# Impedance analyzer



## narithota (Jun 29, 2011)

Hay

we selected AD5934 Impedance analyzer for measuring the impedance of the Li-ION Cell. From the data sheet I found that AD5934 can measure the Impedance in the range of 100Ohms to 10Mega Ohms. But Li-ION cells having very low Impedance in the range of mille ohms.

How i can measure the Impedance of the Li-ION Cell .

Thank you.

Regards

Nari


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## Stiive (Nov 22, 2008)

narithota said:


> Hay
> 
> we selected AD5934 Impedance analyzer for measuring the impedance of the Li-ION Cell. From the data sheet I found that AD5934 can measure the Impedance in the range of 100Ohms to 10Mega Ohms. But Li-ION cells having very low Impedance in the range of mille ohms.
> 
> ...


Can i ask why you selected the AD5934 then?
I dont know anything about it, but if its very accurate you could attach a known resistance (>100 Ohms) in series with the cell and then measure the entire impedance.


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## Tesseract (Sep 27, 2008)

narithota said:


> How i can measure the Impedance of the Li-ION Cell .


You can do this with a reasonable amount of accuracy with a scope, a function generator, a 100:1 current sense transformer and at least 1000uF (1mF) of capacitance (electrolytic is fine and, really, the more capacitance the better here). 

The idea is to apply a capacitively coupled 100mVpp/10kHz sine wave across a series circuit of the cell and a precision non-inductive resistor (smt types for "current sensing" are best). The resistor value should be in the same range as the expected impedance for good accuracy. If the resistor is, eg, 10 milliohms and the ac voltage at the center point of cell and resistor is 50mVpp then cell impedance is also 10 milliohms. You need the 1000uF (or higher) capacitor to block DC current flow.

The best way to get the 100mVpp signal is to use a current sense transformer in reverse: that is, drive the 100 turn "secondary" with the function generator (ie - 10Vpp in to get 0.1Vpp out) and use the single turn primary as the secondary. 

This is by no means a "lab grade" method, but it works. Supposedly, tracking changes in impedance over time is a good way to evaluate a cell's health (ie - it is as useful as testing actual cell capacity/internal resistance, but takes much less time to perform). 

BTW - the AD5934 looks like a cool chip, but it is totally useless for this application.


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## narithota (Jun 29, 2011)

Hay

Thanks for the reply

My application is Portable Battery Management system. I required IC based solution for measuring the Impedance of the LI-ION cell, for this i have chosen the AD5934, But it measure the Impedance range is more than 100ohms.

Can i connect a series resister with cell for measuring the cell Impedance, whether it will be accurate other wise any external circuit for measureing Impedance .

Regards
Nari


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## TigerNut (Dec 18, 2009)

The AD5934 will not give you the milli-ohm resolution that you want for measuring battery impedance changes, especially if you put it in series with a 100 ohm resistor. One approach would be to use the same architecture as the 5934, but you will need to build your own sweep signal generator and A/D system to be compatible with the very low impedance you're trying to measure. An interesting design problem! 

Tesseract is telling you the same thing... just because you can't buy it ready-made in a single chip doesn't mean it can't be portable. But if you want the level of resolution that a chip like the 5934 would give (assuming every other part of the system has similar level of performance) then you will have to spend some money on precision electronics, and a like level of attention to circuit layout and EMI shielding.


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