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Old Dec 06, 2011, 10:47 AM
Airplane Killer
SumthinsFishy's Avatar
Orange County, CA
Joined Oct 2009
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But since you've gone from 1000 to 2000 mah by putting the batteries in parallel, the discharge current capacity (C rating) goes from 20A to 40A (20x1000 vs 20x2000).
Dave
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Old Dec 06, 2011, 07:43 PM
Microbiology at it's best.
beerbrewer's Avatar
Waverly, Nebr.
Joined Apr 2009
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Water analogy conundrum

If I have 2 equal 1000 gal (1000 mah batteries) tanks, the depth means volts (pressure), the dia of or volume represents current capacity (amps), the drain pipe is the current capacity or flow of the system. With this in mind, if each tank drains at 20 gal/min (20C) then would it not be true that the combined output volume of the two be 40 gal/min (40C)? Doubling the dia of the pipe equals a 4x in volume of water (current) for 80 gal/min. The only change is that I am only adding a second equally sized tank with a same dia drain pipe with everything else being the same. Even if the pipe beyond the junction of the 2 tanks is 10x, the max flow rate combined is 40 gal/min from the 2 tanks. With that said, would not my equivalent capacity be 1000Mah @ 40C to the ESC?

Am I trying to tinkle up a rope here and getting all wet? A correct matimatical answer right now would be great since I don't like to get wet when it is cold. It was -3 F this morning in Lincoln, NE.
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Old Dec 06, 2011, 07:56 PM
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^ this all hurts my head

Just go to Hobby King and by the right friggin battery.
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Old Dec 06, 2011, 11:13 PM
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Hurts my head also, don't want to hurt my pocket book either. Just looking for answers.

The Missouri Valley Jet Scramble only allows fan/turbine power, no props allowed. There has never been a Geobat, I want to be the first. They shut down an entire airport for this. I want to to get this right.

Has anyone ever seen $10k jet do a belly skip on an asphalt runway at 240 mph and still stay flying? I have.
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Old Dec 07, 2011, 12:50 AM
Airplane Killer
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Orange County, CA
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I don't know about water, I just know about electronics. Its still 20c. But you have 2000mah instead of 1000mah.
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Old Dec 07, 2011, 08:23 AM
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1 cell = 1300mah 25c 4.2volts, max amps 25*1.3 is 32.5, max power 32.5*4.2 136.5watts


3s1p = 1300mah 25c 12.6volts, max amps 25*1.3 is 32.5, max power 32.5*12.6 409.5watts


1s3p = 3900mah 25c 4.2volts, max amps 25*3.9 is 97.5, max power 97.5*4.2 409.5watts
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Old Dec 07, 2011, 08:51 AM
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From 2006

http://www.rcgroups.com/forums/showp...78&postcount=1
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Old Dec 07, 2011, 09:00 AM
Mr. Breeze
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Tell em big play.
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Old Dec 07, 2011, 10:53 AM
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Thank you everyone, I now know that I need a 40C and can't cheap out on 2 - 20C.

Another 2 things I know are; with nitro smoke good, with electric smoke bad!

Keep'in the smoke in.
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Old Dec 07, 2011, 11:20 AM
Mr. Breeze
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That's funny right there brewer. For sure no smoke in electrics! You are not going to notice the diff in c rating on a plane like the geobat. Maybe on a funjet or another high speed plane, you could benefit from higher c. 20c will do just fine on a small out runner.
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Old Dec 07, 2011, 12:58 PM
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Hey Jazz,

The hope in my ill fated logic was to be able to use a second 20C for the future EDF GB build to power it at its max. I am dreaming of getting 715 grams of thrust with an efflux of 200+ mph which will require a 4S 40C battery along with the compatible ESC.

My first builds are going to use 400-450 motors probably using 3S 20C batteries which are much cheaper. I want to get my foamie building techniques down before I build something crazy using my EDF.
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Old Dec 07, 2011, 02:12 PM
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Quote:
Originally Posted by beerbrewer View Post
Hey Jazz,

The hope in my ill fated logic was to be able to use a second 20C for the future EDF GB build to power it at its max. I am dreaming of getting 715 grams of thrust with an efflux of 200+ mph which will require a 4S 40C battery along with the compatible ESC.

My first builds are going to use 400-450 motors probably using 3S 20C batteries which are much cheaper. I want to get my foamie building techniques down before I build something crazy using my EDF.

I think you're looking at this wrong, go back and look at the calcs in what bigplay posted; you're expecting "4S 40C", but what you need to base this off of is what is the amp requirement. BECAUSE , a 10C 4AH battery can provide the same amp draw as a 40C 1000mAh battery. so, when you talk about 'c' rating, you really need to be understand what is your amp draw at a given voltage, then you know how to size your packs. Both for the ability to deliver the amperage, as well as the length of time you want to fly for. (balanced against the weight of the pack)

Pecanpatch and you are mentioning 400-450 motors, but that number is arbitrary from the manufacture - it's based on brushed can motors, and isn't a good way to try and describe a brush-less set up. You need to know the KV, (RPM per volt) , the amps, and the volts. EDF is going to be significantly different than a propped motor.
if you think you have the right motor, use a calc to make sure you have the watts needed to fly the model: http://www.brantuas.com/ezcalc/dma1.asp

Personally, I'm shooting for 150-200 watts for my first build, I'll be running a 1200mAh 3S pack on a 1750kv motor with an 8x4 prop (TP 2408-21)
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Old Dec 07, 2011, 05:06 PM
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forget the calculations get one of these.

http://www.rcuniverse.com/magazine/a...article_id=468

you can calculate till the cows come home but having an inline meter on a real world setup will always have the final say in my book. Plus it is real handy when chasing a malfunction.
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Old Dec 07, 2011, 05:48 PM
Microbiology at it's best.
beerbrewer's Avatar
Waverly, Nebr.
Joined Apr 2009
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What I have

Ok guys, I will give the numbers and you can tell me if I am all wet.

The motor:
kV 3790
input VDC; 7.4-14.8
max constant I; 25A
Max surge I; 50A
Max constant P; 278W
Max surge P; 740W

EDF w/4S@max
Amps 32.6
Watts 482
Static thrust (g/oz) 721/25.4
Efflux 217.8 mph

Logic to keep the smoke in, 40C battery and ESC
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Old Dec 07, 2011, 06:01 PM
Airplane Killer
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Orange County, CA
Joined Oct 2009
1,709 Posts
We're missing a key piece of information. I believe you said you plan to use a 4s battery (14.8v) but what mah? If you are using a 1000mah, then you need 35C (1 x 35 = 35) to safely run the motor at 32A. If you are using an 1800mah battery 20C is adequate (1.8 x 20 = 36).
Dave
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