Testing car battery voltage

A car battery tester is a simple and quick method for determining the state of charge and health of a car battery. With 40% of roadside breakdowns being battery related, unexpected battery failure is an all to common occurrence!

Factory defects account for only 5 to 7 percent of all warranty claims during the warranty period; with driving habits and poor car battery maintenance being the main reasons behind why a car battery fails. Only a few starter batteries reach their expected design life!

A careful assessment of the condition of the battery, using a car battery tester capable of identifying various problem indicators, can greatly reduce the number of breakdowns. There are different types of tester, from the simple hydrometer to the more advanced impedance testers.

Many batteries include a ‘fuel gauge’ that shows the remaining energy, but these are unreliable because a full charge always shows 100 percent; whether the battery is new or nearing the end of its life. The consumer can mistakenly take this to mean that a faded battery indicating a full charge will deliver the same runtime as a new one. Batteries with fuel gauges (Magic Eyes) only indicate State of Charge (SoC) and not the capacity.

While no practical method exists to quantify all conditions of a battery in a short, comprehensive test, there are car battery condition testers that can provide the most important information to determine the SoC and State of Health (SoH) – the two metrics that, together, provide the best overview of the battery’s condition.

| How to test a car battery

In the previous articles, “How to test a car battery with a multimeter” and “How to measure car battery voltage.” we described how to carry out a general SoC car battery voltage test using a multimeter; In this section we’ll highlight additional testing needed to fully evaluate the battery’s condition in order to avert an unexpected breakdown.

| Testing a car battery’s State of Charge (SoC)

While measuring the battery’s SoC by voltage is simple (As described in “How to measure car battery voltage.”), it is often inaccurate because cell materials and temperature affect the voltage. This is most pronounced after charging or discharging the battery which distorts the voltage reading so that it no longer represents a correct SoC.

To get accurate readings, the battery needs to rest in the open circuit state for at least four hours, although battery manufacturers recommend 24 hours.

An alternative to this method requires the use of a hydrometer (for wet cells) to measure the Specific Gravity (SG) of the electrolyte. This method is only suitable for wet cell car batteries.

For non-sealed batteries, check the specific gravity in each cell and average the cell readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with a digital voltmeter (As described above).

The table below, shows the relative state of charge (SoC) and open circuit voltage for a given average SG reading of a battery.

Table measuring car battery state of charge
Car Battery State of Charge

Temperature Adjustment: You will need to make adjustments to the readings to compensate for temperature. To adjust the specific gravity reading for temperature:

  • Add .004 to the specific gravity reading for every 6°C above 26.7°C
  • Subtract .004 for every 6°C below 26.7°C.

Battery Failure: You should replace the battery if one or more of the following conditions occur:

  • If there is a difference in the specific gravity readings greater than .050, you have a weak or dead cell(s)
  • If the battery will not recharge to a 75 percent or more state-of-charge level
  • If a digital voltmeter indicates 0 volts, there is an open cell
  • If the digital voltmeter indicates 10.45 to 10.65 volts, there probably is a shorted cell

| Load testing the battery

A load test simulates the effects on the voltage of the car battery during starting.

Car battery load tester

A load test can be carried out using a car battery load tester. These testers put a designated load on the battery for a specified amount of time. The load is normally a given percentage of the cold cranking amps (CCA) rating of the The CCA rating is normally shown on the side of the battery.

Before carrying out a load test on the car battery first make sure that the battery’s state-of-charge is at 75 percent or higher.

If the battery is good, the voltage will not drop below the following voltages at the indicated electrolyte temperatures:

Table of car battery voltage under load testing
Load Testing Chart

If the voltage does drop below these values, then first check the battery was fully charged and repeat the test. If the battery voltage again drops below these values then the battery will need replacing.

If you don’t have or don’t want to purchase a designated battery load tester, then you can test the car battery voltage using a multimeter.

| Impedance and Conductance Testing

This test is based on the fact that battery impedance (internal resistance of the battery) increases with age. Impedance measurements need to be compared to previous recorded readings.

It is important to note that while impedance increases as the battery deteriorates the conductance (the ability of the battery to conduct electricity) decreases. Conductance is a direct indication of the battery’s ability to produce current and its capacity, whereas impedance gives an inverse correlation. So by measuring the conductance of a battery you get a indirect measurement of its impedance and an approximation of its state of health.

Impedance and conductance testing are reliable, safe, accurate, fast and they don’t affect the battery performance. They can be carried out while the battery is in use or they can be used to continuously monitor the battery performance, avoiding the need for load testing or discharge testing.

| Using a Monitor

Car battery monitors are small devices that are permanently fixed to the battery or cars electrical system. They provide a continuous voltage reading either on the device through a set of LED’s or digital display. More advanced versions combine a mobile app so the voltage data can be viewed directly on your smartphone.

The devices are inexpensive and simple to use, often as easy to use as a multimeter.

In addition to impedance and conductance these tests will obviously also uncover other cell defects such as shorts, and open circuits.

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