Vehicle Battery Testing and Load Testing

How to Test and Load Test Your Vehicle Battery

Your vehicle's battery is not very demanding, and most often only thought about when it fails. But just a small amount of care and maintenance will help ensure it doesn't let you down when you need it most.

Lack of battery care and maintenance combined with cold weather has a way of bringing out the borderline batteries that were fine in the summer, but don't have the power for a cold weather start.

And not just winter, battery care and maintenance is a year-round requirement. You want to catch a bad battery before it lets you down, usually on one of the coldest days of the year.

Testing and maintaining a battery is fairly simple and only requires a few basic tools. If you only think about your battery once a year, fall would be a good time to go outside and spend some quality time with your battery.

Important Safety Note: Before you do anything with a battery, you need to wear eye protection and keep any open flames away from the battery. This includes cigarettes and other smoking products. Batteries produce hydrogen gas that is extremely flammable. Batteries contain sulfuric acid so I would also recommend wearing latex gloves to keep battery acid from burning your hands.

Testing The Battery

1. Tools

If you have a non-sealed battery, it is highly recommended that you use a good quality temperature compensating hydrometer.

There are two basic types of hydrometers, the floating ball type and a gauge type. Personally, I prefer the gauge type. They are, I think, much easier to read and I don't have to stand there trying to decipher colored balls. Battery hydrometers can be purchased at an auto parts or battery store for less than $20.00.

To test a sealed battery or to troubleshoot a charging or electrical system, you will need a digital voltmeter with 0.5% (or better) accuracy. A digital voltmeter can be purchased at an electronics store, such as Radio Shack, for less than $50.00. Analog (needle type) voltmeters are not accurate enough to measure the millivolt differences of a battery's State-of-Charge or measure the output of the charging system. A battery load tester is optional.

2. Inspect The Battery

Look for obvious problems such as a loose or broken alternator belt, low electrolyte levels, a dirty or wet battery top, corroded or swollen cables, corroded terminal mating surfaces or battery posts, loose hold-down clamps, loose cable terminals, or a leaking or damaged battery case. Repair or replace such items as required. Distilled water should be used to top off the battery fluid level.

3. Recharge The Battery.

Recharge the battery to 100% State-of-Charge. If non-sealed battery has a .030 (sometimes expressed as 30 "points") or more difference in specific gravity reading between the lowest and highest cell, then you should equalize the battery using the battery manufacturer's procedures.

4. Remove The Surface Charge.

The surface charge, if not removed, will make a weak battery appear good or a good battery appear bad.

Eliminate the surface charge by allowing the battery to sit for between four to twelve hours in a warm room.

5. Measure The State-Of-Charge.

To determine the battery's State-of-Charge with the battery's electrolyte temperature at 80° F (26.7° C), use the following table. The table assumes that a 1.265 specific gravity cell average and 12.65 VDC Open Circuit Voltage reading for a fully charged, wet, lead-acid battery.

If the electrolyte temperature is not 80° F (26.7° C), use the Temperature Compensation table to adjust the Open Circuit Voltage or Specific Gravity readings.

The Specific Gravity or Open Circuit Voltage readings for a battery at 100% State-of-Charge will vary by plate chemistry, so check the manufacturer's specifications for a fully charged battery.

Temperature Compensation Table

Open Circuit VoltageApproximate State-of-Charge at 80°F (26.7°C)Hydrometer Average Cell Specific GravityElectrolyte Freeze Point
11.89 or lessDISCHARGED1.120 or less20°F(-7°C)

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average cells readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with a digital voltmeter. This is the only way you can determine the State-of-Charge. Some batteries have a built-in "Magic Eye" hydrometer, which only measures the State-of-Charge in ONE of its six cells. If the built-in indicator is clear, light yellow, or red, then the battery has a low electrolyte level and if non-sealed, should be refilled and recharged before proceeding.

If sealed, the battery is bad and should be replaced. If the State-of-Charge is BELOW 75% using either the specific gravity or voltage test or the built-in hydrometer indicates "bad" (usually dark or white), then the battery needs to be recharged BEFORE proceeding. You should replace the battery, if one or more of the following conditions occur:

  1. If there is a .050 (sometimes expressed as 50 "points") or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s). Using the battery manufacturer's recommended procedure, applying an Equalizing charge may correct this condition.
  2. If the battery will not recharge to a 75% or more State-of-Charge level or if the built-in hydrometer still does not indicate "good" (usually green or blue, which indicates a 65% State-of-Charge or better).
  3. If a digital voltmeter indicates 0 volts, there is an open cell.
  4. If the digital voltmeter indicates 10.45 to 10.65 volts, there probably is a shorted cell. A shorted cell is caused by plates touching, sediment ("mud") build-up or "treeing" between the plates.

6. Load Test The Battery

If the battery's State-of-Charge is at 75% or higher or has a "good" built-in hydrometer indication, then you can load test a car battery by one of the following methods:

  1. With a battery load tester, apply a load equal to one-half of the CCA rating of the battery for 15 seconds. (Recommended method).
  2. With a battery load tester, apply a load equal to one-half the vehicle's CCA specification for 15 seconds.
  1. Disable the ignition and turn the engine over for 15 seconds with the starter motor.

During the load test, the voltage on a good battery will NOT drop below the following table's indicated voltage for the electrolyte at the temperatures shown:

Load Test

Electrolyte Temperature °FElectrolyte Temperature °CMinimum Voltage Under LOAD

If the battery is fully charged or has a "good" built-in hydrometer indication, then you can test the capacity of a deep cycle battery by applying a known load and measuring the time it takes to discharge the battery until measures 10.5 volts. Normally a discharge rate that will discharge a battery in 20 hours can is used.

For example, if you have an 80 ampere-hour rated battery, then an average load of four amps would discharge the battery in approximately 20 hours. Some new batteries can take up to 50 charge/discharge "preconditioning" cycles before they reach their rated capacity. Depending on your application, fully charged batteries with 80% or less of their original rated capacity available are considered to be bad.

7. Bounce Back Test The Battery

If the battery has passed the load test, please go to Tip #8, below. If not, remove the load, wait ten minutes, and measure the State-of-Charge. If the battery bounces back to less than 75% State-of-Charge (1.225 specific gravity or 12.45 VDC), then recharge the battery and load test again. If the battery fails the load test a second time or bounces back to less than 75% State-of-Charge, then replace the battery because it lacks the necessary CCA capacity.

8. Recharge The Battery

If the battery passes the load test, you should recharge it as soon as possible to prevent lead sulfation and to restore it to peak performance.