4 Steps to Measure the Internal Resistance of the Battery

Here is the 4 simple steps that can help you measure the internal resistance of the batter.

Step 1: Measure the Actual Resistance Value

Even if the resistance value has been printed, you still need to confirm that the actual resistance is the same as shown. Usually, the actual value of most resistors has a tolerance of 10% , but it still depends on the type of resistors you are using.

This is just a simple resistance measurement, so I think you can do it :)

Step 2: Measure the No-load Voltage of the Battery

Measure the no-load voltage of the battery by connecting the test leads directly to the battery terminals. It's called "no load" because the input resistance of the multimeter during voltage measurement is very high, usually over 1mohm, so the influence of current consumption can be ignored.

Again, I guess you know what to do as It’s a simple test. So let's move on to the next step.

Step 3: Measure the Voltage Across the Load Resistor

This is the tricky part and you must complete this test as quick as possible, otherwise you may not be able to get an accurate reading number.

Connect a resistor with the terminals of test leads and then connect the test leads to the battery terminals. Make sure your measurement module is in voltage mode!

Once the voltage reading stabilized(stop changing significantly), record the value and immediately disconnect the battery. The contact time should not exceed few seconds to prevent the load resistor from drawing too much current from the battery and affecting the reading.

I found that once the battery is disconnected, the load resistance gets very hot, which will also slightly affecting the reading. But this is inevitable as the reading number can't be the same as the printed number in most cases.

After completing this step, you can proceed to the final step.

Step 4: Calculate

This is the value I got:

- Voltage reading across the battery terminals Va = 3.99V

- Voltage reading across the load resistor Vb = 3.796V

- load resistance R = 4.7 ohm

Since Vb = Va * [R/(R + r)], r = internal resistance, then it can be worked out that the internal resistance of the battery is about 240 mohm.

Because the average reading helps to reduce the error, you can repeat the steps to get multiple values and figure out the average. Here I only read it once for demonstration purposes.

See, it's very simple right? All you need is a multimeter. I hope this article would be helpful for you!