How to measure the Internal Resistance of a Battery? – Why should you do it

Batteries are found and used everywhere! Everyone has seen them and used a battery before. In another previous article – What happens in an electric circuit: Voltage vs Current, we defined and explained what is voltage, current and resistance. However, do you know that every battery has its own opposition to the flow of the current? This is known as Internal Resistance.

We talked about resistance – resistance refers to the measure of opposition to the flow of current. Internal Resistance is essentially an opposition to the flow currently offered by the cells and batteries themselves. In this article, we will be talking about how to measure the internal resistance of a battery:

  • What is internal resistance?
  • How to measure internal resistance?
  • Recommended tools and products to help you.

What is Internal Resistance?

Source: hk-phy

As mentioned, internal resistance refers to the opposition to the flow of current offered by the cells and batteries themselves. All materials resist current flow to some degree, even cells and batteries. This is due to the materials that are used to make the batteries.

The elements that make up a typical battery includes zinc, carbon, lithium, mercury, silver and etc. All of them are not a perfect conductor of electricity. Therefore, it will be difficult to find a battery with zero internal resistance, probably even impossible. 

Internal resistance can be seen as the gatekeeper of a battery. Lower resistance means lesser restriction. If there is high resistance, the battery will heat up and voltage will drop. 

How Internal Resistance affects Voltage and Current?

To understand this better, let’s use Ohm’s Law. Ohm’s law is a formula dictating the relationship between voltage, current and resistance in a circuit. Ohm’s Law states that V=IR. V refers to the Voltage, I refers to the Current and R refers to the Resistance, in our case, the internal resistance.

The key point to understand is that Voltage and Internal resistance is an independent variable, the main thing that is affected is the dependent variable, i.e. the Current. When the internal resistance increase, by V/R=I, the current decreases. When the internal resistance is lower, vice versa, the current increases. They are inversely proportional. However, all of these is only based on the Ohm’s Law formula.

In the real world, Voltage decreases when internal resistance increases. That is also how we can measure the internal resistance of a battery. 

In a nutshell,

Internal Resistance is a resistance in the circuit that is from the cell or battery itself. With higher internal resistance, current and voltage will be lower.

With that in mind, let’s now explore why should we measure internal resistance?

How to measure Internal Resistance?

What you will need:

  • A new Battery (AA Battery)
  • A Voltmeter

0.28 Inch LED digital DC voltmeter – Yellow

  • A Resistor

RESK – Resistor Kit

Steps:

  1. Wire up your Battery and Voltmeter in the following configuration.

[Ignore the Triangle (GND) on the bottom-left]

  1. Take down the measurements from the voltmeter. 1.500V

The voltmeter should show the amount of voltage with regards to your battery specification. This is due to no load being connected to the circuit. This is also known as the open-circuit voltage (VOC).

Open-circuit voltage (VOC) is the voltage when it is not connected to any load in the circuit.

  1. Wire up your Battery, Voltmeter, and Resistor in this configuration.

[Ignore the Triangle (GND) on the bottom-left]

In our example, we will be using a 4-ohm resistor.

  1. Take down the measurements with the voltmeter. 1.446V

The voltmeter should show a lower voltage value. The drop in voltage is caused by the internal resistance of the battery. We can calculate the internal resistance if we take the readings of the open-circuit voltage (VOC) and the voltage across the battery with a load, which is a 4-ohm resistor in our case, attached. 

  1. Use the Ohm’s Law formula and Kirchhoff’s Voltage Law formula to calculate the Internal Resistance.

We will first plug in the value we receive into Ohm’s Law to determine the current flowing through the circuit. 

Abbreviation:

V = Voltage

I = Current

R = Resistance

VL = Loaded Voltage

RL = Resistor Value

V = I · R

VL = I · RL

1.446V = I · 4Ω

I =  1.446V

I = 0.3615A

We will then use Kirchhoff’s Voltage Law to determine the voltage across the internal resistor in the battery. This value is also the voltage drop across the internal resistor.

Abbreviation:

VOC = Open-circuit Voltage

VI = Voltage across Internal Resistor

VL = Loaded Voltage

VOC = VI + VL

1.500V = VI + 1.446V

VI = 1.500 – 1.446V

VI = 0.054

Now, we have the value of the voltage drop across the internal resistor, and the current that’s flowing through the circuit. We can now use Ohm’s Law again to find the Internal Resistance of the Battery.

Abbreviation

VI = Voltage across Internal Resistor

I = Current

RI = Internal Resistance

VI = I · RI

0.054V = 0.3615A · RI

RI = 0.149Ω

From this, we can see the internal resistance of the AA battery is 0.149Ω!

Recommended Tools and Products

I hope you’ve learnt how to measure the internal resistance of a battery and also how and when to apply it! Here are some battery related tools and products which can help you with your IoT projects!

UPS Module for Raspberry Pi Pico – Uninterruptible Power Supply

This Uninterruptible Power Supply (UPS) is a module designed to keep your Raspberry Pi Pico running with a Li-ion battery. It also comes with a voltage/current monitoring chip and a battery status indicator. 

Features

  • Standard Raspberry Pi Pico header
  • Li-ion battery compatibility with dynamic power pats management for a stable power supply
  • I2C bus communication to monitor battery voltage, current, power and the remaining capacity
  • Several Battery Protection Measures i.e. overcharge/discharge protection, over current protection, short circuit protection, reverse protection, and equalizing charge feature
  • Onboard charging, power and battery warning indicator

0.28 Inch LED digital DC voltmeter

This is a supermini 0.28inch LED display. Do not be deceived by its small exterior, it comes with a reverse connect protection feature. This voltmeter can be used to measure mobile phone battery, car battery, and other such applications. There are several colours available as well: Yellow, Red, Green, and Blue.

Specification

  • Voltage Range: 2.5 – 30V
  • Work Current <30mA
  • Size: 30×11.7×9.2 mm
  • Measure speed: 200ms/one time
  • Accuracy 3%

18650 Battery Holder Case – 2 Slot with Switch

This is a battery case for your 18650 rechargeable batteries! This battery holder case can fit two 18650 rechargeable batteries, and it comes with a built-in switch. There are wire leads integrated into the case for soldering/connecting to your IoT projects!

18650 Battery Holder Case – 4 Slot

This is a four-slot battery case for your 18650 rechargeable batteries! This case can fit four 18650 rechargeable batteries. Similarly to the aforementioned battery holder case, there are wire leads integrated into the case for soldering/connecting to your IoT projects!

Summary

I hope you learnt more about measuring the internal resistance of batteries. It can be a fun project for you to try out if you’re new to building circuits. If you have any questions, feel free to leave them in the comment down below!

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