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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
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 |
#5
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2-Batteries
COLIN LAMB wrote:
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. I'm flying with the diode setup. The best diodes I was able to find in the UK were Fairchild MBR1035 Schottky diodes. These handle 35 amps and come in TO-220 packages. Their spec quotes a 0.57 v drop, which seems about right: with 2 x GPS II+, EW-D logger, SDI C4 and B.40 varios all on the C4's internal voltmeter shows 11.6 v on two fully charged 7 AH cells. I need to fit a radio and am thinking of fitting a Filser ATR-500. Can anybody tell me if that will be OK on the end of the diodes or would I be better to discard the diodes and use one battery to drive the ATR-500 and the other to run the GPS, logger, and varios? Another reason I'm wondering about rewiring to separate the batteries is that if we get landed with transponders I assume I'd be better off using one for radio and transponder and reserving the other for the GPS/logger/vario setup. Comments? The other choice of radio would be a Microair 760. I assume that, as its rated for 12-14v it would be quite unhappy on the end of the diodes and just barely usable on a separate battery. I see that Maxim sell a range of solid state voltage boosters (there is a model that can output 14v at 2 amps). Has anybody tried using one of these to drive a Microair radio? If so, how well did it work? Colin Lamb -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#6
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2-Batteries
The Filsner is unlikely to have a problem through the diode setup you have now. (not that I like the diode setup but it should work - BTW you should be able to find lower voltage drop diodes). For the Microair I would not read too much into a nominal voltage spec. It is hard to tell wether the manufacturer is quoting a real absolute voltage range or the nominal voltage. On the other hand I'd not want to inflict anybody with a Microair radio (oops I can feel the flames already). They have a very bad reputation where I fly. The several I've seen installed have had problems, especially apparent heat related problems with the displays getting garbled and then finally the whole radio going out to lunch (and I'm not talking really hot days). Other brands seems to be much more reliable. Why not go with Becker? Personally I'd stay away from trying to get too fancy with regulators etc. I want the supply to be as dumb and simple as possible. Just batteries and circuit breaker or fuse and master switches. As few connectors or solder joints as possible. And I personally I like running one battery at a time - I want to see the health of each battery and know about how much capacity I have in both batteries and know I can switch in a reserve if I run a battery down -- which I may not notice until it is too late. Like many of us in the Western USA I fly over pretty desolate areas and I really want to know roughly what battery capacity in reserve. If you are goigg to install a transponder, have lots of avionics toys, expect long cold flights it may well be you need to look at moving up to larger capacity batteries or installing solar panels. Darryl Ramm Martin Gregorie wrote: COLIN LAMB wrote: 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. I'm flying with the diode setup. The best diodes I was able to find in the UK were Fairchild MBR1035 Schottky diodes. These handle 35 amps and come in TO-220 packages. Their spec quotes a 0.57 v drop, which seems about right: with 2 x GPS II+, EW-D logger, SDI C4 and B.40 varios all on the C4's internal voltmeter shows 11.6 v on two fully charged 7 AH cells. I need to fit a radio and am thinking of fitting a Filser ATR-500. Can anybody tell me if that will be OK on the end of the diodes or would I be better to discard the diodes and use one battery to drive the ATR-500 and the other to run the GPS, logger, and varios? Another reason I'm wondering about rewiring to separate the batteries is that if we get landed with transponders I assume I'd be better off using one for radio and transponder and reserving the other for the GPS/logger/vario setup. Comments? The other choice of radio would be a Microair 760. I assume that, as its rated for 12-14v it would be quite unhappy on the end of the diodes and just barely usable on a separate battery. I see that Maxim sell a range of solid state voltage boosters (there is a model that can output 14v at 2 amps). Has anybody tried using one of these to drive a Microair radio? If so, how well did it work? Colin Lamb -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#8
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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 |
#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
Testing AH of batteries
Battery capacity is measured using a defined minimum voltage. That voltage may or may not correspond to the minimum voltage your glider equipment will operate on. If you use diodes, that minimum voltage will move slightly. Capacity also depends upon a starting voltage - which is significantly higher than 12 volts. I often need to test batterys used in our search and rescue radios and have found that I can test battery capacity quickly without a full discharge. I charge the batteries, then let them sit for a day. Then, I put a load on them and simply watch the decay of voltage over time. In a very short time, you can make a graph that will indicate the trend of the battery and compare with a new battery. A battery with reduced capacity will drop voltage much more quickly. You can determine the capacity of the battery during charge, too. Capacity is the ability of the battery to resist change. That applies either way. it means the battery will drop in voltage or charge more slowly. So, you can simply time the charge of two batteries and learn the comparative capacity of each. Colin |
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