Thread: New Product ESR / IR Meter fo Lipos
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Old Jan 28, 2011, 05:01 AM
Wayne Giles is offline
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Rugby, UK
Joined Feb 2007
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Ibrinson,
I'm sure Mark and John are correct in saying that the readings you are getting are meaningless.
It occured to me that they could be a decade out but they would then be too low. The problem of trying to read anything useful into them is that they are so wildly out that the real figure you are trying to read, say 6 milliohms in the Turnigy 2200 25C case is less than 20% of the reading so it is swamped by the error itself. Therefore they are no use even as a comparitive tool.

Just to carry on John's rant, as he ran out of steam (!), there is a problem with a lot of cheap 'battery measuring and monitoring devices' which we now see at such bargain prices.
I have a small cheap wattmeter which reads cell voltages with errors of up to 9%, a little cell monitor which plugs into a balance connector and reads up to 17% error and have just bought a GT8 charger which shows charging and discharging errors in current (and hence capacity) of 14%.
John is cright in pointing out that when we read a voltage of say 3.732V on a digital display, we assume by implication that it is within 1mV as it has a resolution of 1mV.
We do need to be aware and question readings of many of these cheap instruments.

A UK Lipo supplier who uses a Robbe IR measuring device (not cheap) has just asked why my IR meter reads higher than the Robbe unit. It read 7 milliohms where the Robbe read 4 milliohms.
It turns out that the Robbe meter measures IMPEDANCE, ie it applies a 1kHz AC load and measures the ripple to take a reading.
The instruction book says that it measures RESISTANCE of a battery, which is just not correct.
The difference is that a lipo is a very capacitive component so it produces much lower readings. We are only interested in the resistance ie the volt drop on load.
I am surprised at such a basic error. As an old work colleague said to me; "Just because people manufacture equipment, it does not mean that they either know or care what they are doing!"

End of my rant which has not answered your question of how can you use IR readings.
The answer is that it allows you to compare different makes and it is IR which dictates how much power a pack can deliver and how hot it will get and hence probably, how long it will last.
Individual cell readings will also tell you if you have a weak cell which is usually a pre-warning of the end of life for that cell. A voltage reading will not tell you that.
To quantify the power bit, if you double the percentage voltage drop of a pack under load it will give an approximation of the percentage power loss.

Wayne
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