Below are seven simple steps in testing a car battery. If you have a non-sealed battery it is highly recommended that you use a good quality hydrometer. A hydrometer is a float-type device used to determine the State-of-Charge by measuring the specific gravity of the electrolyte in each cell. It is an accurate way of determining a battery's State-of-Charge and weak or dead cells.

If you have a sealed battery, or to troubleshoot charging or electrical system, you will need a digital voltmeter with 0.5% (or better) accuracy Analog 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. Another way of testing the capacity of lead-acid deep cycle batteries is by using a conductance tester. The most accurate way of testing deep cycle batteries is a full capacity load test.


Visually inspect for obvious problems such as a low electrolyte levels, dirty or wet battery top, corroded or swollen cables, corroded terminals or battery posts, loose hold-down clamps, loose cable terminals, or a leaking or damaged battery case.
If the electrolyte levels are low in non-sealed batteries, allow the battery to cool and add only distilled water to the level indicated by the battery manufacturer or to between 1/4 to 3/8 inch (6 to 10 mm) below the bottom of the plastic filler tube (vent wells). The plates need to be covered at all times. Avoid overfilling, especially in hot climates, because heat will cause the electrolyte to expand and overflow.


Recharge the battery to 100% State-of-Charge. If a non-sealed battery has a difference of 0.03 (or more) specific gravity reading between the lowest and highest cell, then you should equalize the battery. (Section 6.)


Surface charge is the uneven mixture of sulphuric acid and water along the surface of the plates as a result of charging or discharging. It will make a weak battery appear good or a good battery appear bad. You need to eliminate the surface charge by one of the following methods after recharging a lead-acid car battery:

Allow the battery to sit for four to twelve hours to allow for the surface charge to dissipate.

Apply a load that is 33% of the ampere-hour capacity for five minutes and wait five to ten minutes.

With a battery load tester, apply a load of at least one third the battery's amp hour rating for 15 seconds and wait five to ten minutes.


If the battery's electrolyte is above 110°F (43.3°C), allow it to cool. 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 following table assumes that a 1.265 specific gravity and 12.65V DC reading is a fully charged, wet, lead-acid battery. For other electrolyte temperatures use the Temperature Compensation table below 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.


Digital Voltmeter Open Circuit Voltage Approximate State-of-Charge 80°F (26.7°C) Hydrometer Average Cell Specific Gravity Electrolyte Freeze Point
12.65 100% 1.265 -77°F (-67°C)
12.45 75% 1.225 -35°F (-37°C)
12.24 50% 1.190 -10°F (-23°C)
12.06 25% 1.155 15°F (-9°C)
11.89 Discharged 1.120 20°F (-7°C)


Electrolyte Temperature Fahrenheit Electrolyte Temperature Celsius Add or Subtract to Hydrometer's SG Reading Add or Subtract to Digital Voltmeter's Reading
160° 71.1° +.032 +.192
150° 65.6° +.028 +.168
140° 60.0° +.024 +.144
130° 54.4° +.020 +.120
120° 48.9° +.016 +.096
110° 43.3° +.012 +.072
100° 37.8° +.008 +.048
90° 32.2° +.004 +.024
80° 26.7° 0 0
70° 21.1° -.004 -.024
60° 15.6° -.008 -.048
50° 10.0° -.012 -.072
40° 4.4° -.016 -.096
30° -1.1° -.020 -.120
20° -6.7° -.024 -.144
10° -12.2° -.028 -.168
-17.8° -.032 -.192

Electrolyte temperature compensation, depending on the battery manufacturer's definition of 100% State-of-Charge, will vary. If you are using a non-temperature compensated hydrometer make the adjustments indicated in the table above. For example, if the electrolyte is at 80°F (26.7°C), and the specific gravity reading is 1.265 for a 100% State-of-Charge, then when the electrolyte is at 20°F (-6.7°C), the specific gravity reading would be 1.289 for a 100% State-of-Charge. At 100°F (37.8°C), the specific gravity reading would be 1.257 for 100% State-of-Charge. This is why using a temperature compensated hydrometer is highly recommended and more accurate. If you are using an accurate (0.5% or better) digital voltmeter, make the adjustments indicated in the table above. For example, if the electrolyte is at 80°F (26.7°C), and the voltage reading is 12.65 for a 100% State-of-Charge, then when the electrolyte is at 20°F (-6.7°C), the voltage reading would be 12.79 for a 100% State-of-Charge. At 100°F (37.8°C), the voltage reading would be 12.60 for 100% State-of- Charge.

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average the cell readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate (0.5% or better) digital voltmeter. This is the only way you can determine the State-of-Charge. Some batteries have a built-in 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:

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). Applying an equalizing charge may correct this condition. (Section 6.)

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). If you know that a battery has spilled or "bubbled over" and the electrolyte has been partially replaced with water, you can replace this old electrolyte with new electrolyte and go back to Step 3.2 above. Battery electrolyte (battery acid) is a mixture of 25% to 35% sulfuric acid and distilled water when fully charged. It less expensive to replace the electrolyte than to buy a new battery.

If the digital voltmeter indicates 0 volts, you have an open cell.

If the digital voltmeter indicates 10.45 to 10.65 volts, you probably have a shorted cell or a severely discharged battery. A shorted cell is caused by plates touching, sediment ("mud") build-up or " treeing" between the plates.


If the battery is fully charged or has a "good" built-in hydrometer indication, then you can test the capacity of the battery by applying a known load and measuring the time it take to discharge the battery until measures 10.5 volts. Normally a discharge rate that will discharge a battery in 20 hours 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. New batteries can take up to 50 charge/discharge 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.


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