Backup Battery Testing LiFePO4 vs. AGM

[svyolo]

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.

[lr172]

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

[svyolo]

"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.

[rv8ch]

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?...N1506_RevA.pdf

I don't have nor want a dropbox account so I could not read the document.

[emsvitil]

Try this:

https://earthxbatteries.com/wp-conte...AN1506_New.pdf

[svyolo]

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.

[lr172]

Originally Posted by svyolo

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

[rv6ejguy]

Originally Posted by skylor

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.

[rv6ejguy]

Originally Posted by svyolo

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.

[rv8ch]

Originally Posted by emsvitil

Try this:

https://earthxbatteries.com/wp-conte...AN1506_New.pdf

Thanks Ed!