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Battery Testing

 

Types of tests:

Flooded batteries are filled with electrolyte. This electrolyte changes in density according to the state of charge. A fully charged battery will have a high acid content which gives it a higher specific gravity. A discharged battery will be pretty much water, which has a lower density. This density, commonly referred to as Specific Gravity, can be measured with either a hydrometer or spectrometer. The advantage of this method, rather than just checking for battery voltage, is that each cell can be tested. A voltmeter can only give you an average output level across all of the battery cells while a hydrometer will show you if a single cell is bad. Note that a hydrometer test only applies to flooded batteries, not AGM batteries.

 

Hydrometer Test:

Note: When performing this test be aware that you are dealing with battery acid. This acid can cause damage if spilled upon the vehicle. Also be sure to wear proper hand and eye protection because battery acid burns.

1. Do not add water at this time.

2. Fill and drain the hydrometer 2 to 4 times before pulling out a sample from the battery.

3. There should be enough sample electrolyte in the hydrometer to completely support the float.

4. Take a reading, record it, and return the electrolyte back to the cell.

5. To check another cell, repeat the 3 steps above.

6. Check all cells in the battery.

7. Replace the vent caps and wipe off any electrolyte that might have been spilled.

8. Correct the readings to 80o F:

  • Add .004 to readings for every 10o above 80o F

  • Subtract .004 for every 10o below 80o F.

9. Compare the readings.

10. Check the state of charge using Table 1. (Below)

The readings should be at or above the factory specification of 1.277 .007. If any specific gravity readings register low, then follow the steps below.

1. Check and record voltage level(s).

2. Put battery(s) on a complete charge.

3. Take specific gravity readings again.

If any specific gravity readings still register low then follow the steps below.

 

Voltmeter Test:

You will need a Voltage Meter to perform this test (D.C. side)

1. Check voltage level(s).

2. Perform equalization charge. Refer to the Equalizing topic for the proper procedure.

3. Take specific gravity readings again.

If any specific gravity reading still registers lower than the factory specification of 1.277 .007 then one or more of the following conditions may exist:

1. The battery is old and approaching the end of its life.

2. The battery was left in a state of discharge too long.

3. Electrolyte was lost due to spillage or overflow.

4. A weak or bad cell is developing.

5. Battery was watered excessively previous to testing.

Batteries in conditions 1 - 4 should be taken to a specialist for further evaluation or retired from service.

 

Open-Circuit Voltage Test:

You will need a Voltage Meter to perform this test (D.C. side)

For accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.

1. Disconnect all loads from the batteries.

2. Measure the voltage using a DC voltmeter.

3. Check the state of charge with Table 1. (Below)

4. Charge the battery if it registers 0% to 70% charged.

If battery registers below the Table 1 values, the following conditions may exist:

1. the battery was left in a state of discharge too long.

2. The battery has a bad cell.

Batteries in these conditions should be taken to a specialist for further evaluation or retired from service.

 

Common Battery Sizes and Ratings:

Percentage
of Charge

Specific Gravity
Corrected to
80o F

Open-Circuit Voltage

6V

8V

12V

24V

100

1.277

6.37

8.49

12.73

25.46

90

1.258

6.31

8.41

12.62

25.24

80

1.238

6.25

8.33

12.50

25.00

70

1.217

6.19

8.25

12.37

24.74

60

1.195

6.12

8.16

12.24

24.48

50

1.172

6.05

8.07

12.10

24.20

40

1.148

5.98

7.97

11.96

23.92

30

1.124

5.91

7.88

11.81

23.63

20

1.098

5.83

7.77

11.66

23.32

10

1.073

5.75

7.67

11.51

23.02

 

Submitted by Mike Sundberg - 2/06/06
 

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