The Load Test

What Is a Load Test?

A Load Test is exactly what the name suggests! A measured electrical load is applied to the battery for a specific time using a device called a Load Tester.

Load Testers
The Load Tester is commonly referred to as a Volt-Amp Tester, or VAT for short. Its main components include:

• A voltmeter connected to the battery terminals by large cables and clamps
• An ammeter, connected around either the VAT main cables with an inductive current clamp, also referred to as an amp probe.
• An adjustable carbon pile inside the load tester that creates an electrical load. The amount of load applied to the battery is adjusted with a knob on the front of the load tester.

What is a Load?

A Load is any electrical consumer. During normal vehicle operation, many types of load consume electrical power. Examples of loads include: the starter, headlights, ignition system, fuel pump, comfort and convenience items, and heater blower. Even the vehicle computer is a load.

Performing the Load Test – Part One

• Before you perform the load test, make sure the battery is at 75% of charge or better.
• Connect the load tester to the battery, and clamp the amp probe around either of the battery cables.

Performing the Load Test – Part Two

• Rotate the knob on the load tester to apply a load equal to: ½ the battery’s Cold Cranking Amps (CCA) rating
• Apply the load for exactly 15 seconds. At exactly 15 seconds, note the battery voltage and turn off the load.
• A battery at room temperature should have 9.6 volts across its posts at the end of the 15 second load test. If the load tester has an analog voltmeter, connect your DMM to the battery during the test to get an accurate battery voltage reading. The analog meter is not accurate enough.

Important Notes:

• Rotate the load tester knob to apply the correct load as quickly as possible. This ensures accurate test results.
• Measure the battery voltage at exactly 15 seconds, once again to ensure an accurate battery evaluation.
• Do not apply ½ of a CA rating. Use only the CCA rating.

Evaluating the Load Test

Battery voltage should be at least 9.6 volts at the end of the 15 second load test with battery temperature at 70 degrees F. Slightly lower voltage values are normal when temperatures are below 70 degrees F. Refer to the chart to the right to make the corrections for battery voltage minimums when battery temperature is below 70 degrees F.

• Batteries that barely fail when tested at 75% may pass when fully charged. (For example, the battery voltage drops to 9.4 volts.) Recharge to 100% and retest.
• Batteries that pass with room to spare when tested at 75% have a better than average reserve capacity. (For example, battery voltage is 9.8 volts at the end of the load test.)

If the battery fails the load test, continue to watch the open circuit voltage for several minutes. If the battery open circuit voltage bounces back and rises above 12.45 volts, don’t be fooled into thinking its good. It is probably on its last legs.

Temperature and Relative Post Voltage

Temperature affects performance during the load test. Compensate for changes in battery temperature as follows:

Temperature  Load Voltage

70 degrees F  9.6 Volts
60 degrees F  9.5 Volts
50 degrees F  9.4 Volts
40 degrees F  9.3 Volts
30 degrees F  9.1 Volts
20 degrees F  8.9 Volts
10 degrees F  8.7 Volts
0 degrees F  8.5 Volts

Bounce Back Voltage

Here’s why the battery is bad if it bounces back above 12.45V after failing its load test: While the battery acid is strong enough to generate the high no-load voltage, the battery doesn’t have enough good plate material remaining to provide both voltage and current during the load test.

The battery is a pretender and will probably fail to provide enough power under high electrical load conditions.