Thread: Build Log 100% scale, Electric Lazair
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Old Jun 23, 2011, 04:48 PM
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Dale
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Joined Jul 2004
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Jeff,

I have convinced myself that discharging these cells to 3.3v (nominal 100% discharge level) at my expected cruise current does not significantly affect their life. I have discharged to 250 cycles and have retained 75% of their original capacity, this is acceptable to me. I do not plan to discharge to 3.3v in actual use very often and I still believe that actual endurance determination should go to 3.3v/cell.

Watts are used to specifically take the voltage variable out of the equation.

I found that these batteries will not provide 5 amps for 1 hour but interestingly they do provide their nominal whr at <= 5 amps discharge since they start their discharge at nearly 4.2v. Their nominal whr is 5*3.7 or 18.5 whr. This means that the optimism you think you have turns to pessimism since at a 5 amp discharge, they provide more kwhr above 3.7v than below it.

I think that you are pessimistically using 40A for 20 lbs of thrust from my linked data. (also this is static thrust and does not really represent the rpm/amps/volts required to get 20 lbs of thrust at 30 mph). If you look at the graph provided, 20 lbs of static thrust can be interpolated to about 1500 watts. Lets say that this will occur at about 44.5V (another educated guess by looking at the data table). So, 1500/44.5 = 33.7 amps

And at discharge to 100%, using your method would be 67.4/5 or 13.5 times the 4 batteries used in the test per hour of endurance or 54 packs per hour.

And 96 would last 96/54 or 1.8 hrs. This is actually closer than I expected since another large discrepancy between this test data and the 96 pack I have used is that I am using a 16s voltage on the motor and the fact that there are a lot more parallel legs in the 96 pack than the 4 pack of the test. This means that there will be much less voltage sag at the same power levels and the batteries should provide more whr because of this.

I am not as happy with this AmpHr reasoning as my Whr reasoning since more assumptions were made, especially about conditions at 30 mph. In my whr scenario the only use of the test data was to determine that the prop/motor combination seemed to have the efficiency required at the expected cruise thrust.

Am I clearing things up or clouding them over?

Dale
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Last edited by KiloOne; Jun 23, 2011 at 05:01 PM.
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