AUTOMOTIVE BATTERIES

4. HOW DO I TEST A BATTERY?

There are seven simple steps in testing a car battery-inspect, recharge, remove surface charge, load test, bounce back test and recharge. If you have a non-sealed battery, it is highly recommended that you use a good quality, temperature-compensated hydrometer, which can be purchased at an auto parts 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 a very accurate way of determining a battery's state-of-charge and weak or dead cells. To troubleshoot charging or electrical systems or if you have a sealed battery, 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 between $20 and $200. Analog voltmeters are not accurate enough to measure the millivolt differences of a battery's state-of-charge or the output of the charging system. A battery load tester is optional.

4.1. INSPECT

Visually inspect for obvious problems: loose or broken alternator belt, electrolyte levels BELOW the top of the plates, dirty battery top, corroded or swollen cables, corroded terminal clamps, 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 DISTILLED water to the level indicated by the battery manufacturer or to between 1/8 to 1/4 inch (3 to 7 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, becauseheat will cause the electrolyte to expand and overflow.

4.2. RECHARGE

Recharge the battery to 100% state-of-charge. If the battery has a difference of 0.03 specific gravity reading between the lowest and highest cell, then you should equalize it. (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:

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 110o F (43.3o C), allow it to cool. To determine the battery's state-of-charge with the battery's electrolyte temperature at 80o F (26.7o C), use the following table, which assumes that 1.265 specific gravity reading is a fully charged battery:

Digital Voltmeter Open Circuit Voltage Approximate State-of-Charge Hydrometer Average Cell Specific Gravity Electrolyte Freeze Point
12.65 100% 1.265 -75o F (-59.4o C)
12.45 75% 1.225 -55o F (-48.3o C)
12.24 50% 1.190 -34o F (-36.7o C)
12.06 25% 1.155 -16o F (-26.7o C)
11.89 Discharged 1.120 -10o F (-23.3o 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
160o 71.1o +.032 +.192
150o 65.6o +.028 +.168
140o 60.0o +.024 +.144
130o 54.4o +.020 +.120
120o 48.9o +.016 +.096
110o 43.3o +.012 +.072
100o 37.8o +.008 +.048
90o 32.2o +.004 +.024
80o 26.7o 0 0
70o 21.1o -.004 -.024
60o 15.6o -.008 -.048
50o 10o -.012 -.072
40o 4.4o -.016 -.096
30o -1.1o -.020 -.120
20o -6.7o -.024 -.144
10o -12.2o -.028 -.168
0o -17.8o -.032 -.192

TEMPERATURE COMPENSATION

Electrolyte temperature compensation, depending on the battery manufacturer's recommendations, will vary. If you are using a NON-temperature compensated HYDROMETER, make the adjustments indicated in the table above. For example, at 30 o F (-1.1o C), the specific gravity reading would be 1.245 for a 100% state-of-charge. At 100 o F (37.8o C), the specific gravity would be 1.273 for 100% state-of-charge. This is why using a temperature compensated hydrometer is highly recommended and more accurate than other means. If you are using a DIGITAL VOLTMETER, make the adjustments indicated in the table above. For example, at 30 o F (-1.1o C), the voltage reading would be 12.53 for a 100% state-of-charge. At 100 o F (37.8o C), the voltage would be 12.698 for 100% state-of-charge.

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average the readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate 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 or light yellow, then the battery has a low electrolyte level and should be refilled and recharged before proceeding. If sealed, the battery is toast 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), 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). If you are really lucky, 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, 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 is a mixture of 25% sulfuric acid and distilled water. It is cheaper to replace the electrolyte than to buy a new battery.

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

4.4.4. If the digital voltmeter indicates 10.45 to 10.65 volts, you probably have 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. Turn the headlights on high beam for five minutes.

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

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

4.5.4. With a battery load tester, apply a load equal to one half the OEM 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 with the electrolyte at the shown temperatures:
ElectrolyteTemperature F ElectrolyteTemperature 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
0 -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. After the load is removed, 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 more than 75% state-of-charge, then you should replace it because it lacks the necessary CCA power.

4.7. RECHARGE

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


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