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Old Feb 21, 2014, 04:25 PM
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Battery Amps Available as Depleting

My understanding is that to calculate an adequate battery for a given motor you need to:
Battery: mAh * "c" (constant discharge rate)/1000=available amps
5000mA * 25c / 1000 = 125 available amps can be drawn from this battery. If given motors are 25A peak, this battery will work with having additional overhead for system needs.

My question is, how does this play out as the battery depletes? Lets say when the battery is down to 2000mA.

2000mA * 25c / 1000 = 50A. This is not sufficient to power the motors. What is the equation I am missing, or am I just wrong all together?
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Old Feb 21, 2014, 05:06 PM
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Discharge C rating is based upon total battery capacity, not remaining battery capacity. Note that maximum C rating is typically wildly overstated in most cases and drawing the maximum will have you purchasing new batteries (and perhaps building new planes).
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Old Feb 21, 2014, 06:02 PM
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Quote:
Originally Posted by mrforsyth View Post
Discharge C rating is based upon total battery capacity, not remaining battery capacity. Note that maximum C rating is typically wildly overstated in most cases and drawing the maximum will have you purchasing new batteries (and perhaps building new planes).
In the example above, the battery was not at max capacity. It was just an example for math purposes. That being said, I am trying to get an answer for the math. If the math is true at max capacity, it should be true at nominal capacity.

I guess I should consult a battery manufacturer for numbers. Unless an electrical engineer wants to weigh in.
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Old Feb 21, 2014, 06:44 PM
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It escapes me how I could be more clear.

The ability of a battery to deliver current (C rating) is based upon total battery capacity, not percent charged or remaining capacity.

e.g. - A 5000mAh battery with 25C rating can deliver 125A, regardless of charge state (if you happen to believe the 'C' rating).
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Old Feb 21, 2014, 07:05 PM
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Quote:
Originally Posted by mrforsyth View Post
It escapes me how I could be more clear.

The ability of a battery to deliver current (C rating) is based upon total battery capacity, not percent charged or remaining capacity.

e.g. - A 5000mAh battery with 25C rating can deliver 125A, regardless of charge state (if you happen to believe the 'C' rating).
Thanks, I will check into that. Are you a hobbyist who is relating what you have heard, or are you speaking from authority?
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Old Feb 21, 2014, 08:22 PM
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Originally Posted by katman1260 View Post
Are you a hobbyist who is relating what you have heard, or are you speaking from authority?
I'm speaking from ~9 years of direct experience with using/testing lipolys and ~50 years of accumulated common sense.
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Old Feb 21, 2014, 08:38 PM
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katman, trust me - Mark is correct and knows what he is talking about. He has forgotten more about the theory and practice of LiPos than most of us have learned.

Current capability is based on label capacity and C rating of the LiPo.

The label capacity may or may not be accurate and in practice real packs do deliver slightly less capacity as the load current goes up (because the energy losses due to heating go up). In addition the C ratings themselves are fairly arbitrary since they depend on what assumptions the vendor accepts for allowable temperature rise and allowable voltage drop - if they just don't make them up entirely which is fairly common.

There is a large body of data that shows that most manufacturers C ratings are ridiculously optimistic for reasonable pack life and many of us take half the label figure as a good practical guide for sport flying.

Nevertheless theoretical allowable maximum current is calculated from fully charged capacity and C rating. It does not vary with state of charge.

John
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Old Feb 21, 2014, 11:22 PM
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jj604

I hope you are a professional speaking from a position of authority and not a mere hobbyist spreading other hobbyist's wild guesses.

As he appears to take that attitude one wonders why he asked here at all. We are just doctors, cleaners, lawyers, soldiers, garbage wagon drivers, accountants, servers in a burger chain, airline pilots and similar rubbish, aren't we?
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Old Feb 22, 2014, 12:20 AM
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We know that. And mrforsyth/jj604 don't have to prove themselves to the OP, who doesn't seem to want a 'hobbyist'. He even said he would 'check out' the reply, as in:

"What's the time/"
"Five past two"
"I don't believe you, I will check it out with a clockmaker"
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Old Feb 22, 2014, 02:46 AM
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Wow, a little testy are we.
Thanks JJ604 for your reasonable reply. As for the rest, I don't know anyone here and Lord knows, no one on forums just repeats what they have heard elsewhere, so forgive me for not taking the first answer without any data.

My question was reasonable and was not meant to damage sensitive egos. I prefer to know cause and effect rather than just, well someone said this on a forum and I was ridiculed for not taking his statement as fact.

Quote:
"What's the time/"
"Five past two"
"I don't believe you, I will check it out with a clockmaker"
"why has the clock stopped?
because they do that.
OH, Ok."
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Old Feb 22, 2014, 07:24 PM
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battery power available

How much of the battery rated capacity can you use assuming the speed controller shuts down at a safe voltage? I am using 10c2p packs of 2200ma 20c battery to power a motor drawing 22amp at half throttle. Seems using half of the 20c rating should give 7.3amp per cell or 73amp per 10c pack safely as the two packs in parallel double the capacity?
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Old Feb 22, 2014, 10:08 PM
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Quote:
Originally Posted by Big Cubs View Post
How much of the battery rated capacity can you use assuming the speed controller shuts down at a safe voltage?

You should never use more than 80% of the available capacity.

I am using 10c2p packs of 2200ma 20c battery to power a motor drawing 22amp at half throttle. Seems using half of the 20c rating should give 7.3amp per cell or 73amp per 10c pack safely as the two packs in parallel double the capacity?

Your understanding of current and C rate is flawed. The maximum safe current of a 2200mah 20C battery is 44 amps (2.2A * 20 = 44A). So running the battery at 10C would be 22A. The number of cells in the pack does not affect the current rating.

I may be confused about your battery configuration. You say it's a 10S2P (you said 10c2p but 10S2P is the correct convention) 2200mah 20C battery. But then you mention two packs. Do you have a single 10S2P 2200mah 20C battery or do you have two 10S1P 2200mah 20C batteries wired in parallel?

If it is the latter then your capacity is 4400mah 20C.
If you have not already done so may I suggest you read this: http://www.tjinguytech.com/charging-how-tos
It will explain the relationship between series and parallel connections and how they affect capacity.

Glen
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