The internal resistance of the battery is the most important characteristic. It quite accurately determines the overall condition of the battery and the remaining resource. Battery testers calculate the maximum starting current based on the internal resistance. To measure this parameter accurately, you need a special device. The simplest multimeter and Ohm's law can do approximately the same thing.

What is the internal resistance of a battery?

The internal resistance of a battery is the resistance that the battery offers to the electrical current flowing through it. The lower it is, the better. Schematically, it can be represented as an EMF source with a resistor connected in series to it. This is shown in the picture below.

Battery internal resistance diagram
Battery internal resistance diagram

Physically, the internal resistance of a car battery consists of several factors determined by its design. This is least affected by lead, from which the battery plates and terminals are made.

The electrolyte in a charged battery contains a lot of sulfuric acid and is a good conductor of electric current. The electrolyte of a discharged battery contains little acid and consists mostly of distilled water, which is a poor conductor of current.

The most significant factor affecting the internal resistance of the battery is the area of the active lead plates. When the battery is new, this area is maximum, since the plates are not covered with lead sulfate. When sulfates are abundant, they reduce the active area that can contact and interact with the electrolyte. This phenomenon is called sulfation and it greatly affects the internal resistance of the car battery.

A new battery may also have internal resistance that is higher than normal. The main reason is that the manufacturer saved on lead by reducing the number of plates, which directly affects the battery characteristics under consideration. During operation, lead plates tend to break down and crumble. This process also increases the internal resistance of the battery, reducing its other characteristics.

What should the internal resistance of the battery be?

The normal internal resistance of a properly charged car battery with liquid electrolyte is in the range of 4-6 mOhm. For AGM it is lower due to a specific device - 3-4 mOhm. After 4 years of operation, this parameter may increase to 13-15 mOhm. In this state, the battery is still able to work somehow, but you shouldn’t expect much from it. If the internal resistance of an acid battery exceeds 20-30 mOhm, it is considered unsuitable for further use.

How to measure the internal resistance of a battery?

It is impossible to measure the internal resistance of a battery using a conventional ohmmeter. This is due to the fact that the battery is not only a resistor, but also an EMF source connected in series with it. The easiest and most accessible way to find the internal resistance of a battery is to measure the voltage drop across its terminals under a known load. Then, using Ohm's law, calculate the resistance using this formula:

R =  U/I;

where R is the internal resistance of the battery;

U is the voltage drop under a known load;

I is a known load.

This method is programmed into battery internal resistance testers. It can also be used for calculations using a simple multimeter.

Special battery tester

Measuring the internal resistance of the battery using a special tester is the simplest and most accurate way. You need to enter the initial data into the device and connect it to the battery terminals. The tester will measure the voltage without load, then load the battery with a small current, and calculate the internal resistance based on the voltage drop.

Battery tester
Battery tester

Most testers for car batteries simultaneously display the maximum starting current and “health”. How are these readings calculated? To calculate the maximum inrush current, the device uses the measured internal resistance and the user-entered nominal value indicated on the battery label.

It is important to understand that testers of this type do not load the battery with a current of 600 A. They calculate this parameter. In a similar way, the “health” of the battery is calculated as a percentage.

What else do you need to know about these devices? They measure internal resistance quite accurately, and their readings can be trusted. As for the maximum starting current and “health”, then according to these readings you can trust 50 to 50. This is because the device with the described operating algorithm does not directly measure the cold cranking current, but calculates it based on the internal resistance and the nominal value entered by the user.

This means only one thing. If the internal resistance tester shows a high starting current and 100% “health,” this does not guarantee that the battery in front of you is in perfect condition. In half the cases, such a battery may not pass the test with a load fork, as well as the measurement of real capacity by discharging. Especially when we are talking about checking a battery that is not new. When choosing a new battery, such a pitfall is extremely rare.


To measure the internal resistance of a battery, you will need a multimeter and some load of known power. The easiest way is to use a car lamp from a headlight. To find out how much current it consumes, you need to divide the power by the rated voltage of the battery. For example, a 50 W car halogen low beam lamp consumes a current of 50: 12 = 4 A. To obtain more accurate data, you can connect an ammeter in series with the load.

The internal resistance of the battery is measured using the following algorithm:

  1. Turn the multimeter into DC voltage measurement mode within 20 V.
  2. Connect the probes to the battery terminals.
  3. Connect a load to the battery.
  4. Wait for the voltage to stabilize and record it (usually 3-5 seconds).
  5. Disconnect the load.
  6. Record the no-load voltage.
  7. Calculate the difference between the voltage without load and with load.
  8. Divide the resulting voltage drop by the current drawn by the load.

The calculated value will be the desired internal resistance of the battery.

Note. To obtain correct data using this method, measurements should be carried out on a settled battery at a temperature of +20 degrees Celsius. If the battery has just been charged from a generator or stationary charger, the calculations will not be very accurate. The same will happen in the case of a “cold” battery.

10 facts about internal battery resistance

In conclusion, I offer not everyone obvious facts about the internal resistance of the battery:

  1. The lower the internal resistance, the better.
  2. A battery with normal internal resistance can be charged at higher currents with less heat.
  3. In half the cases, a battery with low resistance is capable of delivering a high cold cranking current.
  4. The internal resistance cannot accurately determine the battery capacity. There are also such testers, but you should not blindly trust their readings, since they do not measure the real capacity, but only calculate it.
  5. Not every battery is capable of delivering high starting current with low internal resistance.
  6. Only in half of the cases is the assessment of the remaining battery life based on internal resistance correct.
  7. Measuring the internal resistance is useful when choosing a new battery.
  8. It is advisable to measure the resistance when desulfating the battery. If after the next cycle it decreases, then the procedure can be considered successful.
  9. The internal resistance of a battery cannot be measured with a simple ohmmeter.
  10. The measurement requires a special device or voltmeter with a known load.

Overall, the internal resistance of a battery is an important and useful characteristic. It allows you to exclude the purchase of defective parts when choosing a new battery, and to evaluate the approximate “health” of the battery after several years of operation. This is a good visual tool if you are solving a problem where the electrolyte density does not increase when charging the battery.

Is it worth buying an expensive battery internal resistance tester? If you are involved in battery maintenance professionally, then you definitely should have such a device. It's like with a hydrometer and a refractometer. A simple car enthusiast does not really need such a tester. If you want to find out the internal resistance of your battery, you can do this quite accurately using a multimeter and a light bulb from a headlight.