Thread: New Product ESR / IR Meter fo Lipos
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Old Feb 03, 2011, 01:54 PM
Wayne Giles is offline
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Rugby, UK
Joined Feb 2007
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Jac,

It am not sure which method chargers use to measure IR. I suspect that they measure the voltage step when the charging current is turned on. If so there is likely to be an error in that there is some hysterisis in the first application of current. If you used this method with a NiCad/NiMH cell then the results would be garbage.
Other measurement methods are applying a 1000Hz AC load and measuring the ripple on the cell, but this measures impedance rather than resistance.
As users we are only interested in resistance as that is the parameter which determines the voltage drop when we apply a load (a motor) and hence the power we can get from the pack.
My post saying that IR appears to be independent of charge state is only based on various odd measurements
I will run a test on a pack measuring the IR during a low rate charge and discharge cycle, taking spot measurements and produce a plot of the IR over the whole cycle. It has to be very low rate as high discharge heats the pack and reduces IR whilst charging is endothermic and would increase the IR.
It will take a few days to do and I will postpone analysis about Hyperion's comments until the plot is complete other than to say a heavily discharged lipo will be hotter because more total heat has been generated and it takes some time to get to the surface of the pack.
I agree with Mark's comment about choosing the applications for your lowest IR packs; all lipos seem to increase their IR with age and a rapid rise indicates end of life.
I also agree with Charles' assertion that many suppliers information is useless. Twice recently I have come across the confusion between Impedance and Resistance written into the instructions of IR measuring devices.

Wayne
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