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#21
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At 17:18 05 May 2004, Brian Penfold wrote:
Brian - some great data in your posting. What about the scenario for us pilots who often cloud fly with 'power hungry' artificial horizons? They require considerable 'umpf' to get going and maintain their equilibrium. I changed to 14V on ONE of my two cells (trying to match capacity as you suggest) and have found it to work marvellously, particularly with erecting the AH in cold weather. Any comments for improvement here - would solar cells help? Cheers Pete Harvey I’ll pick up on a point that Derrick raised if I may. He is right but I’d just though I’d expand on the issue somewhat further, with an extract from an article I wrote recently. The ‘nominal’ terminal voltage of a 2 v cell is just that – a nominal voltage. It can actually range from about 1.8v when fully discharged, to 2.2v when fully charged, and can be as high as 2.45 – 2.5v (14.7-15v for 12v battery) when the cell is actually on charge. Add 6 together to get a ‘nominal’ 12v add seven together to get a ‘nominal’ 14v. As you don’t get something for nothing, you have to replace the charge from the battery when the battery discharged. In order to do this you have to apply a potential greater than that of the cell. In normal aviation and automotive applications, this charge is provided by an alternator or generator connected to the prime mover or engine. Because engine speed fluctuate, and because the output of a generator or alternator varies with input speeds, it is necessary to ‘cap’ or regulate the output voltage of the generator to prevent both over voltage conditions and to provide a voltage reference or standard, so that any electrical equipment connected into the aircraft or vehicle operates within the correct supply conditions. The, nominal’ 12v electrical system and it’s associated components and instruments consequently has to be capable of operating across a range of voltages, in particular the generator output voltages required to maintain the battery in a fully charged stated. The output voltages of generators normally is 13.5v ‘nominal’ but can be as high as 14.7-15v under very low load conditions. With me so far? Gliders do not have the luxury of having a prime mover capable of turning a generator and thereby maintaining the battery in a fully charged condition. Most avionic components and instruments are however designed for General Aviation use (powered), and their use in sailplanes and gliders is just seen a different branch of aviation. There is little point in developing a new technical standard specifically for gliders, when the current systems works fine. Most of the instruments and components in general aviation use (including gliding) are designed to meet the normal voltage range input requirements of general aviation. That is to say a ‘nominal’ 13.5v but are generally tolerant to 14.5-15v at the upper end, and may still continue to work at 8v at the lower end. You will find many instruments and components rated at 12v, 14v, 8-16v – but they are generally designed to work on a nominal 12v system. What has this got to do with batteries and the 12v – 14v issue I hear you say….. Well let me continue! The lack of ability for sailplanes and gliders to recharge batteries in flight means that with a load being drawn from the batteries, that eventually they will become discharged and at the extreme your glider electrical system will stop working! The need to minimise the weight of gliders means that practically you need to balance the weight of the battery with the ability (capacity) of the battery to supply all your electrical loads for about a days flying. No point in having a heavy car battery in the fuselage – no point in your battery going flat after 2 hours! A good balance between the two is a battery that will just about last a day –recharge overnight and then provide enough power for the next day…etc. As batteries get older they loose their ability to maintain their charge fully and many owners find after a period of time that the battery doesn’t quite last a day. One fairly typical approach to overcome this has been the somewhat misunderstood practice of adding another (2v) cell in series with the 12v battery to try to give it a bit more power – hence the variety of 12v/14v systems we see around. Does it work?…….read on. Battery capacity is measured by a battery’s ability to provide a relatively constant current over a fixed period of time at a ‘reasonably’ linear rate of decay. Capacity is measured in Amperes per Hour (AH) and will normally be quoted at either a 10 or 20 hour rate. What this means is that battery rated at 7AH (20 hours rate) is capable of providing 7A for one hour, or 350mA for 20 hours (7/20) before becoming fully discharged (and all calculations in between!). Still with me? Adding batteries in parallel general increases overall capacity; adding batteries in series does not increase capacity (and can actually reduce capacity in some instances). So what has this to do with the 12v/14v issue…. Take the following example: A glider has a 7 AH 12v battery fitted and the total load impedance is 400ohms. When a fully charged battery is fitted (assuming a nominal 12v) the battery should be capable of providing a current of about 30mA for 23.3 hours. (V=IxR). If a 2v cell is added in series to the 12v battery to give a nominal 14v, then the total current drawn will be 35mA. The load (impedance) does not increase but the current drawn is greater, nevertheless the capacity of the 12v battery (7AH) is such that it is capable of supplying 35mA for 20 hours, however…and here’s the bit most people don’t understand….if the capacity of the additional 2v cell is not at least 7AH (and very few are – most are around 350mA/H)), the 2v cell will discharge at a far faster rate than the 12v battery. Once it is fully discharged the system not only operates as a nominal 12v system anyway, but the discharged cell acts as an in line resistor to the supply current from the battery. This causes a voltage drop across it reducing the overall system voltage in some extremes to less than 12v. So depending if the right capacity 2v cell is not used , then you can actually degrade your electrical system performance. If you use common capacity cells to create 14v – ie 7 identical cells then unless you have equipment that is extremely sensitive to supply voltage, you may see a very tiny increase in the length of time you can run your sailplane electric’s before the system goes flat. In practice however, the mismatch of 2v/12v cell capacities and the age of your battery will have far more influence on the ability of your battery to maintain your instruments longer. Me, I’ve been an electrical, avionic and systems engineer for the past 25 years and I use 2 x 12v 9AH batteries (switchable between main and standby) QED. |
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I sell the R&R 14 Volt chargers specifically for these battery set-ups....
I have a large charger for batteries over 10aH and a slightly smaller charger for batteries under 10ah please see http://wingsandwheels.com/page25.htm tim www.wingsandwheels.com "F.L. Whiteley" wrote in message ... "Joe Allbritten" wrote in message om... Derrick Steed wrote in message ... Sam Fly wrote Joe, Why do you want 14 volts...Most radios have a 12 volt regulator that drops 14 volts to 12 volts...I for years used 14 volts only to find out is was not required. Have used 12 volts with no problems since... Sam Fly Joe Allbritten wrote: Does anyone know where I can purchase a 14 Volt gel cell battery. I know I can wire 2 or more in series to get 14Volt,(12 + 2 or 6 + 8 etc..) but I thought someone might know where you can purchase a single 14V battery. Thanks in advance. Er,... the terminal PD of a fully charged lead acid cell is 2.2V, so 6 of them in series results in a battery PD of 13.2V. As the cell discharges the PD will gradually fall to some value under 2V (how fast and how long this process takes depends on the rate of discharge and the relative "health" of the cell. So you may end up with you battery actually providing less than 12V for a good part of the time it is in use. I suspect that manufacturers put 14V on their kit to get across the point that is well able to withstand the PD of a fully charge gell cell battery. Rgds, Derrick. Derrick, Sam, Tim, Others Thanks for the reply. The reasons I think I need 14VDC a 1.) That is what I received with the glider when I purchased it 2 years ago. ( 12 +2) 2.) The radio spec sheet calls for 14 VDC. It even states if you put this in a 28VDC aircraft, purchase the 28 to 14 VDC converter. The radio is a IImorrow Apollo 704. I am afraid to try 12VDC without consulting directly with the manufacturer. Reason being that if the voltage is below what it is designed for, some electrical/electronic parts will increase the current draw (amps). Excessive current loads can very damaging to electrical/electronic devices. I know it will burn up motors etc.. but not completely sure about electronics. (I am certianly not an electronic Engineer) Joe Let us know where you find a 14v charger. TIA, Frank Whiteley |
#23
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Frank,
I recieved one with the glider. It doesn't state the Mfg. but it does have a Part number, RR14650A. It is a smart charger just for 14 V gell cell packs. I have seen it at one of the websites that sales soaring supplies. If I figure out which one, I'll email you. mine of course ) http://wingsandwheels.com/page25.htm tim |
#24
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"Tim Mara" wrote in message ... I sell the R&R 14 Volt chargers specifically for these battery set-ups.... I have a large charger for batteries over 10aH and a slightly smaller charger for batteries under 10ah please see http://wingsandwheels.com/page25.htm tim www.wingsandwheels.com Thanks Tim, and those others that provided similar comments. Frank |
#25
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"F.L. Whiteley" wrote in message ... "Tim Mara" wrote in message ... I sell the R&R 14 Volt chargers specifically for these battery set-ups.... I have a large charger for batteries over 10aH and a slightly smaller charger for batteries under 10ah please see http://wingsandwheels.com/page25.htm tim www.wingsandwheels.com Thanks Tim, and those others that provided similar comments. Frank I might add the reason for my request was that we are in the process of realigining part of our club fleet. We use built up 14V, fused battery packs that are interchangeable with all of our club gliders. We currently have three 14V chargers, but have been avoiding including these battery packs with our gliders as we'd also have to part with a charger (believed to be difficult to replace). We also feel most buyers will likely end up replacing our battery packs with 12V batteries and chargers at some point, rather than maybe replacing bad 2V cells or even the fuse. So we'd be unnecessarily giving up a maybe hard to get item in the 14v charger. Appears not to be the case, although our chargers are a bit simpler appearing than what's on offer. Frank |
#26
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Rory O'Conor wrote:
.. Everything works fine with the voltage reading above 12v. The LX5000 starts to run into problems if the voltage indicates 11.8v, and dies if the voltage falls below this. According to Filser the LX5000 works down to 8V where the vario quits. The computer stops working when voltage drops to 4.5V . I optimistically assume that the LX20, Radio and T&S might work down to about 10v, but I would like my LX5000 to keep functioning. We've had the problem of a LX5000 doing lenghty resets after using the starter motor in a DG500M. We've added a 9,6V 300 mAh nicad with diodes that charge it from the 12V battery and power the Filser during brownouts. The LX draws ca. 300 mA with speaker off. We've had the LX running off the Nicad for a while with no problems except a low battery warning. George |
#27
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"Eric Greenwell" wrote in message ... Rory O'Conor wrote: The LX5000 starts to run into problems if the voltage indicates 11.8v, and dies if the voltage falls below this. I optimistically assume that the LX20, Radio and T&S might work down to about 10v, but I would like my LX5000 to keep functioning. I suggest you contact your dealer or the company. According to Filser's LX5000 manual, the power required is 8-16 volts, so your unit needs repairs. Once it is repaired, you might consider running it and all equipment except the horizon directly from the battery. This should make the battery last longer, as you will avoid the loss from the converter. Or he could use a 14 volt battery, which will keep adequate voltage on all the devices far longer than a 12 volt battery will, and still avoid losses (and RFI) from the converter. That way, he wouldn't have to pretend that his radio is still working good at 10 volts. 12 to 14 volt converters are very hard to justify in this application. Speaking of low voltage operation, I once left the master on accidentally. As the battery pack discharged, my Cambridge flight recorder started giving pressure altitude indications that were way too high. It smoothly rose to 5K feet AGL. Since it was in the trailer, which was firmly chained down at the time, I'm reasonably certain this didn't happen. Maybe it's time to start going after some altitude records. Given the propensity for many people to power their panels with undervoltage supplies, perhaps the FAI/IGC ought to evaluate low-voltage performance of the recorders. -Dave |
#28
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David Kinsell wrote:
"Eric Greenwell" wrote in message ... Rory O'Conor wrote: The LX5000 starts to run into problems if the voltage indicates 11.8v, and dies if the voltage falls below this. I optimistically assume that the LX20, Radio and T&S might work down to about 10v, but I would like my LX5000 to keep functioning. I suggest you contact your dealer or the company. According to Filser's LX5000 manual, the power required is 8-16 volts, so your unit needs repairs. Once it is repaired, you might consider running it and all equipment except the horizon directly from the battery. This should make the battery last longer, as you will avoid the loss from the converter. Or he could use a 14 volt battery, which will keep adequate voltage on all the devices far longer than a 12 volt battery will, and still avoid losses (and RFI) from the converter. That way, he wouldn't have to pretend that his radio is still working good at 10 volts. 12 to 14 volt converters are very hard to justify in this application. The LX5000 is a vario system. It should be repaired before it stops working entirely. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#29
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X-no-archive: yes
In article 9p4mc.37093$I%1.2323748@attbi_s51, David Kinsell writes "Tim Newport-Peace" ] wrote in message news:ySGxnTAKeKmAFwtn@sps ys.demon.co.uk... X-no-archive: yes In article , Joe Allbritten writes Derrick, Sam, Tim, Others Thanks for the reply. The reasons I think I need 14VDC a 1.) That is what I received with the glider when I purchased it 2 years ago. ( 12 +2) 2.) The radio spec sheet calls for 14 VDC. It even states if you put this in a 28VDC aircraft, purchase the 28 to 14 VDC converter. The radio is a IImorrow Apollo 704. I am afraid to try 12VDC without consulting directly with the manufacturer. Reason being that if the voltage is below what it is designed for, some electrical/electronic parts will increase the current draw (amps). Excessive current loads can very damaging to electrical/electronic devices. I know it will burn up motors etc.. but not completely sure about electronics. (I am certianly not an electronic Engineer) Joe Not sure about FAA but as I recall, CAA require a radio to operate down to at least 10.5 on 121.500MHz before approval will be granted. A modern radio will do this on all frequencies. So how do you define "operate"? Do you believe it will transmit at that voltage with the same output power and same distortion levels as if it had nominal voltage? Most radios will indeed transmit, but sound absolutely horrible. You've probably heard some of these on the air. -Dave I think you will find that they will work at the lower voltage IF it is maintained, but when a battery is discharged to 10.5 the voltage will fluctuate. This could be what causes the distortion you refer to. However, a battery supplying 11.5 volts to the radio (which is not the same as 11.5v at the battery) should be OK. Tim Newport-Peace "Indecision is the Key to Flexibility." |
#30
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"Eric Greenwell" wrote in message ... David Kinsell wrote: "Eric Greenwell" wrote in message ... Rory O'Conor wrote: The LX5000 starts to run into problems if the voltage indicates 11.8v, and dies if the voltage falls below this. I optimistically assume that the LX20, Radio and T&S might work down to about 10v, but I would like my LX5000 to keep functioning. I suggest you contact your dealer or the company. According to Filser's LX5000 manual, the power required is 8-16 volts, so your unit needs repairs. Once it is repaired, you might consider running it and all equipment except the horizon directly from the battery. This should make the battery last longer, as you will avoid the loss from the converter. Or he could use a 14 volt battery, which will keep adequate voltage on all the devices far longer than a 12 volt battery will, and still avoid losses (and RFI) from the converter. That way, he wouldn't have to pretend that his radio is still working good at 10 volts. 12 to 14 volt converters are very hard to justify in this application. The LX5000 is a vario system. It should be repaired before it stops working entirely. Yes, I'm quite aware the LX5000 is a vario. When I used the term "radio", I was talking about his actual radio, which he earlier said that he optimistically assumed might work at 10 volts. I thought that should be obvious. Dave |
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