LiFePo battery?

[ChiefPilot]

Originally Posted by EarthX Lithium

Any other battery would be discharged to the point of permanent damage and you are out the battery as well because they are not covered under warranty for this.

I don't believe this is entirely accurate. The <non-EarthX brand> LiFePO4 battery I use has had this happen with no apparent ill effects i.e. same time to discharge state as before the incident. I wouldn't make a habit of it, but to say that any other battery would be discharged to the point of permanent damage is perhaps a bit of hyperbole given empirical evidence to the contrary.

[BillL]

Originally Posted by EarthX Lithium

Now for the over charge (over volt situation). How our BMS works is if the internal voltage of the cells are higher than 15-16V, it will switch off and reset all on it's own when the voltage is decreased to a safe level. A couple of scenarios that can happen is if your alternator is charging your battery in this range, you have charging issues and if you have a faulty charger in this range, it can happen. If you continue to charge a lithium battery at this voltage, not only will you destroy it but you will cause extreme internal heat, which is not good either.

Hope that better explains how these features work!

Kathy

There is probably more detailed quantification needed on the over voltage behavior. Here is why. When the typical over voltage failure occurs, and if the plane is equipped with an over voltage protection, it will shut down the alternator. So lets suppose that the scenario is IFR, real weather, the VR fails high, the voltage goes up, the battery removes itself from the circuit (what happens?) then the voltage increases and at 16.9v, the crowbar involves and then the alternator is gone. Are we suddenly without alternator AND the battery? Maybe a little more understanding (testing) is needed to ensure . . . well, we can see the possibilities. It might be that the back up battery or back up alternator picks up, then the main battery returns in a few seconds and it is like any other emergency, but one needs to KNOW what behavior to plan and build a panel and system around.

I don't mean to be negative on the technology, but addressing the new failure modes seems prudent. If one is only flying VFR, then it is fine, the risk is much less.

[Bevan]

Hi Kathy,

Thanks for monitoring this forum and for your reply.

I don't understand your "80%" number. When comparing Ah ratings (with Odyssey in this case) Oddysey's website claims their Ah rating is determined at a 10hr rate. That means a "100Ah battery can deliver 10 amps for 10 hours before the battery voltage drops to 1.67V per cell which happens to be 10 volts for a 12 volt battery." see...

http://www.odysseybattery.com/faq.aspx#7

Are you saying that the actual capacity (to 10 volts) is 80% of the above? Or are you saying that a typical "not new battery" could be said to have 80% capacity left?

Another question is...How long can the EarthX ETX36C battery provide 10 amps before the voltage drops to 10 Volts?

Also, your's and Oddysey's website claims a reserve capacity in minutes but doesn't state at what rate (of discharge). How can a reserve capacity be stated without any reference to a rate? Or, is it assumed to be at 10 amps?

Not trying to be critical, just trying to understand and make a fair comparison.

Bevan

[Bill Palmer]

What my meager brain has learned so far about LiFePo4 batteries (. . . my humble attempt at some overall, but admittedly subjective, clarification based on internet data sources and my personal discussions with some manufacturer?s technical reps):

Battery Capacity:

Most LiFePo4 manufacturers specify their battery capacity in Equivalent Ampere Hours (EqAH). This ?equivalent? battery capacity is reverse-calculated from the LiFePo4?s superior cold cranking amps. The term ?equivalent? is very misleading. The ?lead acid? equivalent AH rating of a LiFePo4 battery is actually slightly less than 1/3 of the LiFePo4?s specified EqAH.

For example, the Odyssey PC680 battery has a 16AH ?lead acid? capacity. The EarthX ETX36C?s (36 EqAH) equivalent ?lead acid? capacity is actually between 11 and 12AH. Bottom Line: one EarthX ETX36C battery can out-crank a PC680, but the PC680 has the greater AH capacity (16AH versus approximately 11.5AH).

The good news: 2 EarthX ETX36C batteries total over 22AH in ?lead acid? capacity and weigh less than 7 pounds total compared to one PC680 with 16AH at 15.4 pounds. The very bad news: 2 EarthX ETX36C batteries cost well over $600, and one PC680 can be purchased for less than $100. You decide the winner!

Discharge Curve:

The LiFePo4 discharge curve is more like a step function (cliff). Everything is fine for quite awhile (for ?most? of the AH capacity . . . check the manufacturer?s data to define ?most?) and then the battery gives up very quickly. A lead acid discharge curve has more of a gradual downward slope with some degradation (volts) starting almost immediately. In other words, a lead acid battery gives up slowly, but degrades along the way. A LiFePo4 battery does not degrade much as it discharges, but it gives up more quickly (due to a lower AH rating).

In the case of an alternator failure, it is debatable (mainly subjective) regarding which battery is better. A LiFePo4 battery is good as long as you can shed load to stretch its AH capacity over time or have two LiFePo4 batteries for greater total AH capacity. If you cannot effectively shed load (for example, your IFR EFIS does not have its own backup power) or you do not have a redundant electrical power system, then the lead acid battery looks much better due to its higher AH capacity per battery. The best way to resolve this debate for yourself is to get each manufacturer?s discharge curves and compare them with your aircraft?s electrical system design to see which one is best; for you. Obviously, relative battery weight(s) and costs are big issues, too.

EarthX Battery Management System (BMS):

In the case of over-voltage or under-voltage (regulator or alternator failure), the EarthX?s BMS will shut down the battery?s LiFePo4 cells to protect them. Solution: Turn-off the alternator, and the EarthX battery will automatically reset and come back on-line. For LiFePo4 batteries without this internal BMS protection system (Ballistic or Shorai batteries, for example), the solution is still the same (turn-off the alternator), but some battery cell damage is probably inevitable for these batteries depending on how quickly the fault is detected and how quickly the alternator/regulator is turned off. What this potential cell damage might mean in terms of subsequent battery performance or safety seems to be mostly unknown. Is there anyone out there with actual failure experience or enough spare cash for destructive testing to give us an answer for these non-BMS LiFePo4 batteries?

Well, that?s all I have. I wish I had more time, energy, and good technical data (confident numbers) to further analyze LiFePo4 batteries, particularly in comparison to lead acid batteries, but I don?t. Who?s up next?!

I hope this overall ?clarification? post helps, anyway . . . use at your own risk!

[grayforge]

Nice Summary!

Another option, besides 2 x ETX36C's would be 2 x ETX24C's. This would provide about a 16AH equivalent power source. Same as the PC680 at just over 5lbs for just over $500. :-)

Interesting that the single ETX48E is well over twice the price of two 24's.

Russ

[Bill Palmer]

I lied . . . (?that?s all I have?). I do have some more Lucy ?5 Cents? Psychiatric Battery Counseling (it?s worth what you?re paying for it!):

Electrical Storage Technology:

A LiFePo4 battery is a dry, electronic system. It?s more like a big, long-acting capacitor and regulator than anything else. It?s definitely NOT your dad?s wet, electrochemical, lead-acid battery. LiFePo4 is silicon-like technology; not chemical (lead acid) technology. LiFePo4 and Lead Acid represent completely different physics. Assumptions and experience with one cannot generally be applied to the other. In other words, if you?re going LiFePo4, throw out everything you ever learned about lead acid batteries, because your experience is no longer applicable and could even be dangerous. If you decide to go LiFePo4, you MUST LEARN LiFePo4. Forget lead acid. Don?t use lead acid chargers, don?t install or treat a LiFePo4 battery like a lead acid battery; don?t even think about going there!

LiFePo4 Installation:

A LiFePo4 battery is a piece of electronics. Firewall forward (heat and vibration) is an issue for electronics. You might think about adding vibration protection, insulation, and cooling if you are planning to install a LiFePo4 battery firewall forward. In my opinion, you cannot blindly insert a LiFePo4 battery in the same location as a typically more durable and environmentally-tolerant lead-acid battery. Although LiFePo4 batteries are designed for relatively harsh operational environments as installed in motorcycles and ATVs, there is probably some airflow to the battery locations in those vehicles, and those vehicles may have much less continuous, strong vibration than your RV with its thunder-thumping Lycoming. Until there is more experience with LiFePo4 batteries operating in harsh environments, you might want to consider installing a LiFePo4 battery behind the firewall as is recommended for external regulators and similar electronics. You might even consider shock-mounting the LiFePo4 battery. Electronics can be delicate, you know!

Now where?s Charlie Brown? I?m going to swap that football for a LiFePo4 battery!

[EarthX Lithium]

I am attempting to copy curves from our manuals and from the Odyssey manuals for ease of reference. Every manufacturer rated Ah is confusing. So what we always recommend is to look at the discharge curves. Here is a comparison of the Odyssey PC 680 and EarthX ETX36.

First, notice the curves are not exactly the same format. The Odyssey curve is Amp on Y axis and hours on X axis, with volts decreasing to 10V, while the EarthX curve is volts on Y axis and Ah on X axis (hours = Ah/Amps).

For airplane applications, a 12 amp discharge rate is common, so below is an example that compares the Odyssey PC680 and EarthX ETX36 discharge at 12 amps.

12amp Discharge Rate Example:
Per the Odyssey discharge table (below to right), line 8, a discharge rate of 12.3amps will drain the battery to 10V within 1 hour, consuming 12.3Ah.


Per the EarthX discharge table (below), lime green line, a discharge rate of 12amps (1C for 12Ah battery is 12amp) will drain the battery to 10.4V within 1 hour (hours = Ah/Amp, 12Ah/12Amp= 1hour), consuming 12.0Ah.



So as you can see from this example, the EarthX?s ETX36 is equivalent to the Odyssey PC680 in discharge Ah capacity.

Note with an EarthX lithium battery, the discharge rate has little effect on the discharge Ah capacity, for the curves for .1C , .5C and 1C are almost identical, meaning the Ah capacity for 1.2amp, 6amp and 12amp discharge rate are almost identical.

Reserve capacity (minutes): reserve capacity is the simply the discharge curve for a 20A discharge rate. So for the EarthX battery it is 12Ah (12Ah/20amp = .6 hour = 36minutes). For the Odessy, per the chart above (line 6), it is approximately 30minutes.

Kathy

[Bill Palmer]

Thanks, Kathy

Okay, you are saying that the Odyssey and EarthX discharge curves show that the actual, equivalent AH rating of the PC680 and ETX36 are nearly identical at around 12AH. (In other words, a 20-amp load will discharge either battery in about 30 minutes plus a little.) Is that correct?

If so, I stand corrected. The PC680 does not have greater AH capacity. I learn something new every day, but only when I shoot my mouth off, forget my own rules, don't finish my homework, and embarrass myself!

Also, if so, why does Odyssey generally spec the PC680 at 16AH and EarthX the ETX36C at 36EqAH?

[rv7charlie]

Sorry, Kathy, but if the graph is labeled correctly, it isn't an equivalent curve (there's no time axis).

[Bill Palmer]

Kathy,

In other words, I believe Charlie would like to see the ETX36 capacity data under the same conditions and in exactly same format, including discharge time, as the PC680 discharge curve and table. Only in this way can we finally obtain a one-to-one (“equivalent”) comparison with no assumptions, calculations, or “if so” required. Can you provide that? I think we would all appreciate seeing that.

Thanks,