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How is IR used to calculate a genuine C-Rating with a LIPO battery?
Time : 2018/1/24    Clicks : 709    Public:Legend of Lipos

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.

Internal Resistance (IR) is an empirical value that describes observed behaviour. 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 overinflated 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.