Amp draw from a Li Poly battery - RC Groups
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Mar 01, 2007, 07:51 PM
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uniquewon's Avatar

Amp draw from a Li Poly battery

I need some help with this. Everything I read about amp draw, ESC and Poly batteries says that even though you are using partial throttle, you are drawing max amps from that battery. I don't understand this at all since most Poly batteries are rated a draw rating like 20C, 40C burst. Well...what would burst be in relation to how a motor is set up with prop, esc and battery.

In other words - if I have a setup that draws 18 amps full throttle and my battery is rated at 10C and is a 2600mah 3S battery, what would burst amps be and how would I obtain them anyway. I'm told that even though I throttle back I am drawing 18 amps from the battery......???????

Help any one with a good explanation of this.

BTW, all of the above assumes I have a setup that is propped for the correct amp draw.
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Mar 01, 2007, 07:55 PM
I havent decided yet
drawing full current at partial throttle is fairly technical. they pull very quick spikes of 18amps but a half throttle the average current ( this is what really matters in this situation) might only be 6 amps. so yes, the battery does spike the full current but the average is all we see for our usage.
Mar 01, 2007, 08:08 PM
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tommyeflight89's Avatar
What you are reading is correct. That is how ESC's work, they switch power on and off to the motor at high frequency through the use of FET's.
This on/off switching occurs at a higher frequency, so your battery really only sees the average amp draw. When you throttle back, the switching occurs at a lower frequency (on/off, on/off etc). Your battery sees a lower average amp draw. I guess the key word here is average, it may be switched on for a milisecond, then switched off for 2 milliseconds. It happens so fast though the battery only sees the average. To prove this, use your watt meter... Low throttle would be 2amps lets say, and full throttle would be 30amps. Your wattmeter is reading the average.

Continuous and burst C ratings come from the battery manufacturer. Noone can tell you them without knowing what battery you are talking about.

Here's an example: We have your setup that draws 18amps at full throttle. You are looking for a little moreperformance, and you can afford the extra current (10Cx2.6ah=26amps max cont.). You find a larger prop, stick it on there, and find that the setup draws 52amps. Full throttle is 52amps, and your battery is rated for 26amps continuous. You could fly the model around hapily at low throttle without damaging the Lipo, but when you want to-open her up! You will be using the battery at it's burst discharge current rating (Let's assume it's 20C-remember that the data is provided by battery company), Don't do this for more than 30s or so or you will begin to damage the Lipo!

Edit: dmiller beat me to it!
Mar 01, 2007, 08:21 PM
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vintage1's Avatar
Whats a given ESC/motor draws at part throttle is HIGHLY complex and depends on details of circuit design, FET switching speed, ESC switching frequency, the inductance and resistance of the motor, and the battery wires, and the size of the capacitor across the ESC input.

One can only be sure of average figures.

Bust capability has little to do with that..burst capability is what the cells will do without internal damage..relying on their thermal inertia to prevent overall overheating.

i.e. if your interest is 3D flying or performance sailplanes, where you only ever use full throttle to do a 30 second climb, or pull out of a hover, then it has some relevance.

Otherwise its just marketing 'pixie dust'
Mar 01, 2007, 09:34 PM
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uniquewon's Avatar
I guess I may have interpreted some other stuff about amp draw wrong. That average is what is important, yes - my watt meter would read that as would most all other meters.

Thanks for the great explanation.

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