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Diesel Generator Lead Acid Batteries General Precautions SECTION 6 – ELECTROLYTE - SG & TEMPERATURE

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6  Electrolyte - Specific Gravity and Temperature

Note: Maintenance-free batteries are sealed and do not require the addition of electrolyte. Some manufacturers of maintenance-free batteries provide an ‘eye’ or some visible means of telling when the battery is discharged or approaching the end of its useful life.

6.1  Checking Electrolyte Level

Caution:NEVER add tap or well water and NEVER allow the battery electrolyte to drop  below the top of the plates.

Caution:Do not add water in freezing weather unless the engine will run long enough (two to three hours) to assure a thorough mixing of water and electrolyte.

Check the level of the electrolyte (acid and water solution) in the batteries at least every month or 100 hours of operation, whichever occurs first. Maintain the electrolyte to the levels indicated in Table 8-1 by the addition of distilled or de-ionised water only and recharge. Replace the vent plugs once filling is completed.

If one cell is low, check case for leaks.

Keep the battery case clean and dry. An accumulation of moisture will lead to a more rapid discharge and battery failure.

6.2  Checking Specific Gravity

Use a battery hydrometer to check the specific gravity of the electrolyte in each battery cell.

Hold the hydrometer vertical and take the reading. A fully charged battery will have a corrected specific gravity of 1.260 at 77°F (25°C). Charge the battery if the reading is below 1.215.


Figure 6-1Checking Specific Gravity



For Filling New   Cells

At end of Charge

Ambient   temperature normally below 32°C


1.270 – 1.290

Ambient   Temperature frequently above 32°C


1.240 – 1.260

Maximum   permissible temperature of electrolyte during charge


113°F (45°C)


113°F (45°C)

Table 6-1Specific Gravity

Table 6-1 shows the specific gravity (SG) of electrolyte, corrected to 77°F (25°C). Correct the SG reading for other temperatures by subtracting seven gravity points (0.007) for every 18°F (10°C) the electrolyte temperature is above 80°F (27C). Apply the correction formula as follows:

1.For every 18°F (10°C) above 77°F (25°C), subtract 0.007 (7 points)

2.For every 18°F (10°C) below 77°F (25°C), add 0.007 (7 points)

For example:

if the Specific Gravity at 77°F (25°C) is 1.260, then the Specific Gravity at 59°F (15°C) is 1.267





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