Battery state of health determination with impedance measurement

Discussion in 'Power Electronics' started by Bhargav Jani, May 20, 2016.

1. Bhargav Jani Thread Starter Member

May 20, 2016
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I have a LCR meter, and lead acid batteries 12V 7AH each 3 of them, i need to observe the impedance spectra all over the frequencies ranges upto 1Mohm, need to build a passive circuitory with some components which will be comnected to lcr meter and then to battery to calculate the r & c of battery....need suggestions for the circuit part as the equivalent circuit model of lead acid alone will not be feasible for such high frequency range

2. Lestraveled Well-Known Member

May 19, 2014
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Measuring the capacitance of a battery with an LCR meter is fairly easy. You put a known capacitor that has low ESR and inductance, in series with the battery and LCR meter. From the LCR meter reading you calculate the battery capacitance. You may have to have a few different values of capacitors to get the most accurate reading. The known capacitor should be approximately equal the the battery capacitance for best accuracy.

Measuring the inductance is a little more difficult. Theoretically you would put a large capacitor in series with the battery and meter. The Xc capacitor of the capacitor should be at least ten times smaller then the expected Xl of the inductance. A large capacitor can have a large inductance. You must choose or make low inductance capacitors to test with. A lead acid battery will have tens of micro-Henrys. Your test capacitor must have less inductance then the battery or else the accuracy of your measurement will be in doubt. Use multiple capacitors to derive an answer.

3. Bhargav Jani Thread Starter Member

May 20, 2016
70
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Thank you sir for your help
Does it mean i can monitor this over large frequency range as my LCR meter provides Test frequencies upto 200KHz ..Is shunt resistor necessary in this case ? or only the Capacitor is enough in series with the LCR meter .? As my ultimate goal is to show increase in impedance how it is affecting battery state of health

4. Lestraveled Well-Known Member

May 19, 2014
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You can make measurements at what every frequency your LCR meter works at. Whether they are accurate or not depends on your selection of the capacitor and accounting for auxiliary inductances. I suggest that you make a "battery simulator" with known inductance that you can test your measurement technique.

5. Bhargav Jani Thread Starter Member

May 20, 2016
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Battery simulator , meaning need to design a circuit with few passive components ? Not so much clear about it sir Could you please elaborate ?
As i do not want to make it complex just 3 lead acid batteries which i will examine one after other with different health conditions and plot the appropriate graph..

Thank you

6. Lestraveled Well-Known Member

May 19, 2014
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Build this circuit. If you can accurately measure the 100uH inductor with your LCR meter and test capacitor, then you can measure the inductance of a battery.

• Battery Impedance.jpg
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7. Bhargav Jani Thread Starter Member

May 20, 2016
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Thank you sir , i will try and rivert back in case of assistance

thank you nice weekend

May 20, 2016
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Mar 4, 2014
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10. Bhargav Jani Thread Starter Member

May 20, 2016
70
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sir here the battery internal resistance is not considered.?.......and shall directly connect the terminals of LCR meter to the diagram u have suggested with the values of capacitor and inductance......
1.battery capacitance measurement of 12V 7AH ? is the 0.1µF value correct to have it in series with battery and LCR METER?
urgent help needed today will be in lab with the expt

thank u

11. Lestraveled Well-Known Member

May 19, 2014
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Please re-read the second paragraph in post #2. The Xc of the series capacitor needs to be much lower than the Xl of the expected inductance of the battery, at the frequency the LCR meter is operating at.

12. ronv AAC Fanatic!

Nov 12, 2008
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You might also try just measuring the battery resistance. With a fully charged battery draw a current of 1C (0.7 amps for your 7AH battery) and measure the voltage drop of the battery. This can be a pulse that you look at with a scope.

13. Bhargav Jani Thread Starter Member

May 20, 2016
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When in AC mode the LCR has some specific range of Voltage and Battery lead acid (DC) 12V 7Ah..
in this case....shall there be series capacitors(electrolytic) connected in between the LCR meter and battery
or single capacitor will do .... I read the paragraph sir about Xc , but there is still some issues ....

thank you

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14. Bhargav Jani Thread Starter Member

May 20, 2016
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coupling capacitors can be used ? as my LCR mode is AC and the load is battery DC ?......what shall be in the intermediate circuit ?

15. Bhargav Jani Thread Starter Member

May 20, 2016
70
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coupling capacitor value ? for high voltage AC from 100V to 240V LCR meter in series with battery (Lead acid)
Any suggestions ?

May 20, 2016
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17. Bhargav Jani Thread Starter Member

May 20, 2016
70
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sir , i need your help
i connected the capacitor of value 650pf in series with the battery and lcr meter, after increasing freq i see the decrease in cap value .can you please let me how do i relate this with the cell impedance of battery what graph shall be plotted .... i took some different values of capacitor upto 100µF.........and the battery Z and angle ....Z is always decreasing with increase in freq it starts from Kiloohms to few ohms .......its too high ..what shall i state from this data ...

18. ronv AAC Fanatic!

Nov 12, 2008
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That's kind of what capacitors do. The higher the frequency the lower the impedance. 1/2piFC.
I still think you want to measure internal resistance not impedance to determine your batteries health.

19. Lestraveled Well-Known Member

May 19, 2014
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What did you read with the test circuit in post #6?

20. Bhargav Jani Thread Starter Member

May 20, 2016
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I got negative inductance after i built that circuit ........So i was not sure what is exactly happening