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Backup Battery Testing LiFePO4 vs. AGM

rv6ejguy

Well Known Member
I spent most of the day making up a test rig to load test my fairly new backup Shorai LiFePO4 battery and for comparison, a 6 year old PC680 AGM.

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.

The Shorai is rated at 18 AH and the PC680 at 16AH.

The first test on the Shorai showed it was pretty much done at the 20 minute mark, getting close to the 12.9V threshold (unloaded)- the point where it's not recommended to discharge below for cell longevity.

The old PC680 soldiered on for over 35 minutes before dropping down into the low 10V range under load (low 11V range unloaded).

10 volts is getting low enough to affect the operation of some components. In particular, the surge current capacity to charge the ignition coils and fire the injectors is diminished. The engine will probably still run down to 9 volts in some fashion though and the ECU doesn't sign off until a bit below 7.5V.

The lower floor level of the hangar was at around 8C during the test to make this a tougher scenario so this may have affected the Shorai more than the Odyssey. I couldn't find any info on how temperature affects capacity on the Shorai but it also affects the PC680 to the tune of 20-30% from the 25C rating.

I want to repeat the Shorai test again when I have time but from this first test, if you are relying on a LiFePO4 for backup power in cooler conditions, you may want to select something larger than 18AH, especially if you have a 6 cylinder system where current draw is higher. In truly cold conditions, you may want to consider an AGM battery instead.

I shot some video which I'll publish at some point when I get time.



Typical LiFePO4 discharge curve



Shorai capacity vs. voltage

As far as our EI systems (CPI/ CPI-2) go, they draw far less current that the full EFI/EI systems so they will run much longer on the same size batteries.

As always, be aware that even colder temperatures, say colder than -20C, severely affect the ability of LiFePO4 batteries to deliver high current for long periods. If you're at altitude on a winter day and the battery is in a location where it's seeing ambient temperatures, don't count on it to last very long if the alternator takes a dump. Also, be aware that batteries lose capacity with the number of cycles they experience. Don't rely on an old, cold battery to keep your engine running.

This test was a bit of an eye opener for me but I'm glad I did it in cool conditions. The lesson is clear here- don't rely on battery AH ratings solely, different chemistries have different characteristics.
 
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Ross;
Since the discharge rate is fairly high, I wonder if you could have run the lithium battery to a much lower voltage.

I know you don't want to ruin a brand new battery. I wouldn't either. But in an emergency, the battery only has to get me home one time. I would thank the battery, and dispose of it.

Looking at the lithium battery discharge curve, it looks like it should have lasted about an hour. I have seen similar charts before.
 
In the last few minutes, the lithium voltage dropped quite rapidly. At 12.9V, there is only 20% remaining so it might have run for another 3-4 minutes at most. You can see the rapid drop off on the curve above. The AGM has a more gradual drop off.

You're correct, since there is no BMS on these, you can run it until dead and I would in a real emergency but I wasn't about to destroy a new battery for the sake of science.

Most battery ratings are done at 25C and they perform slightly better above this temperature and somewhat worse below it. The LiFePO4 seems to be affected relatively more than the AGM in my test and Shorai lists some techniques to improve cold starting so it's obviously a concern.

I had a customer with another brand of LiFePO4 who was stranded twice in Alaska when it wouldn't turn the engine over. He managed to hand prop a cold 400 inch EFI engine to life. Must have had Arnold arms to do that! I guess you do what you have to when you're stranded in the bush.
 
Also I was under the impression that these batteries warm up to a normal internal temperature quite quickly when under load. I have read the recommendation to operate the landing lights for a minute before cranking in cold weather. Is this not actually effective?
 
shorai

"...The first test on the Shorai showed it was pretty much done at the 20 minute mark,..."

I would be suspect of that battery if the test is accurate...it is performing nowhere near it's rating...
 
Remember the ratings are established at 25C. At 11 amps draw, I'd expect it would warm up in a couple of minutes since these are very low mass. This battery weighs only 2.2 pounds.

Experiments often show us things that straight specs don't, hence their value.

I want to repeat the test and see if I get similar results. From the first test, the Shorai didn't perform as well as I expected from the specs.

Your results may vary with other brands but if you're electrically dependent with only one alternator, I suggest you do similar tests at cooler temperatures to verify performance.

I looked at voltage under load as well as with load removed at 5 minute intervals.

The Shorai spins my engine over very well at these temps, maybe 80-120 amps for a few seconds. It seems not to perform so well pulling 11 amps for many minutes however.
 
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With the PC680 considered in the test, maybe the test should be duration per pound.

But then it gets very expensive........:eek:
 
Backup Battery Capacity

Remember the ratings are established at 25C. At 11 amps draw, I'd expect it would warm up in a couple of minutes since these are very low mass. This battery weighs only 2.2 pounds.

Experiments often show us things that straight specs don't, hence their value.

I want to repeat the test and see if I get similar results. From the first test, the Shorai didn't perform as well as I expected from the specs.

Your results may vary with other brands but if you're electrically dependent with only one alternator, I suggest you do similar tests at cooler temperatures to verify performance.

I looked at voltage under load as well as with load removed at 5 minute intervals.

The Shorai spins my engine over very well at these temps, maybe 80-120 amps for a few seconds. It seems not to perform so well pulling 11 amps for many minutes however.

Ross,

Thank you for running these tests and sharing the data. As I’m sure you are well aware, over the last 4 years there have been a few incidents with Reno racers that involved alternator failures and dead engines. In all of these cases the duration of operation under the backup battery was far less than was expected and resulted in engine-outages of electrically dependent engines. One aircraft made 2 semi-successful dead-stick landings in the same race weekend, another made a precautionary landing on an Indian Reservation road due to rapidly falling voltage, and the third made a hard landing on a runway that resulted in a totaled aircraft but fortunately the pilot walked away. All of these occurred in summer conditions. I think they all involved lithium batteries.

The point here is that someone planning an electrical system for an electrically dependent engine needs to absolutely ensure that they have more than sufficient backup battery capacity and the systems need to be tested. Also, the lithium batteries seem to fall a little short of their rated capacities under EFI electrical loads.

Skylor
 
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Thanks for the great info.

Sure makes me glad I chose dual Odyssey 545s for the new project.
 
I was going through Shorai's website and I went through their FAQ's. There was one about AH capacity, but the wording was a bit confusing.

Comparing capacity ratings and weight between 2 different LifePO4 suppliers shows wildly different weight vs AH capacity. I would say one is much more conservatively rated (AH) than the other.
 
Thanks for running these tests! Results were a bit better than expected for the lithium. All of these li manufacturers are quoting their capacity in PB equivilant, which really isn't equivilant. These equivilancy ratings are 3X their actual AH ratings and I suspect your test was closer to 2X because your odyssey is 6 years old. Odyssey spec's close to an hour capacity at 10 amp draw for the 680, which is 3X what you got on the Li. The odysseys only deliver 16 amp hours when drawn at around 2 amps or .1C, if I remember correctly.

Larry
 
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"Shorai LFX are based on a completely different chemistry. Not only do they have less than 1/3 the internal resistance per capacity than do lead-acid, they are also the ultimate "deep-cycle" battery. The internal "completely discharged" capacity of a Shorai LFX is 1/3 the rated "PBeq" capacity. For example, the LFX18 12V series have 6Ah cells internally. But the cells are capable of 80%+ discharge without damage and while retaining more cranking ability. As such, the USABLE capacity(or "reserve capacity") of an LFX18 12V battery is on or very near par with 18AHr-rated lead acid batteries, while providing superior cranking performance and a vast reduction in weight. The Shorai PBeq AHr (lead-acid equivalent) rating system therefore allows users to compare a very different technology from lead-acid, but on a close apples-to-apples basis when making a choice."

After reading this a few times, I believe that it means their battery is rated at 18AH for comparison purposes with respect to cranking performance of a Pb battery, but the actual Shorai capacity is about 6 AH.

They said they do this because their batteries are being marketed as starting batteries, not deep cycle. Their 6 AH battery cranks engines as well as an 18AH Pb battery, so they call it an 18 AH battery. Used as a deep cycle, and the capacity is 6 AH.
 
1/3 AH capacity

From the Earthx FAQ (https://earthxbatteries.com/faqs):
In performance and real life usability for lead acids that are vehicle starter batteries, they only use 30% of the stated amp hours or stated as “30% depth of discharge” before the voltage drops so low you can not start your vehicle. So only 30% of its amp hours are usable, whereas lithium batteries have a 98% depth of discharge. For example, our ETX36 is a replacement for an YTX20 Yuasa series, which has 18ah rating of capacity. The amount of actual useable amp hour is (18ah x .3 = 5.4ah). We use a 12.4ah cell in our ETX36 which is actually 7 amp hours MORE than the lead acid battery we are replacing. Here is a detailed article to help explain this. https://www.dropbox.com/home/EarthX?preview=SLA+vs+Lithium+Ah+AN1506_RevA.pdf
I don't have nor want a dropbox account so I could not read the document.
 
EarthX rates the capacity of their batteries at a 1C discharge rate. That means 100 percent of the battery capacity is used in 1 hour. Kind of a fast deep cycle usage.

Shorai rates there's as the equivelant cranking capacity of a similar Pb battery. So if their 6 Ah battery cranks the engine for as long as an 18Ah Pb, they rate their battery at 18 Ah.

Good to know. 2 of Shorai's 36 Ah batteries should have would have provided maybe 1.7 to 2 hours in Ross's test.
 
After reading this a few times, I believe that it means their battery is rated at 18AH for comparison purposes with respect to cranking performance of a Pb battery, but the actual Shorai capacity is about 6 AH.

They said they do this because their batteries are being marketed as starting batteries, not deep cycle. Their 6 AH battery cranks engines as well as an 18AH Pb battery, so they call it an 18 AH battery. Used as a deep cycle, and the capacity is 6 AH.

This is correct from my understanding and pretty much all of the battery companies follow it. Just wanted folks to be sure they understand this before swapping out odysseys for lithium at similar ratings and have electrically dependent engines or fly IFR.

For pure cranking applications, these batteries are very good and very capable at similar capacities.

Larry
 
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Ross,

Thank you for running these tests and sharing the data. As I?m sure you are well aware, over the last 4 years there have been a few incidents with Reno racers that involved alternator failures and dead engines. In all of these cases the duration of operation under the backup battery was far less than was expected and resulted in engine-outages of electrically dependent engines. One aircraft made 2 semi-successful dead-stick landings in the same race weekend, another made a precautionary landing on an Indian Reservation road due to rapidly falling voltage, and the third made a hard landing on a runway that resulted in a totaled aircraft but fortunately the pilot walked away. All of these occurred in summer conditions. I think they all involved lithium batteries.

The point here is that someone planning an electrical system for an electrically dependent engine needs to absolutely ensure that they have more than sufficient backup battery capacity and the systems need to be tested. Also, the lithium batteries seem to fall a little short of their rated capacities under EFI electrical loads.

Skylor

One of those Reno aircraft was owned by a friend and he was not aware the battery wasn't being charged. As I learned many years ago, have aural voltage warning, not just visual. Visual indications can be missed. You want to know the moment that the charging system isn't, to make the best decisions possible.
 
I was going through Shorai's website and I went through their FAQ's. There was one about AH capacity, but the wording was a bit confusing.

Comparing capacity ratings and weight between 2 different LifePO4 suppliers shows wildly different weight vs AH capacity. I would say one is much more conservatively rated (AH) than the other.

Agreed. I only tested Shorai. Results on other brands are likely to vary.

There are a few different criteria used in ratings and they are not strictly comparable. In light of this, best to do actual tests as I did with your expected current draw. This would also apply for your avionics loads if you're flying IFR.

On my RV-10 project, I had twin alternators and batteries. That setup would give you more peace of mind if something went bump in night so to speak.

Thanks for all the additional research here guys. I had read some but not all of these documents linked to here.

As EFI becomes more and more mainstream, we want to make sure we have robust systems to supply them with electrons for long enough to complete the mission or at least get us down safely.

On our CPI-2, we recommended a 2.9 AH Powersonic AGM battery and supply an optional CNC'd tray and wiring kit for that. This battery was bench tested running the controller and coil pack for, I believe, 90 minutes (4 cylinder system) before voltage was getting critical to charge the coil. With 2 coils firing, probably reduce that to around 40 minutes. The CPI-2 has software options to reduce coil charge time and sign off one coil as it monitors battery voltage to extend run time. The AGM was recommended as we have more experience with them and had no concerns about cockpit mounting them.
 
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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.
_.

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.
 
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.

Valid point. You would need something like a constant-current driver on an LED array to accurately portray the usage.
 
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.
 
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.
 
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.
 
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.

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.
 
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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

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
 
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

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.
 
Oh, yeah....

"...There have been plenty of other threads already on electrical system design/ layout and evaluation..."

That's a fact...
 
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???
 
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.
 
I=V/R, P=IV

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
 
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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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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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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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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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.
 
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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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.
 
just an observation.........the 10 deg c. and doubling rate is a law of chemistry/physics. same law that makes keeping an engine always warm in the winter questionable....it rusts faster.
my observation about all this is that i would think fwf applications should not see a drop in performance. someone needs to put a probe on their battery and go fly in the cold. i'd be glad to do it if i had something flying.
 
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.

Which is, you guessed it, a LiFeO4 battery...:D
 
A few years ago I tested a Shorai LFX18 battery with a CBA IV load tester. I found with a 7A essential bus load I had 50 minutes until voltage fell to 11V. At that point the discharge curve fell off rapidly. Battery-only endurance is not much of a concern to me as I have a dual battery/alternator architecture. But if one wants to use a LiFePo4 battery in a single battery setup then you need to upsize it as the amp-hour ratings can be deceiving.
 
Oh, the tangled webs we weave...

All I know is that when the ?you know what? hits the fan and the only thing that matters is getting the airplane on the ground safely, the most critical thing is to keep that big fan blade turning out front....?EVERYTHING? else is secondary. That?s why I have my electrically dependent ignition and injection on its own separate ?essential bus? and completely separated from everything else in the airplane and I?ll have that big 900 series vented EarthX battery as backup to power this ?essential bus?.

However, in saying that and understanding the importance of everything else in the airplane, including IFR navigational equipment, the ?main bus? has its own separate IBBS backup battery to keep all of the pretty screens lit up and functioning properly.
 
This is great, thanks for testing like this. Correct me if I'm wrong but so far it sounds like the Shorai batteries get less than half of their rated capacity. But this test was also a little harder on the Li batteries because of the cold, but also maybe something to do with the load that was used?

2 36Ah batteries are around 9lbs and $700, hmm.
 
Yes, if you look through this thread, you'll see that these so called 18AH Shorais are closer to 6AH in terms of this sort of current draw over 20ish minutes. Great for starting the engine but limited in endurance for this purpose.

You can go bigger of course for more money and weight.
 
Looking at the Shorai website I found in the FAQ where they say they use 6Ah cells, but it's not clear they mean this is all they use. In their LFX18 example does this mean they could be using 3 12V 6Ah cells in parallel?

Are we attributing the apparent reduction in capacity to the correct thing?
 
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