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Creating a galvanostat/potentiostat

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DanTerp

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Greetings!

I really would appreciate any insight regarding my project. I want to perform a form of electrochemical impedance spectroscopy on consumer electronic batteries (ie. measure the impedance of a battery - real and imaginary). Basically I want to create a device which is capable of doing one of two things:
  1. Draw current from a battery in a sinusoidal manner
  2. Draw current from a battery in such a way that causes the voltage of that battery to vary sinusoidally

device.png

(battery under test on the left, black box right)
The batteries I'll use are Li-ion (3.6V single cell - 4.2V fully charged). My target peak-peak current of 1) would be ~100mA. My target peak-peak voltage of 2) would be 25mV. Remember I only need to achieve one of these. I also want to be able to vary the frequency over a very small range (ideally 10Hz - 0.1Hz).

The ideas I've had so far are:
  • Use a digital rheostat (ie. constantly change the resistance value)
  • Use a fixed resistor with a wienbridge oscillator as a reference voltage
Of course I also need to measure the battery voltage and current. I would like to be able to measure to a resolution of ~250uV and ~1mA.

If you have any advice I would be grateful.
 
Draw current from a battery in a sinusoidal manner
Clarification required. The sine function has both negative and positive halves. Negative current implies you would actually be charging the battery??.
 
hi and welcome,
If I understand correctly, you want a adjustable amplitude/frequency sinusoidal load current of 0mA thru 100mA, superimposed on the battery voltage of 3.6V thru 4.2V.?? ie: the current is always drawn from the battery.

and you want to measure the sinusoidal voltage of the battery as a result of these sinusoidal load currents.?

E
 
By analogy to your digital pot, could you not just modulate the base of a transistor and use a fixed resistor to control maximum load from your supply? A microcontroller with low-pass filter would be a simple way to get the modulation.

What is the end purpose? There is quite a bit of information available on battery life vs. discharge profiles (e.g., pulsed vs. constant).

John
 
A bench audio signal generator (mines goes down to 0.1hz) feeding the base of a emmiter follower transistor with a resistor load in the emmitter circuit ought to do it, the load could well be your rheostat so you can adjust current draw.
Or you could build a wien bridge sinewave osc from an op amp and use that.
 
Clarification required. The sine function has both negative and positive halves. Negative current implies you would actually be charging the battery??.

Alec,
In this case I do not mean a 'non-polarized' sinusoid. I only want to take energy from the battery. The current leaving the battery would have a minimum level of 0mA (or close to) and a maximum of 100mA.
 
hi and welcome,
If I understand correctly, you want a adjustable amplitude/frequency sinusoidal load current of 0mA thru 100mA, superimposed on the battery voltage of 3.6V thru 4.2V.?? ie: the current is always drawn from the battery.

and you want to measure the sinusoidal voltage of the battery as a result of these sinusoidal load currents.?

Thanks for the welcome Eric!

Exactly! Yes, the current is always drawn from the battery.

I should clarify why I posted the battery voltage levels that I did:

A single cell Li-ion battery is often advertised to have a nominal voltage of 3.6V. This doesn't really mean anything except to clarify it is indeed a single cell battery. When the battery is being charged the max voltage is 4.2V, however once the charge current is removed the voltage will relax to some lower value. When the battery is being discharged the cutoff voltage is 3.0V. For my application the battery will always be close to fully charged so the voltage of the battery measured at the terminals will always be between 3.6V and 4.2 (realistically between 3.8V and4.2V).

Yes, I want to measured the load current and the resulting voltage change (over multiple periods - lets say four). With measured voltage and current information in-hand I will perform an FFT to determine the impedance of the battery.
 
By analogy to your digital pot, could you not just modulate the base of a transistor and use a fixed resistor to control maximum load from your supply? A microcontroller with low-pass filter would be a simple way to get the modulation.

What is the end purpose? There is quite a bit of information available on battery life vs. discharge profiles (e.g., pulsed vs. constant).

John

John,

Thank you for your reply. The end purpose is to measure the impedance of a battery when it is near fully charged.

I'm trying to imagine implementing your suggestion. Could you please elaborate on the design and control. Were you imagining something like this?

IMG_20130929_160333.jpg


Daniel
 
A bench audio signal generator (mines goes down to 0.1hz) feeding the base of a emmiter follower transistor with a resistor load in the emmitter circuit ought to do it, the load could well be your rheostat so you can adjust current draw.
Or you could build a wien bridge sinewave osc from an op amp and use that.

Dr. Pepper (if that is your real name :p),

Thank you for your reply. Could you please elaborate on your vision, perhaps with an illustration? Were you thinking something like this? My apologies if i'm horrifically wrong.

IMG_20130929_164134.jpg


Daniel
 
It may not be easy to measure AC values as close as you want at .1 HZ. Have you considered how you would calibrate it?
 
Yes thats pretty much it.
The feedback resistor for a proper wien would be a thermistor, the schematic you drew would have a little distortion but I dont think that'd matter.
I see theres mention of a uC further up, you could program a pic to synthesis a sine wave using its pwm module and a lookup table, and use the same transistor and dump resistor or rheostat.
 
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