Here is a copy paste from the FMA site ( http://www.fmadirect.com/new_applications/LithiumPowerSolutions/index.htm ). I guess I don't know anything more than what this says. The 80% rule certainly seems to be "accepted" by most people who have been using the cells.

Other Criteria for Longevity:

The two most common criteria left for the user to maintain the health of their Lithium pack are discharge current levels and cell temperature. When treated properly, a Lithium pack can achieve 500 cycles before losing only 20% capacity. Lithium packs that have been stressed by over current draw, which creates high temperatures in the cells, have their lifespan reduced to 50 cycles or less. Experimental data shows that Li Po packs decline very rapidly when the 80% capacity level is reached as salts form in the cells and the cell then deteriorates as the crystals grow even when not in use.

Dean,

I don't have an Eagle Tree but very crude estimations. Static I have measured the full battery, full throttle at 55amps on 3 cells. The watt meter shows about 550 watts. If I derate that for unloading in the air by 10% (which is probably conservative) and assume about 26 seconds of high speed that works out to 357 mah for high speed. Now in  reality I don't jump off the throttle right at the exact end of high speed so I'm guessing close to 400 mah for high speed. Then there are 2 or three laps to drop the hook and get set up for low speed at what I'm guessing is 1/2 throttle. Then there is low speed flight, 8 laps. At 4 minutes 15 sec, as I did today, that's 32 second lap times. During a good lap the engine is running pretty evenly with slight increases in power, I'm guessing 10 to 15 % during 1/4 to half of the lap going down wind. The way I fly (not very well I'm afraid) there are probably perturbations every other lap where I have to catch the plane and need 3/4 to almost full power for 3 to 4 seconds. It's hard to be exact and the wind has a lot to do with it. Then a lap or two to setup and land. Total low, or rather, post high speed laps is around 12 or 13.

Based on the above reasoning, I took 1862 mah for the full flight subtracted the 400 mah for high speed and got 1460 mah for the 12 or so post high speed laps. A total of 4 min 15 plus 4 more laps for a total of 15 seconds additional add up to  for 4 min 30 seconds for 1400 mah.  That works out to only a 20 amp average which seems kind of low to me. I may have misjudged some of the "extra" lap times but I'm in the ballpark.

Maybe that kind of data will give you something to chew on!!!!

Thanks for your thoughts and help.

Bob

Here is some info from the "Lipo Care" tab of the MaxxAmps batteries website: http://www.maxamps.com/lipo-care.php

* MaxxAmps says to NEVER discharge below 3.0V and recoomend a limit of 3.2V per cell.  I think the 3.2V number approximates 80%

* They offer add-on warranties (like 3 yr-300 cycles) warranting that the cells will not loss more than 30% (!!!) over the warranty period and state that discharging below 3V per cell voids the warranty.  If you presume that warranties are written by the manufacturer in such a way that they will not to be used, it would also seem to proclaim that near 100% discharges over 300 cycles will deteriorate by 29% or less.

* Oh yeah, MaxxAmps also offers a nice looking set of 2300mah A123's!

Hi Gang,
Bob I was basing my statements on governed use for Stunt. Yes, your average will be very different!
The low voltage limit depends upon current as well. It perplexes me why the sellers don't own up to this.

In Stunt, our worst case currents are around 10C peak, while in pattern use they are maybe 17C peak. In F3A the battery voltage at the end of a vertical climb, in windy conditions that use the full 80% useful capacity, the last maneuver will be just at 3.40V (maybe as low as 3.35). For LiPos in Stunt, I'd be surprised if we ever see the voltage go below 3.4. Should the cutoff be set to 3.0V per cell for battery longevity? No. I'd suggest 3.3V if you have the option.
Dean

I would just caution that whatever LVC you are using, you don't want your motor to stop when you are making the climbout from the 4 leaf clover! That would give both the battery and plane a bad day!

Bob,
Has your motor actually quit like you describe above, or is that just a hypothetical comment?

If it has, I would think you could set up a soft low voltage cutoff that just starts to lower the available power when you hit the LVC. That way if you are that close, you would probably have enough power to make the deck!

I don't know how much you guys have gone in trying to optimize your propeller (I bet you have). I know you want to go fast, and then during the slow part of the flight, want to have power available in order to quickly get out of tough situations. Anything you could do to get a thin light prop--but one still able to hold together would certainly help on the energy conservation point. The light weight feature makes it easier for the motor to spool it up fast when you need the "r's" and need them quick in slow flight (saves power because it can be accelerated faster than a heavier prop). The thinness would help in the high speed part, just to make it easier to turn the rpms you need in high speed -- but without fluttering of course.

Also there are new high "C"  (40-45) packs by Thunderpower, Hyperion , and Polynoname (I think that is right) that have very low internal battery impedance. They should provide more available energy to the motor, but you need to be sure that you the high C rating doesn't translate to extra weight over lower C but higher capacity packs. Just a consideration.

Alan,

No, I haven't had a motor die just before landing, at least not in a contest, but that was prior to the new setup with better balance and the real possibility of much longer flight times. As it stands now I think I'm OK but it's something to watch for.

As for thinner props there is one catch. For the smaller carrier models reverse, CW, rotation is a huge advantage. The plane just stays out on the lines so much better than CCW rotation. This is especially important during low speed and windy conditions. That limits the props available to pushers. The selection is just not that great. I do see what you mean about lighter being better, that's pretty obvious. The other concern I have about light props is that we tend to run at higher RPMs than other applications. My current 15 sized plane has a KV of 1500 and spins pretty high. Some of the lighter electric only props may be suspect at high spin rates. I don't know that for sure but am just guessing. When I look at some of the E props I wonder if they can do 14,000 RPM and hang together.

So far I haven't had to go to the 40 C batteries. That's good because they do tend to be heavier (and more expensive). In fact my current model is flying on Thunderpower Pro Lites and is still within the parameters for the 20 C rating.

Thanks for the good comments though. Every little bit helps and gets me thinking in different directions. I like that!

Bob

Bob,
From the APC site http://www.apcprop.com/v/html/rpm_limits.html


Glow Engine and Thin Electric Props ( includes Folding Electric)

Maximum RPM=190,000/prop diameter (inches)
(For example, a 10x6 glow engine prop should be limited to 19,000 RPM)

So if you believe this, you should be able to put a lot of "r's" on a smaller prop.

From what I see, they have 7x6 (27 krpm), 9x6 (21 krpm), and 10x6 (19 krpm) pushers in the normal electric line---the above rpm limits don't apply to the slo-flight  electric prop (they are much lower).

Of course you might want to try one out in practice before using one in a contest!