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12-26-2019, 04:32 PM
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Join Date: Jun 2012
Location: Frederick, MD
Posts: 821
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Quote:
Originally Posted by rv6ejguy
I made up an array of seven 20 watt Halogen bulbs, which gave a measured current draw of around 11 amps which is close to what a 4 cylinder SDS EFI system draws with one pump running, the ECU, coils and injectors at 2500 rpm.
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When using light bulbs as a load, the load is essentially a pure resistive load. Applying Ohms law, the load will drop linearly as voltage drops. An 11 A load at 14 V will be 9.4A at 12V, and 7.85 A at 10V. There is a variable as the bulb filament resistance changes with temperature, but that is minor in this example. On the other hand, most modern micro based circuits, have a high inductive and capacitive factor, and tend to draw the same power (Watts) as voltage changes, meaning Amps actually increases as voltage drops. Therefore I believe that your duration tests are yielding erroneous results, in the bad direction.
__________________
Dan Morris
Frederick, MD
PA28-140
Hph 304CZ
RV6 built and sold
N199EC RV6A flying
Learn the facts. "Democracy dies in darkness"
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12-26-2019, 08:58 PM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,147
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Quote:
Originally Posted by RVDan
When using light bulbs as a load, the load is essentially a pure resistive load. Applying Ohms law, the load will drop linearly as voltage drops. An 11 A load at 14 V will be 9.4A at 12V, and 7.85 A at 10V. There is a variable as the bulb filament resistance changes with temperature, but that is minor in this example. On the other hand, most modern micro based circuits, have a high inductive and capacitive factor, and tend to draw the same power (Watts) as voltage changes, meaning Amps actually increases as voltage drops. Therefore I believe that your duration tests are yielding erroneous results, in the bad direction.
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Valid point. You would need something like a constant-current driver on an LED array to accurately portray the usage.
__________________
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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12-27-2019, 03:34 AM
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Join Date: Oct 2019
Location: SoCal
Posts: 318
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If you want a more constant Watts load, use a DC to AC inverter and put the light bulbs on the AC side.
As the DC voltage drops, the inverter will draw more amps to compensate for the voltage drop.
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Ed
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12-27-2019, 04:26 AM
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Join Date: Nov 2006
Location: Savannah, GA
Posts: 1,029
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Seems like one would want to size a LiFePo battery not by a single lead-acid-equivalent AH rating, but by using data from the manufacturer (or your own testing) of voltage vs. time based on discharge rate. I know EarthX provides this for their batteries, though I have not conducted my own tests to verify that performance.
I think Syvolo?s post might explain the results?for equivalent cranking performance (very high current, very short duration), the lead-acid battery will have more usable capacity at low (backup use level) discharge rates. For equivalent backup performance, the LiFePo will have more cranking performance. And I think this is because he lithium batteries have less of an efficiency penalty at high discharge rates.
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RV-7ER - finishing kit and systems installation
There are two kinds of fool in the world. The first says "this is old, and therefore good"; the second says "this is new, and therefore better".
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12-27-2019, 04:46 AM
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Join Date: Oct 2017
Location: bellingham, wa
Posts: 202
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Shorai's explanation was a bit wordy, but they did say they used 6 Ah cells in a battery they rate at 18 Ah. For their purposes of a start battery, that has validity. Had it not been for Ross's capacity test, I would have never read the fine print.
EarthX rates their capacity at a 1C discharge rate. So their 12 Ah rated battery is rated for 12 Ah capacity, based on a discharge rate of 12 amps. Their 12 Ah battery is about twice as heavy as Shorai's 6 Ah "18 AH" battery.
That adds up for me.
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12-27-2019, 06:04 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by RVDan
When using light bulbs as a load, the load is essentially a pure resistive load. Applying Ohms law, the load will drop linearly as voltage drops. An 11 A load at 14 V will be 9.4A at 12V, and 7.85 A at 10V. There is a variable as the bulb filament resistance changes with temperature, but that is minor in this example. On the other hand, most modern micro based circuits, have a high inductive and capacitive factor, and tend to draw the same power (Watts) as voltage changes, meaning Amps actually increases as voltage drops. Therefore I believe that your duration tests are yielding erroneous results, in the bad direction.
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Valid point so you can do your own test on whatever gear you will be running on your backup battery- lighting, glass panel, coils and injectors, fuel pumps etc.
I just opened the door on the matter and have learned some additional information from the other posters here. This is an important topic for many people.
Bottom line, I learned that my total advertised AH capacity with 2 Shorai "18AH" batteries is closer to 12 AH in reality. I have maybe 40 minutes of flight time to zero if I lose the alternator. I'm glad I know that now as before I thought I'd safely have double that time. At the time I bought the Shorais, I found it hard to believe these small 2.2 pound batteries could do the same job as the 12 pound AGMs they replaced. For starting the engine, they actually do better, for extended running at moderate current draws, they clearly won't.
On batteries with a BMS, you won't get the full AH rating either as they will shut off before you can tap the full cell capacity- so the lesson is to do your own tests.
Last edited by rv6ejguy : 12-27-2019 at 07:57 AM.
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12-27-2019, 08:19 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
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Quote:
Originally Posted by rv6ejguy
SNIP...
Bottom line, I learned that my total advertised AH capacity with 2 Shorai "18AH" batteries is closer to 12 AH in reality. I have maybe 40 minutes of flight time to zero if I lose the alternator. ...SNIP
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I caution against the myopic approach that the only failure to mitigate is loss of the alternator. While this is perhaps the most likely causality, it does not result in the most severe outcome. Many RVs I?ve seen have multiple single point failure risks (beyond the alternator) that will leave them with a dark panel.
If flying IFR I recommend a careful analysis of your power distribution and avionics, then test your analysis to see if it achieves your design requirements. For example run all the loads you will have for IFR flight in the hangar and see how long you can go. Now add a loss of a master relay, avionics master, common buss bar, etc. and see what you have left. For me my most limiting single causality leaves me with just one of two batteries that will yield 70+ minutes of fully supported IFR flight. The simplistic loss of the alternator yields 3+ hours of IFR flight.
My point - what batteries you use is important, how you build and test your system is more important.
Carl
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12-27-2019, 08:39 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by Carl Froehlich
I caution against the myopic approach that the only failure to mitigate is loss of the alternator. While this is perhaps the most likely causality, it does not result in the most severe outcome. Many RVs I?ve seen have multiple single point failure risks (beyond the alternator) that will leave them with a dark panel.
If flying IFR I recommend a careful analysis of your power distribution and avionics, then test your analysis to see if it achieves your design requirements. For example run all the loads you will have for IFR flight in the hangar and see how long you can go. Now add a loss of a master relay, avionics master, common buss bar, etc. and see what you have left. For me my most limiting single causality leaves me with just one of two batteries that will yield 70+ minutes of fully supported IFR flight. The simplistic loss of the alternator yields 3+ hours of IFR flight.
My point - what batteries you use is important, how you build and test your system is more important.
Carl
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Both components are important but this thread just deals with with the battery discussion. For clarity, let's keep it confined to that topic please.
There have been plenty of other threads already on electrical system design/ layout and evaluation.
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12-27-2019, 08:43 AM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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Oh, yeah....
"...There have been plenty of other threads already on electrical system design/ layout and evaluation..."
That's a fact...
__________________
Bob
Aerospace Engineer '88
RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
EFII System 32 - Done
297 HP Barrett Hung
ShowPlanes Cowl with Skybolts Fitted - Beautiful
Wiring...
Dues+ Paid 2019,...Thanks DR+
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12-27-2019, 10:13 AM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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I wonder if the fact the testing procedure being flawed is relevant or not.
As long as both batteries were tested with the same setup, they were both subject to the same testing error.
If the test was repeated with a correct setup, would it make a difference in the performance delta between the two batteries???
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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