Interested in a LiFePO4 battery?

There's no way to test this, correct? We can't temporarily disable the charging system on the ground and run at medium RPM for n minutes? Pull a fuse or two?

Hi John,

Capacity:

In 80 and 100 hp Rotax 912 (no injection engines), 7 Ah batteries are usually installed in our temperate latitudes (if no complex electronics are installed). For example, the Hawker Powersafe SBS 8, which is a pure lead battery, i.e. one of the best lead batteries. A normal lead-acid battery also loses voltage as its capacity decreases. It is said that only about 50% of the capacity of a lead-acid battery can be used. In the case of this Hawker pure lead battery, I would assume that even 70% can be used - i.e. still just under 5Ah.
With a LiFePo4 battery, approx. 90% of the capacity can be used and the voltage hardly drops at all (my own experience). So to replace a Hawker SBS 8, you need a LiFePo4 of 5.4Ah = 6 Ah.

The fact that ROTAX recommends twice the capacity has to do with the fact that there are fewer and fewer of these "simple" motors and that more and more electronics are being installed. With the aforementioned knowledge, you can decide for yourself how much capacity your LiFePo4 battery should have. With the Rotax recommendation, you are of course on the safe side - no question.

LiFePo4 in general:

Advantage: low weight, high energy density, long service life

Disadvantage: expensive, sensitive to overvoltage, even when charging, limited charging capacity at very low temperatures.

Rotax EarthX battery:
For me the SI-PAC-027 shows that it has a built-in BMS and overload protection - so there is some electronics in the battery. Therefore you will probably not need a special regulator for the battery (if other informations are known please let me know), the standard Ducati regulator should be sufficient. Note: Built-in electronics can break. Even if it is assumed that the EarthX is a high-quality product, there is still a risk of BMS failure, which could lead to the battery being destroyed relatively quickly, possibly while still in flight - an opinion of the manufacturer of my special regulator - maybe some could be right.

Alternative concepts - special regulators for LiFePo4:

The manufacturer of my previously mentioned LiFePo4 regulator (to be installed instead of the Ducati regulator) has the following recommendation:
A high-quality LiFePo4 battery without BMS and without automatic overvoltage cut-off. So only a pure LiFePo4 battery with sufficient capacity, high discharge current for starting and sufficient capacity (depending on the connected consumers). The correct charging voltage and overvoltage protection are ensured by the special controller. The electronics built into the regulator are certainly more robust and less temperature-dependent. There is no BMS in this concept. For safety reasons (failure of the BMS), he would definitely do without a built-in BMS. He points out that in model aircraft construction, the individual cells are balanced separately at least once a year (however, cables from the individual cells must lead to the outside).

Its common here to install such special regulators and use corresponding batteries (e.g. SuperB). Some manufacturers of microlight aircraft recommend this alternative concept (special LiFePo4 regulators - which can also be used normally for a lead-acid battery + high-quality LiFePo4 battery without BMS and overvoltage protection).

Greetings

Original poster here, I ordered the ETX680C today. I was originally concerned about the suggested battery replacement for my Remos GX 912ULS only being 12.4ah and the factory original was 16ah. Upon looking at the FAQ's on the EarthX website it stated the following;

How does the EqAh capacity rating compare to lead-acid Ah ratings?

 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 (12.4ah) 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. (7ah more than the 5.4ah)

I have also read on multiple forums that many users of the standard 912ULS (not SI) are using the ETX680 without problems. 

On a side note,

The nominal LiFePO4 battery voltage is 13.2 volts (four 3.3-volt cells) compared to 12.6 volts (six 2.1-volt cells) for lead-acid batteries

I read the required charge rate of this battery is 14 to 14.6 volts, My plane like many others uses the Ducati Voltage regulator. I know that many on these forums have experienced problems with Ducati regulators but I'm not looking for a debate on that right now. My question is for any Earthx users, have you ever experienced any problem using the stock regulator? Does it seem to charge higher than 14v. I don't remember ever seeing the in flight charge rate over 13.8 volts on my old AGM battery despite the posted charging rate of 14volts, + or - .3 volts on the Ducati regulator. What Voltage reading do YOU see while charging?

Thanks for your interest and replies,

J.T.

It's definitely a difficult question to resolve.  I've never seen any data on the total electrical current demand of a Rotax "iS" engine at a typical cruise power setting.  I've seen varying numbers for fuel pumps alone, but I don't know how accurately they were measured.  This is something that I intend to measure on my airplane, but it will be a while before I reach that point.

To answer your question, yes, you could unplug the stator connectors at both voltage regulators, then use the Battery Backup switch to run the engine and see how long it takes before an EarthX battery starts to flash the fault light (30% state of charge) or a lead-acid battery drops to 11.8V (also roughly 30% SOC).  To get valid data you would need to run the engine at a flight-sustaining power setting and operate all airframe loads that you intend to leave on during a battery-only emergency.

There are two problems, however: 1) you probably won't be able to run the engine at a cruise power setting long enough on the ground without overheating, and 2) the current demand of the ignition system is highly RPM dependent so idle running is a poor simulation.

A good test method with a lithium battery is to use a DC electronic load to test the battery at a constant power (ideally your airplane's measured demand; again, the missing piece of data) and use the load's control software to calculate and display battery capacity.  This can be quite expensive with lab-grade test equipment.  If you're willing to risk Chinese electronics sellers on markets like AliExpress and eBay, and you have the patience for poorly translated instructions and zero customer service, there are lower cost options.

The fact of capacity loss over a battery's life is a good argument for buying more capacity than you need, so natural losses with aging don't put you below your desired endurance figure.  For example, I intend to use an ETX1200 (20Ah), for several reasons: 1) it only weighs 11oz more than an ETX900 in the same case size, 2) it will be loafing when asked to crank a Rotax, 3) the Rotax charging system will treat it very gently vs. the battery's capability, and 4) its excess capacity will allow it to significantly degrade and still meet the Rotax specification and my desired minimum endurance.  I'll use a test equivalent to the measured demand of my engine plus minimum airframe loads to annually check the battery's capacity.  When it can no longer keep me aloft for 45 minutes, I'll buy a new one.  I expect that to take several years.

HI Eric and all

The small single phase stator as on the standard 912 and 914 series needs enough power to run your min load that is required for flight for 45 mins after a failure.  Most of that will be to power your aux electric pump.  Depending on the pump type it may be up to 10A per pump.  In the case of a type 914 where you must have an electrical pump, at least one, working and the other parts that run the TCU and sensors the load is shown in the installation manual as near to 8 amps for maintaining flight.  The battery in that system is not used at all for the ignition as it is self generating.  As long as the engine spins the permanent magnets keep the spark plugs working.  

The iS is different and has a need to have at least one working fuel pump, again up to a 10 Ah draw.  The balance of the bits are very small and you can get an idea in the installation manual what you need.  You have at least the redundancy of a split power supply from the stator and an additional supply from the battery power.  The problem here is the heavy fuel pump draw again.  The power demand from the fuse box and related sensors is actually very small.  The drop dead voltage for operation is at 9V, below that the ECU will shut down.  

With Lithium batteries it is very important to have them with cell balancing.

Cheers