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Old Jan 04, 2011, 08:57 PM
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LIPO C Ratings

Does anyone know if there is a standard that is used to measure the C rating of a LIPO?

As I understand it, the higher the C of a battery, the lower the internal resistance. So as you pull more and more amps from a battery at the C rating, the internal resistance is low enough to stop the battery from overheating and burning itself up.

So a LIPO will actually give way in excess of it's C rating when shorted - so there must be a test that is performed on a battery to determine how many amps it can produce at a minimum voltage and without causing overheating damage to the pack - but what are those numbers?

So a 2200 20C battery should be able to deliver 44 amps - what voltage is that at and what is the max temperature that the battery is allowed to reach?[/LIST]
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Old Jan 04, 2011, 09:47 PM
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A "C" rating is a measure of the pack's highest safe current (amp) level. You get it by multiplying the pack's capacity by the "C" rating.

A 2000mAh lipo with a 20C rating will safely supply 40A of current; a 1500mAh 20C lipo will supply 30A. You are correct.

The voltage is somewhat up-in-the-air; fully-charged voltage for a lipo cell is 4.2V but this will drop as you increase the current load (and go back up when you decrease the current). Generally you'll be best off just not overthinking it and limiting yourself to whatever the said "C" rating dictates; the differences caused by temperature and voltage fluctuations normally won't have too much of an effect.

But then you get into burst ratings, which might be a whole lot higher than the continuous rating but you can only run it at those current levels for X amount of seconds or minutes. None of those values are set in stone; a pack's "C" rating can change over time/with use, and abusing packs (charging them at too high a current level, discharging too low, drawing too much power) will negatively effect both the current capabilities and the overall life of the pack.....
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Old Jan 04, 2011, 11:01 PM
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Originally Posted by TP16 View Post
A "C" rating is a measure of the pack's highest safe current (amp) level. You get it by multiplying the pack's capacity by the "C" rating.
"Safe" is subjective. This doesn't answer the OP's question, which is a good one: how exactly is the C rating measured?

Now clearly C rating has something to do with internal resistance, thermal mass, temperature rise, heat dissipation and airflow over the battery. If I were to hazard a guess: C rating is determined by measuring the output current that produces a temperature rise of T degrees, with some assumed airflow/cooling environment (or perhaps in still air.) That magical measured current, divided by the amp-hour capacity, gives the C rating.

Different manufacturers/brands might differ in their choice of T or their assumed airflow/cooling environment. They may even differ on their determination of mAh ratings for the same physical battery.
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Old Jan 05, 2011, 09:56 AM
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Thanks rafe_b - that's exactly my point

There are $10 lipo packs and $50 lipo packs all made from Chinese cells.

If they are both rated at say 20C I assume that one will do better (generate less internal heat and give a higher voltage at 44 amps) than the other?

So I am wondering if there is some measurement standard that says that a pack that is run at it's max C rating MUST give (say) a minimum of 3.5V per cell at that amperage.

It's not so important while I train with my high wing, but I really want to get into jets eventually and it seems to be much more important due to the fact that they run at full or at least very high throttle settings most of the time.
So as I am buying LIPO packs like they are going out of fashion (and that the Chinese have cut back their production of Lithium by 10%) I am trying to see whether it's better to buy 4 cheap packs or 1 expensive pack.
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Old Jan 05, 2011, 10:12 AM
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Quote:
Originally Posted by steveclv View Post

There are $10 lipo packs and $50 lipo packs all made from Chinese cells.

If they are both rated at say 20C I assume that one will do better (generate less internal heat and give a higher voltage at 44 amps) than the other?
There is no standard, unfortunately. Many many threads have been generated trying to get a standard at least identified so we could all speak the same language when we discuss C ratings. At least one of our vendors here on RC Groups has devised their own tests to determine what they sell their packs at.

As for a $50 pack delivering a higher voltage at a specified amp draw, again, there is no definitive answer on that, as just as many people swear by the $10 pack as swear by the $50 pack relative to voltage, amp draw, and battery life.

All you'll find really, is mostly anecdotal evidence, and very little in the way of controlled tests. A problem considering that the vendors and suppliers keep changing the names of their batteries and battery chemistries every few months it seems.

Sometimes a certain vendor will make excellent batteries that everyone raves about, then 6 months later, we start hearing reports that the new batteries from the same vendor don't perform even half as well as they used to perform.

In the end, you pay your dime, and take your chances.
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Old Jan 05, 2011, 11:21 AM
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It's not rocket science. But it's a field that's certainly full of marketing hype and probably a lot of dubious specmanship. And I believe this confusion applies to specs on battery capacity as well as C rating.

There are computerized battery-test systems advertised in the AMA monthly magazine and other RC mags. You can buy an IR temperature gauge for $20 to monitor the battery's temperature rise. But who's to decide what a "safe" temperature rise is? What sort of airflow is assumed? Nobody's saying.

Even within a single online source, you get to decide... should I buy the 20C battery for ten bucks, or the 40C version for fifteen bucks? For most of my own planes, the higher C rating isn't needed, but it might buy me "peace of mind."
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