail "open" the other would still provide power. Most likely overkill. |
#9
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2-Batteries
COLIN LAMB wrote:
"If both batteries are on-line all the time, how do you know when one is getting weak and needs to be replaced? Or do you replace both batteries when voltage is marginal at the end of a flight?" Well, that is a problem. You do not know. Everything is automatic. If you have one battery that has lost most of it's capacity, the good battery will do all the work. The only way you will know is when the total capacity of both batteries has been reduced. And, then, you will not know whether one battery has done 50% of the work and they are both down in storage ability, or one battery is still 90% and the other one is 10%. So, you will have to determine the capacity of each battery separately. You could test the battery capacity by switching either off during discharge, or even by using 2 ammeters - but it is becoming more complex once we try to extract more information. You can also determine the capacity during charge. Colin During any one flight, you don't need to know. What is a good idea is to test the batteries if you think you are losing capacity. During the week charge them up Monday and Tuesday, and Wednesday put a load on each one and see how long it lasts. Ideally, you'd like the load to test the batteries for as long as you'd normally fly. Say 4-6 hours unless you are typically flying for longer. If either one can't go the 4-6 hour distance and mainain a suitable voltage, then it's due for replacement. After the Wednesday test, charge them back up so you're ready for the weekend assuming they pass. I'd certainly do this in the spring and before any big contest during the year. 7Ah batteries are cheap in the grand scheme of things. |
#10
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2-Batteries
wrote:
By all means a second battery should be installed in our electrically driven modern sailplanes. After many years of quickly flipping my 3-position battery switch, and trying not to have my logger to momentarily dropout, I thought all loggers just started again after a momentary power loss, and continued the same flight file. It sounds like yours isn't behaving properly, or are some brands a bit touchy? -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * "Transponders in Sailplanes" http://tinyurl.com/y739x4 * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org |
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