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Mechanic

Car And Deep Cycle Battery

Frequently Asked Questions


How Do I Test a Car Battery?



4.1. Inspect4.2. Charge4.3. Remove Surface Charge4.4. Measure State-of-Charge (SoC)4.5. Capacity Load Test4.6. Bounce Back Test4.7. Recharge4.8. Refill

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.)

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.

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.

SoC/DoD



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

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


Electrolyte Temperature (Fahrenheit)

Electrolyte Temperature (Celsius)

100% SoC

75% SoC

50% SoC

25% SoC

0% SoC

120°

48.9°

1.249

1.209

1.174

1.139

1.104

110°

43.3°

1.253

1.213

1.178

1.143

1.108

100°

37.8°

1.257

1.217

1.182

1.147

1.112

90°

32.2°

1.261

1.221

1.186

1.151

1.116

80°

26.7°

1.265

1.225

1.190

1.155

1.120

70°

21.1°

1.269

1.229

1.194

1.159

1.124

60°

15.6°

1.273

1.233

1.198

1.163

1.128

50°

10.0°

1.277

1.237

1.202

1.167

1.132

40°

4.4°

1.281

1.241

1.206

1.171

1.136

30°

-1.1°

1.285

1.245

1.210

1.175

1.140

20°

-6.7°

1.289

1.249

1.214

1.179

1.144

10°

-12.2°

1.293

1.253

1.218

1.183

1.148

-17.8°

1.297

1.257

1.222

1.187

1.152


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.



HOW DO I USE A HYDROMETER?

A hydrometer is an inexpensive float-type device used to measure the concentration of sulfuric acid (Specific Gravity) of battery electrolyte ("battery acid"). From this reading you can easily and accurately determine a non-sealed battery's State-of-Charge. A hydrometer is a glass barrel or plastic container with a rubber nozzle or hose on one end and a soft rubber bulb on the other. Inside the barrel or container, there is a float and calibrated graduations used for the Specific Gravity measurement. The following is a list of instructions on how to correctly use a battery hydrometer:

BATTERY HYDROMETERS


Hydrometer


E-Z Red SP101 Hydrometer

  1. If the battery's electrolyte is above 120° F (48.9° C), allow it to cool.

  2. If the battery has been charged or discharged within the last four hours, remove the Surface Charge.

  3. Wear some glasses, preferably safety glasses, in the unlikely event that a battery explosion or electrolyte spill might occur.

  4. While holding a clean hydrometer vertically, squeeze the rubber bulb, insert the nozzle into the electrolyte in the cell, and release the bulb. The electrolyte will be sucked up into the barrel or container allowing the float to ride freely. Start with the cell that is closest to the POSITIVE (+) terminal.

  5. Tap the hydrometer to dislodge any air bubbles on the float.

  6. Squeeze the rubber bulb to release the electrolyte back into the battery's cell.

  7. To increase the accuracy of the measurement, in the same cell, repeat this process several times so the float will reach the same temperature as the electrolyte. If you are measuring a large battery, stratification can occur when the more concentrated electrolyte settles to the bottom. If you notice a difference in the readings from electrolyte taken at the top and bottom of the cell, average the two readings.

  8. At eye level and with the float steady, read the Specific Gravity at the point the surface of the electrolyte crosses the float markings. The Specific Gravity reading should be between 1.100 and 1.300.

  9. Release the electrolyte back into the cell from which it was taken and record the reading. Be sure to avoid spillage.

  10. If the hydrometer is not temperature compensating, measure the electrolyte temperature and use the appropriate temperature row and SoC column in the Specific Gravity vs. Temperature at Various States-Of-Charge (SoC) for a Wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) Battery Table to determine the SoC. If the hydrometer is temperature compensating, determine the State-of-Charge from the hydrometer or the 80° F (26.7° C) temperature row and SoC column in 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.

  11. Repeat the process for each individual cell. The Specific Gravity reading should not have a difference of more that 30 "points" (.030) between the lowest and highest reading or 10 "points" (.010) below the battery manufacturer's recommended temperature value with the battery fully charged. If so, try and equalize the battery by following the battery manufacturer's procedures or the procedure in Section 9. If equalizing does not help, replace the battery.

  12. Determine the battery's State-of-Charge (SoC) by taking the average of the cell readings, but the battery's performance will be based on the weakest cell.

  13. Throughly rinse the hydrometer with water after using it.



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


Approximate
State-of-Charge
(SoC)

Approximate
Depth-of-Discharge
(DoD)

Approximate Electrolyte Freeze Point

100%

0%

-77°F
(-67°C)

75%

25%

-35°F
(-37°C)

50%

50%

-10°F
(-23°C)

25%

75%

5°F
(-15°C)

0%
(DISCHARGED)

100%
(DISCHARGED)

20°F
(-6.7°C)






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


Electrolyte Temperature (Fahrenheit)

Electrolyte Temperature (Celsius)

100% SoC

75% SoC

50% SoC

25% SoC

0% SoC

120°

48.9°

12.663

12.463

12.253

12.073

11.903

110°

43.3°

12.661

12.462

12.251

12.071

11.901

100°

37.8°

12.658

12.458

12.248

12.068

11.898

90°

32.2°

12.655

12.455

12.245

12.065

11.895

80°

26.7°

12.650

12.450

12.240

12.060

11.890

70°

21.1°

12.643

12.443

12.233

12.053

11.883

60°

15.6°

12.634

12.434

12.224

12.044

11.874

50°

10.0°

12.622

12.422

12.212

12.032

11.862

40°

4.4°

12.606

12.406

12.196

12.016

11.846

30°

-1.1°

12.588

12.388

12.178

11.998

11.828

20°

-6.7°

12.566

12.366

12.156

11.976

11.806

10°

-12.2°

12.542

12.342

12.132

11.952

11.782

-17.8°

12.516

12.316

12.106

11.926

11.756


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 which only measures the State-of-Charge in ONE of its six cells.

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


Electrolyte Temperature (Fahrenheit)

Electrolyte Temperature (Celsius)

100% SoC

75% SoC

65% SoC

50% SoC

25% SoC

0% SoC

120°

48.9°

12.793

12.563

12.463

12.313

12.013

11.773

110°

43.3°

12.791

12.561

12.461

12.311

12.011

11.771

100°

37.8°

12.788

12.558

12.458

12.308

12.008

11.768

90°

32.2°

12.785

12.555

12.455

12.305

12.005

11.765

80°

26.7°

12.780

12.550

12.450

12.300

12.000

11.760

70°

21.1°

12.773

12.543

12.443

12.293

11.993

11.753

60°

15.6°

12.764

12.534

12.434

12.284

11.984

11.744

50°

10.0°

12.752

12.522

12.422

12.272

11.972

11.732

40°

4.4°

12.736

12.506

12.406

12.256

11.956

11.716

30°

-1.1°

12.718

12.488

12.388

12.238

11.938

11.698

20°

-6.7°

12.696

12.466

12.366

12.216

11.916

11.676

10°

-12.2°

12.672

12.442

12.342

12.192

11.892

11.652

-17.8°

12.646

12.416

12.316

12.166

11.866

11.626


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.




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.

 

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. A battery's internal resistance can be computed using the following formula: Internal Resistance = Voltage Drop / Load Current.

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

Electrolyte Temperature Fahrenheit

Electrolyte Temperature Celsius

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


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.

 

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.

 

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.

 

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.

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