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› Capacity to time Formula question...
01-23-2007 05:30 AM  10 years agoPost 1
Razmo

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Just flipping through this months RC Heli magazine.

There is talk of batteries and their capacity. Their example is that a 2000mah pack will last about 6 minutes drawing 20amps.

20/2000 = 1/10th of one hour (6 minutes)

What I can't make sense of is the more amps I draw the longer the battery lasts. What am I not understanding here?

Raz

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01-23-2007 06:27 AM  10 years agoPost 2
maxpower097

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The formula is faulty. Your gonna get I higher number if you increase your amps. That can't work. Basically you dont have enough info either. You will need to calculate the volts too. For instance the more volts or higher cell count you use, the less amps you will draw. So say you fly a 3lb heli and are drawing 40A on 6S. On 12S you would draw like 25A. So on 6S you will get 5min for 2000mah. On 12S you will get 10min for 2000mah. All numbers are fictional just to show how it works. So basically Volts = power Mah = capasity.

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01-23-2007 07:33 AM  10 years agoPost 3
Rob_T

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You have the calculation backwards.

capacity / discharge rate = duration .

So for the example you had

2 / 20 = 1/10 . Note the 2000 mah battery is written as 2 (for 2ah) so that the units are amps on both lines of the left side.

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01-23-2007 11:59 AM  10 years agoPost 4
stickyfox

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One thing I learned in engineering school that I actually still find useful is that working with units helps keep you out of trouble, pain tho it is.

Battery capacity is measured in milliamp-hours (mAH). Current is measured in amps, or thousands of milliamps (A or mA).

If you multiply mA times hours, you get mAH. So:

20,000 mA * 0.1 H = 2,000 * mA * H or 2000 mAH.

The answer came out in mAH, which is what you were looking for.

If you try to work units into the equation you posted, you get:

20,000 mA / 2,000 mAH = 10 (1/H)

An answer in something like inverse hours should be an indication that something didn't go right.

(Also don't forget to convert mA to A or vice versa before you start crunching numbers.)

-fox

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01-23-2007 01:04 PM  10 years agoPost 5
MikeC

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One thing I learned in engineering school that I actually still find useful is that working with units helps keep you out of trouble, pain tho it is.
Very good point! If you put the units into the conversion, the proper units will cancel out. If the conversion is incorrect the units left won't make any sense.

One other thing you have to consider when calculating runtime is the capacity will not be the same at all currents. At higher discharge currents the capacity will be lower and vice versa.

Mike

Century Swift, TREX 450, Blade MCX, Spektrum DX7

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