Understanding LIPO Battery C Ratings?

Have you ever wondered about the authenticity of the C rating applied to your shiny new battery pack?  You might be shocked to learn that many manufacturers are rather careless when handling the truth and prefer to massage the ratings just  a little. It’s a vicious cycle because as one vendor slaps on a new label others will follow. Some vendors are blatantly dishonest and should be called out for fabricating the capability of their product. 

Some manufactuers don't like being called on their claims. When you make a claim like 150c you can expect to be taken to task and thats exactely what happened to halarious effect. 
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https://mainlandrc.wordpress.com/2012/01/22/maxamps-boycott/

Generally the C rating should be considered a guide only. This is because marketing departments can be a little creative when it comes to discharge currents. The concept of a C rating is to provide consumers with a simple guide to measure one battery against another which  is a great idea. However, in practice you’ll find the minute one vendor releases a 65C battery their competitor is ready with a 75C Label.  I’ve seen just that happen and people in forums blindly post that battery Z is better because it’s a 75C.  Like a bug to a lantern the gullible are soon relieved of the money padding their wallet.

Some may recall when Turnigy launched their very impressive graphene battery packs. The market scrambled to come up with something as good and some companies opportunistically just added the word graphene to the title of some batteries on their site. In fact they hadn't even bothered to add the work graphene to their labels a year on. Again, the like bugs to a lantern many were drawn to the allure of a cheap high performance pack.
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Lipo battery discharge current is often expressed as a C-rating in order to create a standard measurement  to rate one battery against another.  C stands for capacity, so as an example if we have a 1000mah battery that is rated at  1C this means that the battery is rated to discharge a current of 1amp (1000x1/1000=1). 
If we have a 5000mah battery that is rated at  65C this means that the battery is rated to discharge a current of 325amp (5000X65/1000).  Most 5000mah batteries would be stressed at 100-150amps and could not possibly supply 325 amps.

Manufactures can be a little boastful when it comes to the ratings of batteries. Some manufactures are better than others and some are simply deceptive as per the above examples.  The RC aircraft community is particularly cautious of manufacturer claims and relies on independent scientific testing to verify claims. Unfortunately, the RC car market or multirotor community is not so cautious and buys into the marketing claims of unscrupulous vendors.

Take note of the type of rigorous review testing that car packs are subject to. Its all about feels and looks.


There is no shortage of people happy to make a fool of themselves reviewing something they know nothing about.


That said, scientific testing can generally be relied upon. Repeatable, scientific testing is far more important than what it feels like in the air. Companies with bogus claims avoid scientific reviewers because the results are generally not to their liking. It is also difficult to skew a scientific test


Recently the RC community has become saturated with bogus reviews offering favourable reviews with a links to purchase items via affiliate link. In return they  receive a payment when you buy something. Think hard about what you are being offered because if it lacks a scientific basis you’re likely to be buying something that may not be fit for purpose.  

MAX amps claim a true 150C on one of their car packs and this is nothing more than a bogus claim. They are not expecting to be pulled up on that claim however using the Wayne Giles ESR meter quickly exposes the actual true rating.  


 

What makes a good lipo battery?

Not all batteries are created equal, so in reality a high C rating is only part of the picture.  A good analogy is Beer!  Similar look, similar alcohol content, and even the bottle might look similar. but that’s where it ends because there is much that needs to be considered when determining a good battery from a poor battery.  Something as simple as poor soldering makes a good battery a poor battery no matter how good the cells in the pack.  
 

What to look for?

  • Low internal resistance (Verifiable by a Wayne Giles Universal ESR Analysis Meter)
  • Balance lead wires are silicon coated so that they are not prone to breaking
  • High C Rating (Can be verified with the aforementioned ESR meter)
  • High Cycle Count  (500-1000+ Charge / Discharge)
  • Reputation for quality
  • Good warranty support
  • Excellent construction
  • Scientific peer review
 
Avoid
 
  • Reviews that focus on how it feels or looks.
  • Reviews that avoid a scientific approach to testing
  • Small operators or new operators
  • Deals that sound too good to be true
  • Ridiculous claims (e.g  150C when the largest players in the market are offering 65C)
  • Illegal shipping (If it damages an aircraft or kills people, YOU as the importer are liable)
 
How is IR used to calculate a genuine C-Rating with a LIPO battery?
 
There have been a few contenders over time but the original is still the best. The Wayne Giles designed ESR meter is without doubt the gold standard in measurement. https://hobbyking.com/en_us/catalog/product/view/id/70221/s/wayne-giles-designed-universal-esr-meter-for-100-10000mah-1-6-cell-lipo

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Internal Resistance (IR) is an empirical value that describes observed behavior. When used in a normal application such as in an RC model, a LiPo battery behaves as though it were a voltage source in series with a small resistance inside the battery (hence Internal Resistance). A LiPo is not a perfect voltage source containing an internal resistor. It just behaves as though it was and that allows us to make some predictions about the performance. The equations involved and their solution is not for that feint hearted and we’ll leave that for the boffins.

We must remember that in reality there is no resistor within the batter so with this in mind we only use some resistor rules. (two cells in parallel have half the IR of one) but not others (IR varies dramatically with temperature in a way a resistor does not). That is why the Effective Series Resistance (ESR) term is really preferable. Importantly, IR is a measured number not an intrinsic property, so that different methods of measuring IR give slightly different numbers.

Other IR Measurement Methods such as FMA PL8 charger, iCharger 3010B, all use the Kelvin method by measuring cell voltage through the balance leads to avoid errors caused by resistance of the battery leads and connectors. The Hyperion Super Duo which may or may not, however, Hyperion recently revised their firmware and it resulted in more accurate values consistent with the ESR meter and iCharger. It is noted that although results are similar  they are consistent across different batteries if using the same method. Based on a range of test results, my conclusion is that the ESR meter, PL8 and iCharger produce close, but not identical, numbers for IR

When comparing IR numbers you can only reliably do so if you know at least the test temperature AND the method used to measure them. This is very important because as we discussed earlier, temperature has a significant impact on the results.


Why use IR if it is so prone to measurement variables?
 
C rating cannot be relied upon as manufactures are known to overinflate  is largely  meaningless since that also needs to be specified under particular conditions - and never is, and never will be, since it is not in the manufacturer’s interest. Practical experience by a number of knowledgeable folks over several years and in several countries suggests that IR is a good measure provided the device and method used is correct. It is fairly easy to do with the right equipment and it’s easy to understand with relatively few controlled conditions. Practical experience to date has shown it is a good guide to real performance.
 
What is the importance in practice?

The aim of the Max Current calculator is to give a very simple tool to estimate the maximum current any particular battery is capable of while maintaining a decent cell voltage and limiting the internal thermal heating.

It is not a theoretically derived tool but one based on a number of years of observation of the performance and heating of batteries under test and in flight. The conclusion is that placing a limit on the power dissipated in the battery as a function of its capacity is a remarkably good guide to the maximum current rating. This has proved true over a range of commonly used battery sizes down as far as the tiny single cells used in the Micro fliers and the like and hopefully can be extended by others experience.       

In the end we all want reliable batteries that will do what they say they will and deliver what they claim. 

Enjoy.
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