Discharge rate and capacity

[hyrulejedi86]

I was bored today and looking around hobbyking when I came across a page that goes over some basics of batteries. They posted the following:

"Discharge (C)
Discharge is the amount of power the battery can 'push' out and the number shown '20C' is an multiplication of the capacity. For example; A 20C battery can discharge at 20 x 2,000mAh which is 40,000mAh or 40Amps.
This is an important number if you know your motor requires a certain power level. In addition to this, batteries have a 'Burst' rate, which is the amount of power the battery can discharge for a short period, usually 10-20 seconds. A typical battery label may show 20-30C, this would mean a 1,000mAh battery can discharge 20,000mAh constantly or give a sudden and short 10-20 second 30,000mAh (30A) burst of power.
Tip: A higher 'C' rated battery will last longer if run at a lower 'C' rate. Example: a 30C battery run at 20C maximum will have a longer cycle life than a 20C run at 20C each flight. "

It's the bolded tip that I'm curious about. Is this true? And if so Why?

[kookie_guy]

Their translation to english is a little wrong....

First off, the battery does not "push" out power. It supplies the power that an ESC PULLS. The motor pulls power from the ESC, the ESC pulls power from the battery. Rest of it is correct.

Second, the tip is worded wrong as well. You don't decide at what C you run the battery. Your ESC is what will pull the amps. If your ESC is pulling 100A, and you have a 20C 5000mah pack, that means you are running that pack at it's rated discharge rate (5000mah x 20 = 100A). This will shorten the battery's life. However, if you run a 40C 5000mah pack, it's now capable of a 200A discharge. Your ESC still only pulls 100A. So the pack will not heat up as much, and essentially have a longer life.

Remember, just because a battery is capable at supplying 200A, does NOT mean that it will. It will only supply what the ESC requires. So they are correct in the bold tip, just worded it wrong. Rule of thumb is, buy the highest C rated packs you can afford. Even if you don't need that kind of discharge right now, who knows what will happen in the future.

[hyrulejedi86]

Ahhhh, Well, all of what you said I already know but I see where the confusion was. They did word it badly, I thought they were a saying that the higher C pack would last longer in a single flight than a just sufficient C rated pack. But it was saying that it would allow the battery to last longer as in the number of charges and discharges before it reaches the end of it's useable life.

Well said! And Thanks, but this leads me to a new question.

How are two battery packs that are the same capacity but different C ratings able to safelty put out their respective amperages. I.E. what is different in a 20C 5Ah pack vs. a 40c 5Ah pack that allows one to put 100A and the other to put out 200A? They both hold 5Ah worth of electricity why can one more easily put out more amps?

[Fluid]

They are not able to do so. You do not want to use packs with different discharge rates in the same boat at the same time. The lower "C" rated pack will heat up faster than the other due to its higher internal resistance, and the higher-rated pack will deliver power "faster" since it has a lower internal resistance. Depending on the discharge rate, the high C pack can over-discharged while the other can overheat. Only use packs with the same capacity and C rating - and the same brand and label.


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[hyrulejedi86]

I wasn't referring to using the two packs in the same setup. I ment my question based on an individual pack use, what makes the two packs different (chemistry, wires, internals, resistance anything?) so that they can put out such different amps.

[kookie_guy]

I believe it's the internal resistance, but I am not certain. Could also be beefier internals. But again, I'm not certain.

[Boaterguy]

in many ways batteries can be linked to gas tanks. Capacity is the size, or how much the gas tank can hold.
the discharge rating is the amount of Gas that can be coming out of the tank, so kinda like the size of the whole, although that is determined through the relation to the size of the pack (capacity)
I don't know what changes the C rating, but I don't think it is internal resistance, I'm pretty sure its size of the packs and the chemical makeup of the pack.