Thread: New Product ESR / IR Meter fo Lipos
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Old Nov 14, 2013, 03:36 AM
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Melbourne, Australia
Joined Jul 2005
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Well, this just raises all the issues about how difficult it is to make reliable comparisons!

One of the reasons I am reluctant to make public my IR vs/ temp readings is that it is too easy to read into them conclusions that are invalid.

I particularly don't want people to draw the conclusion that one brand is "better" than another from them.

One of the things Mark, Wayne and I have emphasised is that IR is a useful "quick and dirty" measure of LiPo performance. The LiPo tool is only ever claimed to be a "rule of thumb" based on extensive experience. It still remains in my view far and away the best non-destructive really quick way to establish likely pack performance and is particularly useful to keep track of LiPo life cycle behaviour but I cannot emphasise enough:

IR is a derived number from voltage drop under load that is a measure of likely pack performance - it is NOT an intrinsic property of the pack. People who talk about "reducing the IR to improve pack performance" misunderstand this. There is a lot more to pack performance than just IR - but it does remain one of the most useful single parameters we have.

The only real controlled test of a pack is its voltage and temperature trace under varying loads. Comparison of real life discharges such as Joe (MSGUY) has recently done is equivalent and for most practical users is more convincing provided all the variables are tightly controlled. The "I use Brand X and Brand Y and Brand X gives me more punch" do not fall into this category unless we can be sure all tests were made under identical conditions.

The thrust test in the video is unconvincing unless it is continued to LVC. It shows initial thrust to be higher but that may be due to an initial high voltage retention under load. For example I have found my initially superior Turnigy A-Spec drop off more rapidly at a 35C rate than my Revolectric packs which hold a more constant voltage and actually deliver higher Watt hours.

The number of variables is substantial:
1) Temperature affects voltage and current delivery
2) Temperature varies with discharge rate and during the discharge and depends on the physical cooling arrangements
3) Consequently so does pack voltage and current
4) Pack history affects performance and capacity - most of us fly for fun and testing packs for 50,100, 200 cycles is not fun.
5) Pack chemistry is largely a mystery and so we get anomalous behaviours. Different size cells may be different chemistries. Different production batches may have different cells. Voltage vs. time curves have different shapes.
6) Developments in packs may change the results. I just tested a Revo 70C at approx. 35C rate and it shows different initial voltage curve to a Revo 60C. Yet to find out what it does at high rates.

and so on and so on

Bottom line is, I posted the IR vs. Temp curves to make the point that the response of different packs is quite different as temperature changes and this will affect your judgment of what is "best". A pack with relatively low IR below 25deg C if only used at moderate rates may produce excellent power and low temperature rise with consequent long life. A pack with higher IR at room temperature, but which has an IR that drops off rapidly with heating, may perform well at high rates where the internal heating kicks in and lowers the IR.

These packs were my ordinary flight packs and while most were relatively new a couple had some milage under their belt. They were included deliberately to show the difference that can arise between different packs. Specifically my Gens Ace 25C packs have not lived up to the perormance some other people have seen and it is entirely possible that are from a different manufacturing batch. My three Turnigy A-Specs on the other hand have been consistently superior performers with low temperature rise at high current rates - BUT they are significantly lower capacity than claimed on the label.

It's all a bit of a lottery really.

John
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