J2M
Senior Heliman
Location: UK
| Hi Steph,
" you are probably pulling 4-6 amps in the hover and possibly 17 -18 amps full climb this is no where near the 24 amps constant and not touching the 30c burst then why are your packs getting anywhere near 50c or above surely the packs should not even be seeing 30c"
Agree with you on hover current but I'm not sure where you get the 17-18Amps full climb figure. This would equate to only about 375W which is nowhere near enough to get the flight performance that I have demonstrated on video. Ready-to-fly, my Rex weighs 830g or 1.83lb. I don't have in-flight figures for my 6S Rex as I just haven't had the time or inclination to strap on my Eagle Tree data logger but from past experience with everything from Hornets to Ion-X's, I am confident that the flight performance equates to a set-up with at least 250W/lb peak power and probably more. Hence my estimate in an earlier post of at least 500W peak power and peak current of at least 24A (20C) or more (assuming 3.5V per S under load).
You do raise an interesting question about why packs get hot (in this and other applications). I've already posted that a 7 minute flight drains 1.0Ah from the Rex packs indicating that average current consumption over a flight is only 8.6A even under continuous 3D. Looked at this way, it may seem surprising that the packs are at all warm. (By the way, the same has been true of every data set that I've analysed from my data logger. No matter how hard you fly the heli, it is difficult to get average current to be more than double hover current.)
I think you have to step back and ask what is the source of heating. It's my belief (and battery experts please step in to correct if necessary) that the principal source of pack heating for lipo and most other secondary cell chemistries is I2R dissipation (ohm's law) due to the pack's internal resistance rather than any chemical reaction effects. If this is true, then pack heating is proportional to the square of current so that short duration high current peaks during climbouts or 3D manouevres have a disproportionate effect on pack heating. The conclusion must be that if you move to packs with a higher peak current capability and use that capability, you'd expect them to run hotter - there has to be a price for performance!!
Which brings me back to the FlightPower Evo20s. The 3D performance of an electric heli is most easily measured by just one number - peak w/lb figure (and I'm talking about peaks lasting for meaningful periods of time, not a few milliseconds as quoted by some e-flight vendors). In turn peak w/lb comes down to current delivery and voltage stability of the battery pack ( assuming all other factors are equal). All I was trying to highlight in my posts is that the Evo 20 packs deliver in spades on these two measures and the performance of my Rex has been transformed simply by fitting them. I sent the video to the FlightPower people because I was amazed at the performance I was getting from the little Rex. I have absolutely no doubt these packs can deliver 20C and more. If you don't want to believe it or are concerned about pack heating, just buy something else!!
One more thought, perhaps I've misunderstood your post but it seems that you think that the Evo20s or any other lipo isn't a genuine 20C lipo unless it doesn't get warm under that sort of load. At that rate of current draw, the pack will, by definition, be empty in 2.5 minutes or less. I defy you to find any pack from any manufacturer anywhere, lipo or otherwise, that won't get seriously hot under that sort of duress!! |
