Best way to measure IR

I'm working on my tester device and trying to design the internal resistance/discharge part.


Most circuits that I have found used two resistors in series, then measured the voltage drop over one of them, then I also found one who used two resistors in parallel and measured the voltage dropacross them both.





I'm wondering which way is the best, and what's the reasons are for using both of them.

The first circuit makes no sense AO/A1 just sees one resistor .... Neither of those circuits is very clear ......
.

Connect a voltmeter across the cell ...right down the reading , lets call it Voc (open circuit voltage)

Now put a resistor across the cell ...the voltage will drop to Vwr (Voltage with resistor )

The amount of voltage drop tells you the internal resistance of the cell ... if it halved ...say dropped from 4 V to 2V the internal resistance is the same as the resistance you used ... if it dropped by a 1/4 , from 4 to 3 , then internal resistance is a third of the value of the resistance you used ...

(Voc-Vwr / Vwr)R= Int resistance ... you have to know accurately the resistance you are using ....

There is an easier way ...

Are you testing capacity of cells as well ??? many people use that small device with red led display and three buttons ...



the voltage drop when you start the discharge cycle can easily give you the int res ..... Voltage drop/ discharge current = int res

So if you set discharge at exactly1A then voltage drop is internal resistance ... if it drops from 4V to 3.9V when you start discharge , that's 100mV drop so internal resistance is 100mOhms

As I understand it, the most accurate way to measure IR is using two different value resistors and calculating based on the differences between the readings.

Julian Ilett did an interestingvideo about it:

Srry for the unclearexplanation of my problem, I'll try to give a better one.


The purpose of this circuit is to both being able to measure the internal resistance of the cell, as well as testing the capacity. I want to create a circuit that can be fully controlled by a microcontroller so it can be automated.

Both requires you to know the current running through the resistor. Most people I've seen uses two analog pins to measure the voltage on each side of the resistor compared to the ground.
I'm trying to see if you can make a shortcut and only use one analog pin.

Here is a circuit similar to the first one I posted.




Since the Arduino have the same common ground as the cell would you really need the A0 pin?

To getthe Voc (open circuit voltage):
-D13would beLOW, to allow no current through the mosfet
- Then the Voc can be measured with A1

Then to get the Vwr (Voltage with resistor):
- D13 would be HIGH, to allow current through the known resistor
- Then the new voltage Vwr would also be measured with A1

Since the Arduino and negative terminal of the cell are both grounded, the measured A1 voltage would be equal to the voltage drop over the resistor. This should only work because the voltage is measured arecompared the ground, if they weren't grounded I would guess you need both A1 and A0.

So this would be the final circuit which should be able to measure both internal resistance and the capacity:




It seems to make sense to me, but the fact that everyone uses two analog pins for this circuit,makes me thing I'm wrong.

---

watts-on said:

As I understand it, the most accurate way to measure IR is using two different value resistors and calculating based on the differences between the readings.

Julian Ilett did an interestingvideo....

Click to expand...

That is actually interesting how he uses two different resistors, wonder what the accuracy of that is?

I've seen people also sending a high frequency pulse through the cell, but for me it doesn't really matter as long as the results are consistent.

I've just realized that this device does not give reliable readings ....

The voltage reading under discharge bears no relation to the voltage of the cell ,you have to put another voltage meter across the cell to get int res ... that's assuming current readings from this gadget are reliable ...

It also does not allow discharge greater than 2,1A even though it has printed on it 3A max discharge!

Well before you press 'OK' and start the capacity test you can see the Voc, then after you start you can see the voltage is different. This is from from my understanding because of the voltage drop over the battery's internal resistance. So I guess you can still use the HW-586 as internal resistance tester, how reliable it is.

But if I'm using this device correctly I only use one of the two given resistors connected to R terminals, which means the circuit in the first image of my post could be used(Just ignore that it has two resistors in paralle). Then you also wouldn't need two analog pins to measure the voltage across the resistors as long as your Arduino and Cell have common ground. Please someone correct me if I'm wrong.

My guess is that two analogue pins are normally used because the FET hasresistance too, which will skew the results.

Ok I'm new to this I got a new lipo balance charger I discharged about 10 18650 battery to 3.0 volts I just checked with my field piece meter mostly show around 3.4 to 3.5 v did they self charge that much or is my lipo imax b6 not dropping to 3.0 v im discharging used ncr 5.0a 3400 mah batteries to 3.0 and getting 2400 to 2650 mag can someone give me advice.

watts-on said:

My guess is that two analogue pins are normally used because the FET hasresistance too, which will skew the results.

Click to expand...

Yeah I have thought about that as well, but don't think the resistance of it would make much of a difference in the measurements since it only have a resistance of max0.035? which shouldn't do much to the results.

---

DavidHuffman said:

Ok I'm new to this I got a new lipo balance charger I discharged about 10 18650 battery to 3.0 volts I just checked with my field piece meter mostly show around 3.4 to 3.5 v did they self charge that much or is my lipo imax b6 not dropping to 3.0 v im discharging used ncr 5.0a 3400 mah batteries to 3.0 and getting 2400 to 2650 mag can someone give me advice.

Click to expand...

Hello David,
Think you'll get more response if you made your own post here on this thread

But since we are at it, a battery should not be able to charge itself in any way, if they really did then I like to buy all of them haha
The should for sure stay at 3V if that's what you decharged them to. 0ne thing that could have happened is that, when a current is drawn by a battery the cell would have a voltage drop. A higher current would result in a higher voltage drop. So when you are decharging the battery the voltage drops to lets say the 3 volts, but when you stop drawing current the voltage would stabilise again. All though the Imax B6 should be able to do it properly, and what I mean is it lowering thecurrent the closer it get to the cuttoff voltage(3V in your case).

I have a question about original post.

If goal is to quickly determine if a cell is bad using IR, is their a minimum voltage that cell should be charged to before placing under the 1 amp load

For example if cell is discharged to 3.3 v (measured not under load), should you charge to 3.5v or 3.8 v before putting under load to calculate IR

Again just looking for quickest way to determine a pass / fail

From my understanding does the internal resistance change depending of the battery voltage, why I don't really know. So you want to test the batterys at the same voltage. Don't think it matters whether it's a 3.3V or 4.2v, the test takes no time so the battery shouldn't be damaged in any way.