Last Updated on March 12, 2003
Written and compiled By Bill Darden (freely reproducible public domain)

Re-produced here by www.optimabattery.co.uk for your information, and it will be updated regularly.

 

4. HOW DO I TEST A BATTERY?

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, which can be purchased at an auto parts or battery store for between $5 and $20. 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.

Hydrometer

[Source: Popular Mechanics]


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. A digital voltmeter can be purchased at an electronics store, such as Radio Shack, Jameco, Frys, etc., for between $50 and $400. 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 CCA (Cold Cranking Amp) or capacity of lead-acid car batteries is by using a conductance tester, such as a Midtronics, costing between $100 and $600. The most accurate way of testing a car battery is a full capacity load test.

4.1. INSPECT

Visually inspect for obvious problems such as a loose or broken alternator belt, 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.

Electrolyte Levels


4.2. RECHARGE

Recharge the battery to 100% State-of-Charge. If 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. (Please see Section 9.)

4.3. REMOVE SURFACE CHARGE

Surface charge is the uneven mixture of sulfuric 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:

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

4.3.2. Turn the headlights on high beam for five minutes, shut them off, and wait five to ten minutes.

4.3.3. With a battery load tester, apply a load at one-half the battery's CCA rating for 15 seconds and then wait five to ten minutes.

4.3.4. Disable the ignition, turn the engine over for 15 seconds with the starter motor, and wait five to ten minutes.

4.4. MEASURE THE STATE-OF-CHARGE

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 table assumes that a 1.265 specific gravity and 12.65 VDC 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 or less

DISCHARGED

1.120 or less

20°F
(-7°C)


STATE-OF-CHARGE

[Source: BCI]


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°

-.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


TEMPERATURE COMPENSATION


Electrolyte temperature compensation, depending on the battery manufacturer's defination 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 (.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 cells readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate .5% (or better) 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.

Magic Eye

[Source: Popular Mechanics]


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:

4.4.1. 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). Applying an EQUALIZING charge may correct this condition. (Please see Section 9.)

4.4.2. 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). 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 4.2 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.

4.4.3. If a digital voltmeter indicates 0 volts, there is an open cell.

4.4.4. 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.

4.5. LOAD TEST

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 the battery by one of the following methods:

4.5.1. Disable the ignition and turn the engine over for 15 seconds with the starter motor.

4.5.2. With a battery load tester, apply a load equal to one half of the CCA rating of the battery for 15 seconds.

4.5.3. With a battery load tester, apply a load equal to one half the OEM cold cranking amp specification for 15 seconds.

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:


Electrolyte Temperature F

Electrolyte Temperature C

Minimum Voltage Under LOAD

100°

37.8°

9.9

90°

32.2°

9.8

80°

26.7°

9.7

70°

21.1°

9.6

60°

15.6°

9.5

50°

10.0°

9.4

40°

4.4°

9.3

30°

-1.1°

9.1

20°

-6.7°

8.9

10°

-12.2°

8.7

-17.8°

8.5


LOAD TEST

[Source: Interstate Batteries]


4.6. BOUNCE BACK TEST

If the battery has passed the load test, please go to Section 4.7 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 (please see Section 9.) 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.

4.7. RECHARGE

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.

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