4. HOW DO I TEST A BATTERY?

Last Updated on November 14, 2006

INDEX:

4.1. Inspect

4.2. Charge

4.3. Remove Surface Charge

4.4. Measure State-of-Charge (SoC)

4.4.1. Specific Gravity vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Car Battery Table

How Do I Use a Hydrometer?

Electrolyte Freeze Points Table

4.4.2. Open Circuit Voltage vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Car Battery Table

4.4.3. Open Circuit Voltage vs. Temperature at Various States Of Charge (SoC) for a Wet "Maintenance Free" (Ca/Ca) Car Battery Table

4.4.4. Interpreting the SoC Measurements

4.5. Capacity Load Test

Capacity Load Test Table

4.6. Bounce Back Test

4.7. Recharge

4.8. Refill

While working with car and deep cycle lead-acid batteries, please help to prevent blindness and wear glasses in the unlikely event of an explosion. Below are eight simple steps in testing a car or deep cycle battery. Alternatively, some auto parts or battery stores in the United States and Canada, like Auto Zone, Sears, Wal-Mart, Pep Boys, etc., will test your battery, charging system and starter for free. If you have a non-sealed wet battery (with filler caps), it is highly recommended that you use a good quality temperature compensating hydrometer, like an E-Z Red SP101, which can be purchased online or at an auto parts or battery store for less than $10.

If you have a sealed battery or need to troubleshoot a charging or electrical system, you will need a digital voltmeter with 0.5% (or better) DC accuracy, such as a Fluke 73-3 or 175. A digital voltmeter (or multimeter) can be purchased at an electronics store for between $20 and $200. A good, free digital multimeter applications manual for testing electrical systems is available on-line from Fluke at http://us.fluke.com/usen/support/appnotes/default?category=AP_AUTO(FlukeProducts). Analog voltmeters are not accurate enough to measure the millivolt differences of a battery's State-of-Charge or output of the charging system. Do not use a 12-volt test light to troubleshoot vehicle electrical circuits, except for testing the parasitic load at the battery, because you might damage the emissions computer or other sensitive electronic devices. A good source of information on measuring voltage and for maximum voltage drops can be found at Exide's Caring For Your Battery. A battery load tester is optional. Another way of testing the CCA (Cold Cranking Amp), amp hour (AH) or Reserve Capacity (RC) of lead-acid car or deep cycle batteries (with a 50% SoC or greater) is using an electro-chemical impedance spectroscopy (EIS) tester, such as a Cadex Spectro CA-12 or a conductance tester, for example a Argus or Midtronics. A sulfated sealed battery's voltage often will read higher than the SoC actually is, so capacity testing maybe required to determine the battery's actual condition.

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4.1. Inspect

Visually inspect for obvious problems such as a low electrolyte levels; loose, corroded or swollen cables, corroded battery terminals or posts; loose or broken alternator belt; frozen battery; loose hold-down clamps; dirty or wet battery top; or a leaking, cracked, bulging or damaged battery case. If the electrolyte levels are below the tops of the plates, add enough distilled, deionized or demineralized water to cover the plates and recharge the battery, allow to cool to room temperature and then top off the levels. The plates need to be covered at all times to prevent sulfation and reduce the possibility of an internal battery explosion. Please see Section 3.2 for electrolyte fill level information.

If electrolyte has been spilled, please see Section 9.14 for more information on adding electrolyte or adjusting the Specific Gravity within a cell.

4.2. Charge

Charge the battery to 100% State-of-Charge in a well ventilated area. 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 or if a cell is .010 or 10 "points" below the reading for a fully charged cell, then you should equalize the battery using the battery manufacturer's procedures. (Please see Section 9. for more information on equalize charging.)

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4.3. Remove Surface Charge

Surface charge (or "counter voltage") is the uneven mixture of sulfuric acid and water along the surface of the plates as a result of charging or discharging as the electrolyte has an opportunity to diffuse in the pores of the plates. It will make a weak battery appear good or a good battery appear bad. Larger wet lead-acid batteries (especially over 100 amp hours) could also have electrolyte stratification where the concentration of acid is greater at the bottom of the cell than near the surface. Open Circuit Voltages (OCV) will read higher than they actually are. Stratification can be eliminated by an equalizing charge, stirring or shaking the battery to mix the electrolyte.

The surface charge can be eliminated by one of the following methods after recharging a lead-acid car battery:

• Allow the battery to sit (or rest) without discharge or charge for between six to twelve hours at room temperature, if possible, to allow for the surface charge to dissipate. (Recommended method.)
  
  
• Turn the headlights on high beam for five minutes, turn them off, and wait ten minutes.
  
  
• With a battery load tester, apply a load at one-half the battery's CCA rating for 15 seconds and then wait ten minutes.
  
  
• Disable the ignition, turn the engine over for 15 seconds with the starter motor, and wait ten minutes.
  
  
• Apply a load that is 33% of the ampere-hour capacity for five minutes and wait ten minutes.
  
  
• With a battery load tester, apply a load is one third the battery's amp-hour rating for five minutes and wait five minutes.
  
  

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4.4. Measure the State-of-Charge (SoC)

The State-of-Charge acts like a "battery fuel gauge", but it only measures the state of the battery's charge and not it's storage capacity or state of health to produce rated current. For storage capacity measurements, please see Section 4.5, below. For example, a 50% SoC reading does not necessarily mean that a 100 amp hour (C/20) battery will produce 50 amp hours at five amp discharge load (with five amps being the 20 hour discharge load) because the battery might not have a 100 amp hours of storage capacity to begin with. Depth-of-Discharge (DoD) is the inverse of State-of-Charge (SoC) as shown in the following graphic.

[Source: Andre Packwood]

To measure a battery's State-of-Charge, perform the following steps:

• If the battery's electrolyte is above 120° F (48.9° C), allow it to cool to room temperature.
  
  
• Measure the electrolyte temperature (Recommended). If the battery has not been substantially charged or discharged within the last four hours, the ambient or surrounding air temperature can be used.
  
  
• Measure Specific Gravity of each cell of wet, non-sealed (with filler caps) lead-acid batteries with a hydrometer or Open Circuit Voltage of wet and VRLA batteries with an accurate (.5% or better) digital voltmeter.
  
  
• Based on the battery type, select the appropriate table. For wet, non-sealed (with filler caps) Low Maintenance (Sb/Ca) or Standard (Sb/Sb) car lead-acid batteries, use the Specific Gravity vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Car Battery Table or use the Open Circuit Voltage (OCV) vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Car Battery Table. For wet, sealed "Maintenance Free" (Ca/Ca) lead-acid batteries, use the Open Circuit Voltage (OCV) vs. Temperature at Various States-Of-Charge (SoC) for a Wet "Maintenance Free" (Ca/Ca) Car Battery Table. If you are unsure about the battery type, please see Section 7.1 for more information.
  
  
• Based on the electrolyte (or ambient) temperature and the measurement, determine the State-of-Charge from the appropriate temperature row and SoC column in the table selected. Some interpolation may be required.
  
  

A downloadable Temperature Compensated Battery State-of-Charge (SoC) Table is available. When printed, this Excel spreadsheet produces a single page that contains table with the Specific Gravity and Open Circuit Voltage measurements by temperature vs. various States-of-Charges. This table is for wet Low Maintenance (Ca/Sb), wet Standard (Sb/Sb), and wet "Maintenance Free" (Ca/Ca) car lead-acid batteries. The file size is approximately 22 KBytes.

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4.4.1. Specific Gravity vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Car Battery Table

Using a hydrometer to measure the Specific Gravity is the most accurate way of determining a wet, non-sealed (with filler caps) lead-acid battery's SoC. When the SoC measured by a hydrometer does not materially agree with the SoC measured by an accurate digital voltmeter, it is probably due to sulfation. If you suspect that a battery is sulfated, it probably is, especially if it has not been charged in a while or has been continuously undercharged. For more on sulfation, please see Section 16. This table has a baseline that assumes that a 1.265 Specific Gravity (SG) reading for a fully charged (100% SoC), wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) lead-acid battery at rest at 80° F (26.7° C). The Specific Gravity readings for a battery at 100% SoC will vary by plate chemistry, so if possible, check the battery manufacturer's specifications for their State-of-Charge definitions for the battery being measured. If the baseline is unknown at 100% SoC, please see Section 9.5. How Do I Know When My Battery Is Fully Charged? Depending on the plate chemistry, the Specific Gravity can range from 1.215 to 1.300 for a fully charged wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) car batteries at 80° F (26.7° C) and tend to be higher in deep cycle batteries.

Specific Gravity vs. Temperature
at Various States-Of-Charge (SoC)
for a Wet Low Maintenance (Sb/Ca)
or Standard (Sb/Sb) Car Battery Table

For example, if the electrolyte is at 20° F (-6.7° C), the Specific Gravity reading would be 1.289 for a 100% State-of-Charge because the liquid is more dense at the colder temperature. At 100° F (37.8° C), the Specific Gravity reading would be 1.182 for 50% SoC and a reading of 1.104 or lower at 120° F (48.9° C) would indicate a discharged battery.

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Electrolyte Freeze Points
at Various States-of-Charge
for a Wet Lead-Acid Battery Table

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4.4.2. Open Circuit Voltage vs. Temperature at Various States Of Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Car Battery Table

If the battery is sealed, then use an accurate (.5% or better) digital voltmeter to measure the battery's Open Circuit Voltage (OCV) to determine the SoC. When the SoC measured by a hydrometer does not materially agree with the SoC measured by a digital voltmeter, it is probably due to sulfation. If you suspect that a battery is sulfated, it probably is, especially if it has not been charged in a while or has been continuously undercharged. For more on sulfation, please see Section 16 This table has a baseline that assumes that a 12.65 Open Circuit Voltage (OCV) reading for a fully charged (100% SoC), wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) car lead-acid battery at rest, 80° F (26.7° C), and with the negative terminal disconnected. The OCV readings for a battery at 100% SoC will vary by plate chemistry, so if possible, check the battery manufacturer's specifications for their State-of-Charge definitions for the battery being measured. Depending on the plate chemistry, the Open Circuit Voltage can range from 12.22 to 13.00 for a fully charged wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) car battery at 80° F (26.7° C).

Open Circuit Voltage (OCV) vs. Temperature
at Various States Of Charge (SoC)
for Wet Low Maintenance (Sb/Ca)
or Standard (Sb/Sb) Car Battery Table

For example, if the electrolyte is at 20° F (-6.7° C), the Open Circuit Voltage reading would be 12.566 for a 100% State-of-Charge. At 100° F (37.8° C), the Open Circuit Voltage reading would be 12.248 for 50% SoC and a reading of 11.903 or lower at 120° F (48.9° C) would indicate a discharged battery.

4.4.3. Open Circuit Voltage vs. Temperature at Various States Of Charge (SoC) for a Wet "Maintenance Free" (Ca/Ca) Car Battery Table

If the battery is sealed, then use an accurate (.5% or better) digital voltmeter to measure the battery's Open Circuit Voltage (OCV) to determine the SoC. This table has a baseline that assumes that a 12.78 Open Circuit Voltage (OCV) reading for a fully charged (100% SoC), wet "Maintenance Free" (Ca/Ca) car battery at rest, 80° F (26.7° C) with the negative terminal disconnected. The OCV readings for a battery at 100% SoC will vary by plate chemistry, so if possible, check the battery manufacturer's specifications for their State-of-Charge definitions for the battery being measured. Depending on the plate chemistry, the Open Circuit Voltage can range from 12.6 to 13.1 for fully charged wet "Maintenance Free" (Ca/Ca) batteries and tend to be higher in deep cycle than in car (or starting) batteries. Some sealed wet "Maintenance Free" batteries have a built-in hydrometer, "Magic Eye", which only measures the State-of-Charge in ONE of its six cells.

"Magic Eye" Built-in Hydrometer

[Source: Popular Mechanics]

Open Circuit Voltage (OCV) vs. Temperature
at Various States-Of-Charge (SoC)
for a Wet "Maintenance Free" (Ca/Ca)
Car Battery Table

For example, if the electrolyte is at 20° F (-6.7° C), the Open Circuit Voltage reading would be 12.696 for a 100% State-of-Charge. At 100° F (37.8° C), the Open Circuit Voltage reading would be 12.308 for 50% SoC and a reading of 11.773 or lower at 120° F (48.9° C) would indicate a discharged battery.

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4.4.4. Interpreting the SoC Measurements

If the State-of-Charge is BELOW 75% using either the Specific Gravity, voltage test or the built-in hydrometer does not indicate "good" (green or blue), then the battery has a low charge and needs to be recharged before proceeding. If the battery is sealed, the battery could have low electrolyte, especially in a hot climate. You should replace the battery, if one 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 per the battery manufacturer's procedures may correct this condition. (Please see Section 9.)
  
  
• 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" (green or blue), which indicates a 65% SoC or better).
  
  
• If a moderate load is applied and if there is no or very little current flowing there is an probably an open cell or a completely sulfated battery. Without a load, a voltmeter reading may or may not indicate an open.
  
  
• 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.
  
  

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4.5. Capacity Load Test

Capacity load testing is used to determine how good or bad a car or deep cycle battery is. The primarily purpose of a car battery is to start an engine, so the battery ability to produce high current is the most important capacity test. In addition, some car batteries are only rated in their amp hour capacity. If this is the case, the capacity testing for deep cycle batteries below should be used.

4.5.1. Car Batteries (High Current Method)

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 car battery by one of the following methods:

• With a battery conductance tester, test the battery. Please note that the accuracy of conductance testing improves with the battery at a 50% or more State-of-Charge. Most auto parts and battery stores have battery conductance testers and some stores will test battery capacity for free. (Recommended method).
  
  
• With a battery load tester, apply a load equal to one half of the CCA rating of the battery for 15 seconds.
  
  
• With a battery load tester, apply a load equal to one half the OEM cold cranking amp specification for 15 seconds.
  
  
• Disable the ignition and turn the engine over for 15 seconds with the starter motor.
  
  

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

Capacity Load Test

[Source: BCI]

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4.5.2. Car and Deep Cycle Batteries (Low Current Method)

Motive and stationary deep cycle batteries and car batteries with amp hour or Reserve Capacity ratings can be capacity tested using a slow discharge load test. A DC ammeter and an adjustable resistive load, for example, 12-volt lamps or variable resistor, are required for this test. Please note that this test will not test the battery's ability to produce enough high current to start an engine, but normally, if a battery fails this test, it will also fail the high current load capacity test in Section 4.5.1 above.

If the battery is fully charged, the surface charge has been removed, and you know the Amp Hour rating of the battery, then you can test the capacity of a battery by applying a specific load and discharging the battery of it's rated amp hour capacity as defined by the battery manufacturer. Normally the discharge load is the resistance that will discharge a battery in 20 hours (C/20) for car (SLI) and motive deep cycle batteries and eight hours (C/8) for stationary deep cycle batteries. For example, if you have an 100 ampere-hour (C/20) rated battery, then an constant load of five amps would discharge the battery to it's rated amp hour capacity in approximately 20 hours (100 AH / 20 Hours = 5 Amps). To determine the capacity, at 80 degrees F (26.7 degrees C) measure the number of hours it takes to discharge a fully charged battery at the discharge rate to 10.5 volts. As the battery discharges, the resistance will have to be decreased to maintain the constant discharge load, at five amps in this example. Do not discharge the battery below 10.5 volts because you could damage the battery.

If the battery is fully charged, the surface charge has been removed, and you know the Reserve Capacity (RC) rating of the battery, then you can test the capacity of a battery by applying a constant 25 amp load and discharging the battery of it's rated Reserve Capacity in minutes as defined by the battery manufacturer. For example, if you have an 120 minute RC rated battery, then at 80 degrees F (26.7 degrees C) measure the number of minutes it takes to discharge a fully charged battery with a constant 25 amp load to 10.5 volts. Do not discharge the battery below 10.5 volts because you could damage the battery.

A battery with 80% or more of it's manufacturer's original rated capacity is considered to be good for most applications. Some new batteries can take up to 30 charge/discharge "preconditioning" cycles before they reach their rated capacity. If the battery passed the Capacity Load Test, then skip the next test, Section 4.6 Bounce Back Test and go to Section 4.7. Recharge below.

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4.6. Bounce Back Test

If the car battery has passed the high current load test, please go to Section 4.7. Recharge below. If not, remove the load, wait ten minutes, and measure the State-of-Charge. If the battery bounces back to less than 75% SoC then recharge the battery (please see Section 9.) and load test again. If the car battery fails the load test a second time or bounces back to less than 75% SoC, then replace the battery because it lacks the necessary high current (CCA) capacity.

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4.7. Recharge

In a well ventilated area, you should recharge your battery to 100% SoC as soon as possible to prevent lead sulfation and to restore it to peak performance.

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4.8. Refill

When the non-sealed wet battery (with filler caps) has cooled to room temperature, recheck the electrolyte levels and, if necessary, fill to the correct levels with distilled water. Please see Section 3.2 for electrolyte fill level information.

Impressum

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