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Old 05-05-2009, 08:15 AM   #16
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A 40C 5000 can deliver more current than a 30C 5000 given an equal load because the 40C has a higher voltage at that same load. The higher voltage translates into an increased current which means more torque at the coils at the same load. Back emf is an induced voltage, but it can also be thought of as a current source.

Pablo, the C rating is an ability of the battery to retain its voltage, not capacity, under a high current draw and it is a measure of internal resistance.
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Old 05-05-2009, 08:44 AM   #17
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Originally Posted by mattnin View Post
Pablo, the C rating is an ability of the battery to retain its voltage, not capacity, under a high current draw and it is a measure of internal resistance.
Mattin, the following is from Danny/SMC :
"The standard method to test C rate is look at the capacity of the cells at full C rate discharge. Our cells have to remain at 90% of there rated capacity at a full discharge. Some manufacturers use 80% to get there C rate."

According to this it actually is about retaining capacity not voltage. It may be that in general cells with a higher C rating happen to have a lower IR too, but IR may not directly correlate to C rating.

The point I'm really trying to make is that if all we do is look at labels that claim a high C rating we don't always get the best performing battery. Like you have started to do, we need to test them against each other to find out which perform the best.
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Old 05-05-2009, 09:22 AM   #18
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I am pretty sure there are several typos on Danny's part because that quote does not make any sense. A full C rate discharge basically means he is emptying the battery at 40C x capacity (remember, capacity has a time component to it). And then he says once this is completed, there is still 90% capacity!

I agree that we cannot trust the C rating from different manufacturers though. They all come up with their # a different way. I thought C rating was derived by drawing the max amount of current until full discharge and the battery never reached its cutoff voltage.
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Old 05-05-2009, 10:13 AM   #19
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Quote:
Originally Posted by mattnin View Post
I am pretty sure there are several typos on Danny's part because that quote does not make any sense. A full C rate discharge basically means he is emptying the battery at 40C x capacity (remember, capacity has a time component to it). And then he says once this is completed, there is still 90% capacity!

I agree that we cannot trust the C rating from different manufacturers though. They all come up with their # a different way. I thought C rating was derived by drawing the max amount of current until full discharge and the battery never reached its cutoff voltage.
I think he means at the full 40c discharge(208amps) the battery should still perform to at least 90% of its rated (5.2 amp hour) capacity.
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Old 05-05-2009, 10:21 AM   #20
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C ratings are all just hype. Until there is an industry standard like we had with nihms this debat will go on forever. True you can feel a difference between certain companies packs and others but are we really going that much faster with the so called better packs? Guys are claiming 2 or 3 laps faster with brand X lipo's but my question is did you just drive better on this run with no crashes???? It's all hype. IMO Happy racing
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Old 05-05-2009, 06:20 PM   #21
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The maximum current test for a battery is a short circuit test. Batteries can deliver more current than the C rating continuosly, but they do damage to their internal structure (which effects capacity) and in the extreme they get hot and dangerous. As Danny mentions SMC have their own 90% of capacity retention measure for C ratings.

I think the important point is, given two batteries of identical C rating (using the same rating method) the better one is the one that has lower IR since it will hold a higher voltage under the same load.

As a side note, on a 10sec lap of a technical track using a 10.5t motor, i average 20A from the battery with a max between 50-70A. With a 13.5t these numbers will be lower again so it's no real wonder you see diminishing returns between 28C and 40C as neither battery is really working hard in the first place.
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Old 05-05-2009, 08:15 PM   #22
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I couldn't have said it better! It sure would be nice to see a scientific side-by-side comparison of several leading manufacturers best batteries. Even something as simple as a dyno test with the battery being the power.

Quote:
Originally Posted by STLNLST View Post
C ratings are all just hype. Until there is an industry standard like we had with nihms this debat will go on forever. True you can feel a difference between certain companies packs and others but are we really going that much faster with the so called better packs? Guys are claiming 2 or 3 laps faster with brand X lipo's but my question is did you just drive better on this run with no crashes???? It's all hype. IMO Happy racing
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Old 05-05-2009, 09:51 PM   #23
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Originally Posted by Pablo Diablo View Post

It may be that in general cells with a higher C rating happen to have a lower IR too, but IR may not directly correlate to C rating.
DINGDINGDING we have a winner. Do you really think that there would be any performance difference between 2 batteries, one that can put out 150 amps continuous (5000 30c) vs one that can put out 200 amps continuous(500 40c) In an onroad application? No, especially not in a stock/superstock class, where theres probably not even a 100 amp spike. Its the IR that makes the performance difference, always has been, always will be. Lower IRs flow current faster and drop voltage less under load, and that will be the difference maker. IR has always been the most important number on Sub-C data labels (assuming the runtime isnt poo). Now that there is a new # for everyone to drool over, it seems like people have forgotten about IR
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