I am continuing to fly the SOS using the K2 3S1P pack and Master Airscrew10x8 standard wood prop to check the life of the K2 cells. I have decided to charge the K2 for the rest of its life at 7.5 amps, as waiting longer than 20 minutes is "just too long", when at the field! This should provide a good "torture test" for them as well.      On the 17th of June I did a couple of discharge tests to try and establish a baseline capacity for the pack. Unfortunately, I do not have a CBA discharger. I first used my SR Batteries charger and discharged to 7 volts at a 1-amp discharge rate, as high as the unit will go. I found the results of the test, not the cells' performance, unsatisfactory with the capacity of 2224mAh and the bounce back resting voltages of the cells, after the load was removed, were 2.795, 2.616, 2.652. Next I charged the pack again and used the unmodified AF 109 charger to discharge the pack. The AF 109 uses a 1.25-amp rate, and I was surprised to find that it took the pack down to 7.07 volts, almost the same as the SR charger. It showed 2405mAh taken from the pack and the bounce back resting voltages were 2.791, 2.668 and 2.658, which was very similar to what I found using the SR charger.      At this time I am not satisfied with the tests I'm using for capacity, but have recorded the results here so that I have a record of what I've seen. 06/21/08 Flying Update      The Son of Swallow was flown four times using the 3S1P pack made up of the K2 Energy cells and the Master Airscrew 10x8 standard wood prop. The performance is good, equal to the A123 pack using the 10x7 prop, but the power system did require a little higher average throttle setting than with the A123 Systems, Inc. 26650 cells. This was demonstrated by the higher average current draw when compared to the A123 pack, which results in approximately the same flight time as the A123 pack. It is also shown by the approximately 135-degrees F/57.2-degrees C to 140-degrees F/60-degrees C battery temperature upon landing on an 80-degree F/26.7-degrees C ambient day. Because the pack was "coming down" hotter, I did wait for the K2 pack to cool some between each 7.5 amp charge. The K2 pack's temperature, after each charge, was about 105-degrees F/40.6-degrees C. All flights were taken with the pack "hot off the charger." In many ways the K2 pack's characteristics reminds me of the Emoli pack I flew two years ago. 06/23/08 Balance Update      These cells seem to be staying in balance very well. I meausred the cell voltages of the resting pack that had been discharged with the Son of Swallow's last flight on Saturday, June 21. The cell voltages were; 3.234, 3.234, 3.230. 06/25/08 Flying Update Five flights on June 24. About 80-deg ambient temperature. The pack was consistently at 140-deg F, or higher, after a 6 minute timed flight. I waited for the pack to cool before recharging between each flight.      Even though I had only partially charged the pack at home and finished at the field, the first flight had noticably less power available. Once the pack had "warmed up", the performance was up on flights 2 through 5. I was consistently putting in about 2000mAh according to my AF 109. On flight 5 I decided to bump the timer up to 7 minutes. I went into a loop at about the 6-minute mark and the power was obviously down at that point. I just landed without flying to the 7-minute mark. Even though these cells have a higher listed capacity, 2500mAh, their "usable" capacity is about the same as the cells from A123 Systems, Inc., which are rated at 2300mAh.      The last flight was about 2 p.m. yesterday and using the CellMeter-8 the cells read 3.122v, 3.094v, and 3.088v on the morning of June 25. 06/28/08 Update - testing done on 06/27/08      I had not been able to get much more than 2000mAh, according to my AF 109, returned into the K2 Energy pack at the field when charging at 7.5 amps. When this testing was done, it was four days since the K2 pack was flown. I checked the resting cell voltages (3.120v, 3.091v and 3.085v). I decided to balance the pack with my A123 Blinky. The resting voltages, after being AF 123 Blinkied, were; 2.971v, 3.001v and 2.952v.      The pack was then charged at 2.5 amps (1C). The ambient temperature at the end of the charge time was 88-deg F and the pack's temperature was 93.1-deg F. The AF 109 indicated that 2329mAh had been returned to the pack. Before the start of the test using the MA 10x7 wood prop, the K2 pack's cell voltages read; 3.319v, 3.401v and 3.372v. After the test run, the pack's temperature was 110.5-deg F.      I captured full throttle data every five seconds on the Hyperion Emeter and recorded the data to the spreadsheet between each five data captures. The test sequence was run until the Castle Creation's Phoenix 45 hit the 6v LVC that I had it set for. I forgot to gather the voltages of the cells after the test sequence.      I had left my A123 pack partially charged. 902mAh were Zip charged into the pack according to the AF Whattmeter. The ambient temperature was still 88-deg F. The A123 pack's temperature was 95.3-deg F after the Zip charge. The same test was run using the MA 10x7 wood prop to the LVC for this pack. The A123 pack's temperature after the test was 107.5-deg F. The voltages after the test sequence were 2.811v, 2.863v, 2.803v.      A second set of data was collected for the K2 pack. The prop was changed to the MA 10x8 wood and the K2 pack recharged at 7.5 amps. 2083mAh was returned to the pack. Its temperature after the 7.5 amp charge was 103.1-deg F. The voltages after the test sequence were 2.982v, 2.998v, 2.957v.      The graph below shows the volts. The voltage of the K2 pack with the 10x7 load and 10x8 load can clearly be seen to taper off in voltage after about the mid-point of the run.

I have not previously shown RPM, because it does not present well with the other data, so here is a graph of the RPM, for those who may be curious. The RPM obviously parallels the volts.

I had noted earlier that both packs fly the Son of Swallow for 6 minutes of very aerobatic flight when the MA 10x7 wood is used with the A123 pack and the MA 10x8 wood is used with K2 pack.      The K2 pack with the MA 10x8 wood averages 32.24 amps for the 150 seconds, while the A123 pack averages 33.45 amps for 150 seconds.      I noted in my first flight report that I did not care for the way the plane flew with the K2 pack and MA 10x7 prop. The reason for that can be seen in the following graph that shows the Watts in.

Based on Watts in per pound, it looks like the A123 pack with the 10x7 should be far superior to the K2 pack with the 10x8, but flight testing has proven this NOT to be the case! Why? Pitch Speed. Here is a graph showing the pitch speeds.

The preceding two graphs, and my flight experiences, cleary demonstrate that, for planes that "fly on the wing" and that are "relatively clean", the pitch speed, when used in the ratio of pitch speed to stall speed, can even override the watts in per pound rule of thumb.      The Son of Swallow has a pitch speed to stall speed ratio of 4.09:1 when using the A123 pack with the 10x7 prop, 4.15:1 when using the K2 pack with the 10x8 prop, and 3.73:1 when using the K2 pack with the 10x7 prop.      I honestly cannot perceive any difference in the flyability, performance or duration of the Son of Swallow whether I am flying the K2 pack with the 10x8 or the A123 pack with the 10x7.      The next graph is similar to ones that I presented earlier and shows the volts and amps, but this time all the way to the 6v LVC.

Finally, I measured the resting voltages for the two packs approximately 18 hours after they were depleted in the tests. K2 - 3.004v, 3.028v, 2.983v A123 - 2.947v, 2.990v, 2.944v Note: The following flight tests were completed using the K2 Energy pack with the CC Phoenix 45 at low timing, using the Hyperion (mislabeled) Z3019-10 and the Master Airscrew 10x8 standard wood prop. 07/04/08 Flying Update      Three flights on the pack today in the SOS. Sunny with temperatures in the low 70-deg F range all morning and early afternoon.      The first charge was done at the field so that the pack would be "warmed up" before the first flight of the day. All charges were done at 7.5 amps and there was a ten minute "cool down" time for the pack between flights. Charge #1: pack temp. beg. chg. 73.6F, after chg. 99.2F, input 2200mAh, after flight 125.2F Charge #2: pack temp. beg. chg. 101.0F, after chg. was missed and mAh, I was talking, after flight 137.8F Charge #3: pack temp. beg. chg. 106.4F, after chg. 117.2F, input 2100mAh, after flight 131.7F Pack voltages after about one and a half hours from the third flight were; 3.203v, 3.202v, 3.194v.      Each flight was a timed 6 minutes and the LVC was never reached during any of the three flights. The charging was done manually using the AF 109 charger and the stated mAh were shown in Ah on the AF 109 screen. 07/05/08 Flying Update      Four flights on the pack today in the SOS. Sunny with temperatures in the low to mid-70-deg F range all morning and early afternoon.      Again, the first charge was done at the field so that the pack would be "warmed up" before the first flight of the day. All charges were done at 7.5 amps and there was a ten minute "cool down" time for the pack between flights. Charge #1: pack temp. beg. chg. 73.0F, after chg. 106.1F, input 2040mAh, after flight 131.6F Charge #2: pack temp. beg. chg. 113.7.0F, after chg. 118.1F, input 2000mAh after flight 135.2F Charge #3: pack temp. beg. chg. 105.8F, after chg. 118.6F, input 2010mAh, after flight 143.6F Charge #4: pack temp. beg. chg. 117.3F, after chg. 121.5F, input 2140mAh, after flight 148.6F Pack voltages after about one and a half hours from the fourth flight were; 3.142v, 3.145v, 3.121v.      Each flight was a timed 6 minutes but today the LVC kicked in near the end of the forth flight, cutting it just slightly short of the six minute mark. I can tell I was pushing it harder on the fourth flight, as the pack temperature after landing was the hottest of the day. The ambient temperature had not risen much as it was only about 75-deg. F at the end of the fourth flight. The charging was done manually using the AF 109 charger and the stated mAh were shown in Ah on the AF 109 screen. 07/06/08 Flying Update      Good day of flying with temperaturer in the mid-70's F. Only got three flights, even though I was at the field quite a while. Helped a friend find a plane that went down in the woods, and that took quite a while.      The first charge was a partial charge at home at 2.3 amps and then topped at the field at 7.5 amps. Put in 1250mAh at home and another 800mAh at the field Charge #1 see note above, after flight 131.1F Charge #2 pack temp. beg. chg. 108.3 F, after chg. 123.4, input 2100mAh, after flight 138.0F Charge #3 pack temp. beg. chg. 109.8, stopped charge, looked for plane and then finished charge, after flight 136.5F 07/11/08 Flying Update and flight time clarification      Flew the SOS three times after completing the Mid-Am preparations, but did not record any data.      I've noted that I've been flying this pack for 6-minutes. That means that I start the timer on my Hitec Eclipse 7 just before taking off, then throttle up and take off. The timer on the transmitter beeps a few times at 5 minutes and 45 seconds. I generally finish any maneuver I'm doing and then set up for a typical "pattern" type landing, in no hurry. Wheels down varies from a little less than 6 minutes to a little over. If the grass permits easy taxiing, I generally taxi back to the flight station that I'm using. The typical flight, to keep "pressing the pack" includes a lot of verticals; stall turns, consecutive loops, loops with snaps, vertical eights, etc. 07/23/08 IR test      Since I have been beta testing the K2 Energy 26650 2500mAh LiFePO4 cells, I wanted to get a direct comparison of the internal resistance to the A123 26650 2300mAh LiFePO4 cells I had been using. Based on my comments from above, I knew that it was higher for the K2 Energy cells because of the pack's higher measured heat after charging and flying, but I wanted to know by how much. Using a FMAdirect CellPro 10S answered that question!      I ran three charge cycles at the same time on the two 3S batteries, one at 2.3 amps, one at 5.0 amps and one at 7.5 amps, using the 3.65v A123 default on the charger at the time I tested. By checking the saved .TXT files via the spreadsheet method I confirmed that the K2 Energy cells had a lot higher internal resistance, when compared to the A123 cells.      The spreadsheet shows that the IR for the K2 cells is more than three times greater than the A123 cells when charged at the same rate at the same time. This clearly confirms why the K2 pack gets "hotter" during charge and discharge, and why it is "harder" to get energy out of the pack. It also explains the greater voltage depression of the K2 pack under load.

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