Battery load test

A battery load test determines how well the battery performs under load. The test determines the battery’s ability to deliver starting current and maintain sufficient voltage to operate other systems. In other words, if the battery can sustain a high discharge for some time and the voltage is still relatively high, then you know the battery has good capacity and is in good condition. If the voltage drops quite quickly, the battery capacity is low.

The battery load tester serves to carry out the load test. It contains an ammeter, a voltmeter, and a high power variable resistor. Depending on the strength of the battery, the amount of load for testing is determined. The tester draws current from the battery through the resistor. An ammeter measures the amount of current drawn. The maximum draw current and battery voltage drop define the battery state.

There are several prerequisites for the battery test. The battery electrolyte should be at room temperature, between 21°C and 27°C. A cold battery shows a significantly lower capacity. Never test a sealed battery if its temperature is below 15°C. A load test is performed only on a fully charged battery.

Battery load tester

When testing the battery load, use the following test parameters:

Test load = half of the starting current of the battery
Loading test time = 15 seconds
Results: Pass = 9.6 V or greater; Error = less than 9.6V

If the battery fails the load test, another test is required before a final judgment is made on the battery. Charge the battery with a 20-hour charging current and repeat the load test. If the test results are still weak, the battery is sulfated and must be replaced with a new battery. If it passes the retest, test the vehicle’s charging system to see if it is properly charging the battery.

Some manufacturers emphasize that their batteries should not be load tested, but instead tested for conductivity. They claim that load testing can damage the battery. Therefore, always check the service information for a given battery before performing a load test.

To perform a battery load test, follow these steps:

Connecting testers

Use an infrared temperature gun to measure the temperature on the side of the battery and check that it is within the test parameters. Connect the red clip to the plus pole of the battery and the black to the minus pole.

Testing

If using an automatic load tester, enter the starting battery current. When using a handheld load tester, calculate the load, which is usually half the starting current (eg 700A/2 = 350A – test current), and set the load.

Results analysis

Test for 15 seconds. Read the test voltage on the voltmeter and immediately turn off the tester. If after 15 seconds the test voltage is 9.6 V or higher, the battery is good. When the test voltage is less than 9.6 V, charge the battery and repeat the load test. If the voltage is lower than 9.6V again, the battery is faulty. However, if the repeated test voltage is higher than 9.6V, check the power supply system.