Proof C Ratings are Useless

Tagged: LiPo / ESR / Internal Resistance /

What is ESR and IR?

IR is "Internal Resistance". Internal resistance, or IR as we call it, is the impedance that converts (wastes) some of the energy into heat. The wires, connectors, and even the cells themselves all contribute to IR.

ESR is equivalent series resistance. You can read a lot more about this on Wikipedia.For our purposes all we need to know is ESR, or an ESR meter, are very useful for measuring a batteries potential.

How does IR affect performance?

You can have two batteries that both take a full 1300mAh charge and balance at 4.2V per cell. However they may perform very differently. One may get hot under load. So hot it can risk catching fire when pushed hard. You have probably used batteries that just feel less powerful than others. They lack "punch" as we say. Why is this? You may say, "One has a higher C rating than the other". While that should be a good answer, it isn't. C ratings are essentially useless. See this post that shows how performance does not correlate with C rating.

It comes down to IR. How efficient is the battery at getting that power from the cells to the load? A battery with a higher IR will convert more of the energy to heat. That means your pack gets warmer and your motors get less juice. Under extreme loads, high IR can mean too much heat is created and your pack puffs, starts to smoke, or can even catch fire. Even if you don't start a fire, you will feel the difference in voltage. You won't have the punch you expect because your battery is making more heat, not more power for the motors.

For a MUCH more complete explanation, Wayne Giles post on RC Groups is a great read.

How can you test IR?

An ESR meter is the best way to test your pack's IR. Progressive RC sells them for $90 (at the time of publishing). These were specifically designed for our hobby LiPos. The newest version also includes some pretty nifty software that will calculate the C rating and Amp draw you can expect from the pack. It uses the IR to determine how much heat will be created and the C rating/Amps are the max before you risk damage to the pack or an unsafe operating load.

Some good chargers like the iCharger can take IR measurements. Those are better than nothing, but even the iCharger is 10-20% off. Lesser chargers or chargers that don't do IR as well will have a higher margin of error.

No matter what tool you use, you must measure IR consistently. Just a few degrees F will change the results. A standard approach is:

  • balance charge the pack to 4.2V at 1C and 72F
  • Let the pack rest for one hour at 72F
  • measure the IR using an ESR meter (or your charger's IR measurement)

Follow those steps and you now have a very useful number, how many milli ohms (mOhms) resistance your pack/cells have.

What does the number mean?

So your pack has an IR of 30, and the cells are 10/1010...is that good? It depends. Keep in mind, lower IR numbers are better.

  • IR goes down as temperature goes up
  • older/abused packs have a higher IR
  • IR goes down as the total capacity of the pack goes up
  • Pack total IR goes up as you add longer or thicker wire or less efficient connectors.
  • the cell chemistry/formulation will affect IR
  • the physical size of the pack also has an effect on IR

IR of 10/10/10 on a 500mAh 3S pack is very good. On a 1300mAh pack it is ok, but not great (5 is about as good as it gets). On a 2800mAh pack that is poor (there you can expect 2-3mOhm).

Some really bright and helpful people made this website: http://www.jj604.com/LiPoTool/

That lets you input your IR value of the worst cell (higher IR) and the capacity. It then calculates the Amps you can safely expect as well as "Figure of Merit" (FOM). Higher is better, 1 is average. From that page: Figure of Merit is a calculation that uses measured cell internal resistance and normalizes it to cell capacity.It is very useful when comparing packs of different sizes and from different manufacturers.The larger the calculated FOM, the better.For more background information on FOM, go here.

A 500mAh pack with 10mOhm has a FOM of 2.4. A 1300mAh pack would have a FOM of 0.92. A 2800mAh pack would have a FOM of 0.43.

When you compare packs of the same capacity the numbers are even more helpful. A 1300mAh pack with a cell IR of 10 has a FOM of 0.92 and can safely draw 28A (21.5C). A pack with an IR of 5 per cell (very good) has a FOM of 1.85 and can safely draw 39A. Note that the very good 1300mAh pack is just 30C! For a pack to have a true 70C rating like some claim, the measured IR would need to be 1.6 or lower. That is unheard of.

I don't get it, show me a pretty graph!

I'm a visual person and a good chart can communicate much more than what has already been written here. So here you go:

IR vs V

It can be hard to pick out the discreet lines in the graph, but you can probably see how the voltage drop as the cell IR goes up. There are a few subtle exceptions where a higher IR performs better. However that can be attributed to other variables like total pack IR (connector and wires), temperature (just a degree will change results slightly), and sample size.

Oh, by the way. All the fancy brand high C packs that cost much more sit in the middle dotted line region. The lesser known packs (SMC, VCANZ, Bonka) are at the top. The Turnigy Graphene is the top performer with its secret sauce that has the forums buzzing.

Conclusions

C ratings are not helpful. They are bloated by the factories (of which there are only a few) and the packs we buy are rebranded and often the same cells. Some put 45C on the label while another will put 60C on their pack made with the same cells. Some manufacturers even sell the same cells with different C ratings. They use different wire or assembly methods to increase the weight to give a perception that the pack is different in some meaningful way. But it isn't. You are paying for the same quality, but a different number on the label.

The only way to really tell what you are getting is to measure IR with an ESR meter. Then you can know how healthy/strong your packs are and track them over time. You'll then know what to buy and what to avoid.

If we as consumers can gather the data we can produce some helpful charts and resources to inform each other. What batteries actually perform the best, regardless of what it says on the label.

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