Internal Resistance of a Battery: How to Measure It

The Definition of Internal Resistance

Resistance is a familiar concept in engineering. Moreover, internal resistance happens when an object opposes the level of current flowing inside a conductor–resulting in heat generation.

Interestingly, battery cells offer some kind of resistance to current flow. Since batteries don’t have perfect electric conductors, finding a zero internal resistance battery can be tricky. 

Also, the battery’s internal resistance is like the guardian of that battery. Hence, the lower the internal resistance, the fewer restrictions you’ll encounter while using that battery.

If you have high internal resistance, your battery will overheat and experience a voltage plunge.

How Does a Battery Work?

Batteries come with two terminals created with different metals and one electrolyte between the terminals. Electrolytes are chemical solutions that allow electrons to flow. In other words, electrolytes are electric conductors. 

Additionally, electrolytes also react with metal and keep them in their respective ions. One battery terminal is a cathode, usually a metal oxide with many positive charges. Or metallic ions with zero or little electrons.

The second battery terminal is the anode, usually metal with many negative charges. Or metallic ions with an overflow of electrons.

Now, the electrolyte chemical reaction oxidizes the cathode, thus allowing it to lose electrons. These electrons then travel through the circuit before reaching the anode.

Additionally, the battery converts chemical energy into electrical energy until it runs out of reactants. Hence, the battery dies. But, you can get more reactants through external electrical energy that converts into chemical energy.

In other words, it recharges the dead battery and allows you to repeat the process of converting the stored chemical energy into electrical energy.

Measuring the Internal Resistance of a Battery: The Ideal Way

Internal resistance measurement involves you getting a few components like:

• New battery (preferably AA batteries)

AA Batteries

• Resistor

• Voltmeter

Voltmeter

Steps

1. First, connect the voltmeter and batteries by clicking the voltmeter to the battery terminals. Connect the red voltmeter terminal to the battery cathode and the black terminal to the battery anode.

2. Note the measurements you get from your voltmeter at 1.500 volts. The voltage load of the voltmeter provided depends on your type of battery. You get the readings due to the open-circuit voltage (VOC) or simply not adding any load to the circuit.

3. Next, add your resistor to the battery and connect your voltmeter again. You should witness a voltage plunge because of the battery’s internal resistance.

4. Also, note the measurements from your voltmeter at 1.446 volts after adding the resistor. Since there’s a voltage plunge, you should get a value lesser than your initial measurements. Thus, you can measure the battery’s internal resistance by using the values you got from the VOC (open-circuit voltage) and the battery voltage when connected to a load–in this case, a resistor.

5. Next, you can use Kirchhoff’s voltage law formula and Ohm’s law formula to measure your battery’s internal resistance. First, we’ll use the value obtained after connecting a load to calculate the amount of current flowing into the circuit:

Where:

V is voltage

R is resistance

I am current

VL is the loaded voltage

RL is the resistor value

Next, use Kirchoff’s voltage law to calculate the volts of the battery’s internal resistor. It’s the same as the voltage plunge value within the internal resistor:

Where:

VOC is an open-circuit voltage

VL is loaded voltage 

VI is the voltage across the internal resistor

Once you have the values for the current flowing into the circuit and the internal resistor drop of voltage, you can utilize Ohm’s Law to calculate your battery’s internal resistance.

Where: 

RI is internal resistance

Hence, the AA battery’s internal resistance is 0.149Ω.

How Internal Resistance Affects Voltage and Current?

You can understand this better by using Ohm’s law. Ohm’s law uses a formula that shows the relationship between current, voltage, and resistance in a circuit. 

Keep in mind that voltage and internal resistance are stand-alone variables. The dependent variable (current) is the variable affected by internal resistance. 

In other words, when the internal resistance increases, there will be a current decrease. 

Alternatively, there will be a current increase when the internal resistance reduces. Both the current and internal resistance are inversely proportional. But these only work based on Ohm’s law.

Without the law, the voltage will decrease when internal resistance increases.

How Does Internal Resistance Affect Battery?

A low internal resistance would mean the battery would encounter fewer problems when delivering the required power spikes.

Contrarily, If you have a high mW reading, it can cause your battery to die earlier than usual. Why? Because the battery cannot deliver energy efficiency while storing some energy. In other words, large internal resistance would harm the battery’s lifespan and performance.

Does the Internal Resistance of a Battery Change?

As a batter gets older, its internal battery increases. Because of this increase in internal resistance, the available power at the battery’s terminal will also decrease.

Hence, if you want reliable operations from your battery, it’s necessary to monitor its internal resistance before it gets too high.

Components That Impact the Internal Resistance of a Battery

Two primary components affecting internal resistance include electronic resistance and ionic resistance.

The electronic resistance includes the resistance of all the battery materials, like internal components and metal covers. 

It also involves how all these materials make contact. Also, you can see the effect of electronic resistance immediately after connecting a load to the battery.

On the other hand, ionic resistance deals with how the battery resists current flow based on electrochemical factors like ion mobility, electrode surface area, and electrolyte conductivity.

Rounding Up

Measuring the internal resistance of your battery is essential when building circuits. Internal resistance is responsible for battery operations. Plus, its state determines if you’ll have a good or bad battery.

Additionally, high battery internal resistance can cause your battery to overheat and die faster than the estimated time. So, you can use your measurements to decide if you want to change your batteries for your circuits.

If you have any questions, feel free to contact us, and we’ll be happy to help.