Measuring Specific Gravity

Bulletin # 6


Before measuring the specific gravity, completely read Bulletin # 7 on Basic Battery Safety.

The most accurate and direct way to test the state of charge of a battery cell is to determine the specific gravity of the battery electrolyte. The higher the specific gravity of the electrolyte the higher the state of charge. The best way to truly monitor your system over it lifetime is to regularly take and record specific gravity readings.

Unfortunately hydrometers are not easy to use. Testing can be time consuming, there are possibilities for error and safety must be considered. For these reasons we present this bulletin.

Hydrometer Types                                                                  

Hydrometers come in many sizes and shapes. We recommend a hydrometer with a float, contained in a glass vessel with a rubber bulb to draw the acid into the tube. Stay away from floating colored balls as the extra inaccuracy results in very subjective testing. The hydrometer should give you a numeric reading directly from the instrument. A good hydrometer is accurate to +/- 0.005 points so 1.265 could read from 1.260-1.270. The instrument accuracy should be known.


Checking Calibration                                                              

As with all measuring equipment drawing conclusions from the results is not worth the effort if the equipment is not calibrated.

The correct way to check the calibration of a hydrometer is to test the hydrometer against a known master that is accurate to one more decimal point. These hydrometers are; however, very expensive ($100-150) and can be easily broken.

The simplest and cheapest way, when facilities exist, is to measure out a volume of acid and weigh it. A small graduated cylinder and an electronic balance are perfect. The specific gravity is then calculated as follows:

SG = Mass (g) / Volume (ml)

The metric units must be used to convert to the specific gravity scale based on water. The hydrometer is calibrated if it agrees with the sample within its manufacturing tolerance.


Method of Use                                                                         

Exact procedures is instrument dependent and this is given as a general procedure and assumes a hydrometer with glass float and body.

bulletPut on eye protection and rubber gloves. See bulletin 607, Basic Lead Acid Battery Safety Principals
bulletIt is recommended to disconnect the battery especially if on a high rate of charge / discharge.
bulletRemove vent cap. Carefully insert the hydrometer into cell, not pushing down on the top of the plates.
bulletCarefully draw liquid into the hydrometer and avoid "bumping" the hydrometer. Be careful the float is not flooded (too much liquid) or sticking to the sides of the glass tube.
bulletObtain a reading by looking directly at the float.
bulletRepeat steps 3-5 to reconfirm reading.
bulletRECORD the cell number and result.
bulletIf it is very warm or very cold correct the specific gravity for temperature. If the ambient temperature is fairly consistent and original gravities are taken when the batteries are put into service temperature correction is not as critical and only necessary if problems arise. Make sure electrolyte is not hot if just taken out of service. Let it reach room temperature.

An easy procedure is to number the cells starting with the positive cell and move from cell to cell towards the negative terminal. If this is part of a preventive maintenance program it is helpful to number the batteries.

Temperature Correction                                                        

SG of acid is temperature dependent. If the temperature is very cold or very hot this can lead to incorrect readings. To correct for temperature use the following equations the equations or below 70F subtract points (0.03 per 10F) and above 70F add points.

bulletCorrection factor = (0.331 x T/F - 23) / 100 or 0.03 pts per 10 F
bulletCorrection factor = (0.595 x T/F - 12.5) / 100 or 0.03 pts per 5 C

This is valid for 0-130F or -17.8- 54.4C

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