I am going to use the voltmeter-ammeter method for finding the internal resistance of a battery.Why is it necessary to turn off the switch after each reading is taken?And what assumptions might i have made in this experiment.
PS:I am using battery bought from the supermarket, and typical school- laboratory moving-coil voltmeter and ammeter.

 

Normally, when a test is done, you measure the open circuit voltage of the battery (switch to circuit turned off. This becomes your battery voltage.

Then add a load of known resistance. Measure the battery terminal voltages again (it will also be voltage drop across the resistor). From the voltage across the resistor and the resistor value, you can calculate the current flow using ohms law.

Now, subtract the closed circuit voltage (with load) from the open circuit voltages of the battery. Divide your voltage difference by the current flow you calculated above to get internal resistance.

Very easy and no ammeter needed.

 

What is the voltmeter-ammeter method for fining the internal resistance of a battery?
You should not short circuit a battery. Provide a load instead.

You need to disconnect the load to allow the battery to recover.

 

What is the voltmeter-ammeter method for fining the internal resistance of a battery?
You should not short circuit a battery. Provide a load instead.

You need to disconnect the load to allow the battery to recover.

This is one method. http://data.energizer.com/PDFs/BatteryIR.pdf

 

I use LiPo batteries in my RC airplanes. The batteries are 3, 4, 5, and 6 cells with mAh ratings of 1500 to 6000 (1.5 to 6 Ah). The batteries have labels indicating the discharge rate in C. They vary from 20C to 60C.

I want to measure the internal resistance of the batteries. I'll be using a Volt/Amp/Watt meter in series between the battery pack and the load resistor. I'll first read the no load voltage, then apply the load, and read the voltage and current. To calculate IR, I subtract loaded voltage from unloaded voltage and divide that by the amps. I can estimate the individual IRs by dividing the total IR by the number of cells. OK so far.

I used a 1 ohm 250 watt resistor on some 3 cell packs (12.6 volts fully charged). I waited about 10 seconds for the voltage to settle before taking the readings. The IR calculations are way too high. I assume I should use a much higher resistance?

My question is how much of a load to put on the battery and how long to leave it connected before taking the reading?
Jack

 

I used a 1 ohm 100 watt resistor on some 3 cell packs (12.6 volts fully charged). I waited about 10 seconds for the voltage to settle before taking the readings. The IR calculations are way too high. I assume I should use a much higher resistance?
My question is how much of a load to put on the battery and how long to leave it connected before taking the reading?
Jack

I would say the resistance is too big ... under this test you only draw about 10A (the ammeter, unless a clamp type, will have a significant resistance) ... this all comes down to what ammeter you have , clamp types can measure very high and are not in the circuit . the higher the current the more accurate should be your measurements . I would try to measure at the current levels you are using when the battery is in use.

 

If you use a load resistance it is easy. Otherwise you need to check the battery processes and how they change.

 

https://www.eetimes.com/author.asp?section_id=36&doc_id=1320427

Internal resistance and maximum internal resistance: The internal resistance of a battery, measured in ohms, is the effective DC resistance acting in series with the voltage-generating part of the battery (for the AC resistance, see Impedance below.). The maximum specification is the worst-case internal resistance for a charge or discharge when operating the battery safely and within the manufacturer's recommendations. To measure the DC internal resistance, I use the two-tier DC-load method. Select a load resistance that will lightly load the battery, apply the load for 10 seconds, then measure the terminal voltage V1 and the current I1with the load still connected. Then apply a higher load (lower resistance) for three seconds and measure the terminal voltage V2and current I2. The DC internal resistance = (V1 - V2)/(I2 - I1). The two loads must be different enough to produce a very clear voltage delta while remaining inside the manufacturer's recommendations. Confidence in the number increases by making an additional measurement using another completely different resistor-pair. Note that the internal resistance (and impedance) of a battery are the primary causes of loss-of-capacity and inefficiency.

https://batteryuniversity.com/index.php/learn/article/how_to_measure_internal_resistance

 

The internal resistance is high when the battery is charged and low when the battery is discharged. The idea is that when the battery is discharged (less than 11.2V for a car battery) the resistance drops, it can even drop to less than 1Ohm. Then the battery drains a lot of current when charging. From there we have Ohm's law and like this we can see how the resistance changes and is it linear or not.

 

The internal resistance is high when the battery is charged and low when the battery is discharged. The idea is that when the battery is discharged (less than 11.2V for a car battery) the resistance drops, it can even drop to less than 1Ohm. Then the battery drains a lot of current when charging. From there we have Ohm's law and like this we can see how the resistance changes and is it linear or not.

I am sure that you mean this other way around. The lower the internal resistance of the battery, the more able it is to deliver current to a load. A high internal resistance is indication of a bad battery, or one needing a recharge.

Also this is the reason that you need to limit the current when charging a battery.

A'perfect' battery would have an internal resistance of 0 Ohms and would be able to deliver an infinite amount of power (Voltage * infinite Current) to a 0 ohm load.

Also, with the exception of SLA batteries, it seems the better batteries are made, their internal resistance stays very constant even as they are reaching the end of their capacity, so I am wondering how useful other than confirming that the battery meets the manufactures specs, measuring it is.