(Cross Posted from the "other" discussion)
I've gathered some more data, based on some of the concerns raised in this discussion. The main issues brought up are
2: Performance and capacity
3: Weight and Balance.
Warning! Info dump to follow!
I looked at a few scientific research papers on the safety of LiFePo4 battery cells, and I was actually able to understand some of the words! After uncrossing my eyes, I actually searched something asked for in this discussion; what do these cells do when they are abused. I found some interesting videos:
- Full Destruction of a charged cell
- Penetration Test 1
- Penetration Test 2
- Over Charge
Also, here is some information from Wikipedia, which I know is NOT a infallible source:
And Finally, from http://www.mpoweruk.com/lithiumS.htm
Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of Lithium-ion technology made with other cathode materials. Lithium phosphate cells are incombustible in the event of mishandling during charge or discharge, they are more stable under overcharge or short circuit conditions and they can withstand high temperatures without decomposing. When abuse does occur, the phosphate based cathode material will not burn and is not prone to thermal runaway. Phosphate chemistry also offers a longer cycle life.
Recent developments have produced a range of new environmentally friendly cathode active materials based on Lithiated transition metal phosphates for Lithium-ion applications.
2: Performance and Capacity
Some of the questions brought up relating to this were the operating temperatures that a LiFePo4 battery would encounter on the firewall of an RV10. I had asked the EarthX rep this question at Osh, and his response was this: "The operating range of the EarthX battery is wider than a comparable Odyssey SLA battery". This looks to be mostly true, as the Odyssey manual lists the range without a protective metal case to be only good to 113°F, but 176°F with an added shield. I'd imagine that you could add a similar, or even the same shield to the EarthX to get even more temperature range.
Also, a few of our fellow RV10 pilots are going to stick some temperature probes on the firewall in a few places to see what the requirements really are. As it stands today, it doesn't sound like temperature is going to be an issue.
Odyssey Operating Temperature Range
PC545, PC680, PC925, PC1200 and PC1700 without metal jacket: -40°C (-40°F) to 45°C (113°F),
PC545, PC680, PC925, PC1200 and PC1700 with metal jacket: -40°C (-40°F) to 80°C (176°F),
EarthX Operating Temperature Range
Operating Temperature* -30°C (-22°F) to +60 °C (140°F)
Storage Temperature -40°C (-40°F) to +60 °C (140°F)
EarthX Discharge Versus Temperature
We use a similar Cold Cranking Amp test standard (SAE test performed at 0°F, but 3
second discharge time) as the lead acid battery manufacturers. As such, our battery with a
similar CCA rating as a lead acid battery should provide the same cranking performance
at 0°F. But, below 0°F an equivalent lead acid battery will outperform a lithium battery
As far as capacity, the EarthX battery that I am considering has 12ah. According to Skytec's specs, the 149-NL starter that I'm going to use has a draw of 125-185 amps. The 12ah battery will give me almost 4 minutes of solid cranking at the highest spec'd starter draw. As far as electrical emergency capacity, between whatever is left in the starter battery (say after 2 minutes of engine crank) and a 6ah TCW backup battery, I'll have 12ah, which at 6 amps of draw (rough estimate) gives me two hours of endurance buss time. And that is if both the primary and secondary alternators fail.
3: Weight and Balance
Taking the suggestion of a few people in this discussion, we figured out an example weight and balance for a generic RV10, found the arms of the battery tray and the firewall, and ran the numbers with a 26lb battery in the tail, and a 4lb battery on the firewall. Even with the battery on the firewall and 26lbs removed from the tail, it's difficult to get the plane into a CG past the forward limits. Pretty much any weight you carry in the plane pushes the CG back. So, worst case scenario would be a light pilot on low fuel. Even with the stock battery in the stock location, you'll be forward of the CG limit. Adding ~40 lbs in the baggage area will keep the plane within CG in this light configuration, and have the benefit of being removable when carrying capacity is needed. If even a single 65lb person is in the back seat, this extra weight isn't needed.
The real problem is hitting the aft CG before you hit gross weight, because, as I stated before, pretty much every pound you add pushed the CG rearward. Because of this, I personally would prefer to have a CG as far forward as possible to give me more options.
Basically, having the battery in the tail only moves the cg aft 1" in a single pilot no fuel scenario
but allows you to carry 30lbs more baggage in a full gross scenario.
The one thing these numbers don't include are the weight and CG changes from removing the large #2 power cable from the tailcone to the firewall.
Thehe WnB spreadsheet I've used for my numbers is here:
Thanks to Justin Twilbeck for throwing this together for me. (It even has a calculation for the loading needed to cause a tailstrike! Good stuff!)