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  #31  
Old 12-27-2019, 10:41 AM
RVDan RVDan is offline
 
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Quote:
Originally Posted by Mike S View Post
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???
The discharge characteristics of the two different chemistries are drastically different. The testing errors will be different. For example, for lead acid chemistries the voltage decreases continuously throughout the discharge period. As the discharge proceeds voltage continuously drops until at about 20% state of charge (SOC) where voltage becomes to low to be usable. LiFePo chemistries do a very good job of holding voltage constant until about 15% SOC and then at about 10% SOC drop rather quickly to unusable voltages (see the chart in Post 1). Also lead acid technologies drop voltage based on the actual discharge current, whereas LiFePo tends to hold a more stable voltage. For LiFePo batteries, voltage under load is not a reliable indication of the SOC until the very end. For lead acid batteries, voltage under load can be used to determine the SOC with some limited degree of accuracy.
For battery ratings one has to pay attention to the discharge time used to develop the rating. In aviation we typically use a 1 hr rate (C1). A lot of commercial batteries use a C5 or even a C20 rating. The longer the discharge time the higher the AH ratings as the batteries do better with low discharge rates.
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  #32  
Old 12-27-2019, 10:45 AM
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skylor skylor is offline
 
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Default I=V/R, P=IV

Quote:
Originally Posted by Mike S View Post
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???
Except that since the lead acid batteries are lower voltage than the LiFePO batteries, they are providing a consistently lower current to a constant resistance test rig vs. their lithium counterparts. I think this accounts for some disparity between Ross’s test subjects, but not all.

One note of caution: Someone mentioned elsewhere in this thread that actual power loads would be constant power - i.e. increasing current with reduce voltage because of the DC/DC power supplies used in digital electronics. This ignores the fact that probably the largest power consumer of an EFI setup is actually the fuel pump which, if driven by a brushed DC motor, would actually result in lower current draw with lower voltage. HOWEVER (and this is really the caution part) since LiFePO batteries operate at higher voltage than Pb-acid batteries, LiFe backup batteries may actually need to have more capacity than their Pb counterparts to provide equivalent run-time of an EFI system.

Skylor
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  #33  
Old 12-27-2019, 11:00 AM
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rv6ejguy rv6ejguy is offline
 
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The Odyssey was my main battery for 6 years in my RV and was starting to show signs of slightly slower cranking on a cold engine. It already had hundreds of cycles on it. The Shorai was new back in the Spring with almost zero cycles and no deep cycles since is was for backup only.

Apples to apples with the same load, the old Odyssey lasted about twice as long. A fresh one would have gone even longer.

Folks can split hairs on the testing technique but clearly a true 6AH battery of any chemistry isn't going to last too long while 11ish amps is being drawn from it. Whether 10 or 12 amps is the average current draw during the discharge test isn't very germane here. The point is to understand some of the important info uncovered in this thread and be able to pick an appropriate backup battery and/or 2nd alternator for your mission. I'm not going to be pushing the time in flight, I'm going to get down on the ground in under 30 minutes and with the type flying I do and the locale, I'm ok with that. If I'm flying a long ways away from airports, I'd certainly install a larger backup battery.

I'd encourage folks to run their own tests with some other batteries and report their findings here for everyone in the community to benefit from.
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Last edited by rv6ejguy : 12-27-2019 at 11:05 AM.
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  #34  
Old 12-27-2019, 12:33 PM
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Mike S Mike S is offline
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Dan, Skylor---thanks for the responses.

If I am reading them correctly, each battery needs a different setup to measure its true potential.

But, I think the best way to find the best battery for a given specific purpose would be to make a test rig that closely duplicates that purpose, and see how each battery performs with that test setup.

If one battery performs best doing task "A", and the other battery performs best doing task "B", but they are actually being asked to perform task "C"---------then ????

Good discussion, gets the old brain cells working.
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  #35  
Old 12-28-2019, 06:12 AM
vic syracuse vic syracuse is offline
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I ran a test between the EarthX and the PC925 a few years ago and posted it here and in KP. The EarthX outperformed the PC925. It was not at a cold temp, but the power remaining was amazing after cranking for a start and then running the entire aircraft load for 45 mins and then cranking again.


Edit: post #69 on this thread. http://www.vansairforce.com/communit...X+PC925&page=7
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Last edited by vic syracuse : 12-28-2019 at 06:18 AM. Reason: Added info
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  #36  
Old 12-28-2019, 07:12 AM
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Mark33 Mark33 is offline
 
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Default Shorai VS. Earthx (Ah measuring method)

Ross,
I’m not familiar with the particular Shorai battery that you’re using and/or if they’re using a different method of measuring Ah as compared to an EarthX. Can you please give me a little bit of incite and compare it in size to an equivalent EarthX? I’m planning on using the vented 900 series EarthX rated at 16Ah as my backup battery and I’d like to get some sense as to how much reserve power/time I’ll have if all else fails and I’m relying on this battery to get me safely on the ground.
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Last edited by Mark33 : 12-28-2019 at 08:03 AM.
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  #37  
Old 12-28-2019, 07:25 AM
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rv6ejguy rv6ejguy is offline
 
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Good stuff Vic.

An engineer who worked on lithium battery development and testing for 10 years sent me an email a couple of days ago with a lot of useful information and observations. One paragraph stood out with regards to temperature and I hope he doesn't mind me quoting him here:

"Battery capacity or the energy stored in the cell is not diminished with
cold. What changes is the ability to get power (which is not the same as
energy) out of the cell. This is due to the rate of ionic diffusion
through the plates of the cell being roughly cut in half for every 10C
drop in temperature. Another way of stating it is the internal
resistance of the battery is doubling for every 10C drop in temperature."

This is the significant part and the reason I bring up the temperature angle. What seems like fantastic performance at 25-30C will be drastically different at -20C. Folks who fly in cold places need to be aware of this.

He also stressed that the charging rate in cold temps needed to be reduced for best cell longevity as well. We generally hit them with high current after startup in aircraft.
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Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 441.0 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi2.htm



Last edited by rv6ejguy : 12-28-2019 at 07:35 AM.
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  #38  
Old 12-28-2019, 07:32 AM
humptybump humptybump is offline
 
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From what I've read, the Shorai and EarthX use very different formulas for rating their battery capacity. The Shorai method is "generous" if you are looking at it for running electronics.

Like Mike suggested, I ran one of my early VFR "glass" panels, prior to install, with a Shorai 18Ah. The results showed about 8.5 Ah usable. At the same time, the same battery would start the O320 at 40F with no problems. My conclusion was it had it's rated CCA but should be considered at less than half its amp-hours for running electronics.
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  #39  
Old 12-28-2019, 07:33 AM
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rv6ejguy rv6ejguy is offline
 
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Quote:
Originally Posted by Mark33 View Post
Ross,
I’m not familiar with the particular Shorai battery that you’re using and/or if they’re using a different method of measuring Ah as compared to an EarthX. Can you please give me a little bit if incite and compare it in size to an equivalent EarthX? I’m planning on using the vented 900 series EarthX rated at 16Ah as my backup battery and I’d like to get some sense as to how much reserve power/time I’ll have if all else fails and I’m relying on this battery to get me safely on the ground.
From some of the research other posters did here, it seems that the Shorai 18AH rated battery that I am using would have a rating closer to 6AH in terms of the slow draw specs generally used to rate lead acid AGM batteries whereas an EarthX rated at 18AH is actually close to 18AH. You can see the difference in physical size and weight between the Shorai and EarthX. Even the AGM is affected somewhat at cold temps so looking at battery AH ratings or tests performed at 25C won't give the true picture at -20C.

Remember that the EarthX has a BMS which will limit the lower end of what you can tap off for power and the above info on cold weather performance.

Also consider typical length for your mission and current draw for all critical systems you'd have on if the alternator caved on you.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 441.0 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi2.htm



Last edited by rv6ejguy : 12-28-2019 at 07:39 AM.
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  #40  
Old 12-28-2019, 08:12 AM
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Mark33 Mark33 is offline
 
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Thanks Ross,

My electrical architecture is such that the only thing in the airplane that?ll be dependent on the backup battery will be the electronic ignition and electronic fuel injection which is powered via the ?essential bus?. Everything else is on the main bus, with its own IBBS backup battery.
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