Lead Acid Battery Maintenance Hints

Lead Acid Battery Maintenance Hints

Off-grid solar energy systems are not as low maintenance as grid-tied solar power systems.  The use of batteries in off-grid or grid-interactive (grid connected with a battery backup) systems does require regular maintenance.  The maintenance for off-grid solar systems is not too difficult but should be done with the proper knowledge and equipment.

The following is a step by step guide on how to maintain your battery bank by measuring the specific gravity (SG) of your batteries.  SG is the primary metric used to determine a battery’s “health.” Specific gravity is the ratio of the mass to volume of a substance.

SG = Mass (g) / Volume (mL)


A hydrometer is a device used to measure the SG of the battery electrolyte.  There are various hydrometer types but we recommen using a glass hydrometer with a float and a rubber head to draw the battery electrolyte in.  Digital hydrometers may be used, but will likely be more expensive than their glass counterparts. Additionally, digital hydrometers may need to be calibrated. The hydrometer should be accurate to +/- 0.005.

Using a hydrometer

Battery maintenance for off grid solar energy systems should occur at least every 3 months, although every month checking the liquid levels is best practice.  The following is the procedure for using glass hydrometers with a float:

1.    Put on safety glasses, rubber gloves, long sleeves and covered shoes

2.    Please do not wear your Sunday clothes as small droplets of the electrolyte will at least discolour your clothing or worse.

3.    Shut off the charge controller input first than the output breaker. Also turn off the main battery breaker.

4.    Clean the battery of any dust, dirt or debris. Also make sure your hydrometer is clean of dust or other foreign materials.

5.    Remove a cap on the top of a battery and insert the hydrometer tail into the liquid (be sure to not push down on the interior plates)

6.    Draw liquid into the hydrometer up to the measuring line (be sure to not overfill past the line)

7.    If bubbles are present in the hydrometer, gently tap the glass to release them

8.    With the hydrometer held level, obtain a reading and record it.  Discharge the fluid back into the battery.  Be careful to not splash the electrolyte.

9.    Repeat the steps 2-3 times per battery to minimize any errors in reading. It is a good idea to record your readings on a battery maintenance log.

10.    Apply temperature corrections if necessary

Temperature Correction

Above 21 degrees Celsius add value to the SG reading.  Below 21 degrees Celsius subtract value from the reading.

Correction factor = (0.595 x Cell Temperature [in degrees Celsius] – 12.5) / 1000

Specific Gravity

The following values are for Rolls Surrette batteries at 25 degrees Celsius.  See the manufacturer’s specifications for other batteries types.


Specific Gravity at Set Charge  for Rolls Surrette Batteries

If battery electrolyte levels are low, fill up to the original line using only 100% distilled or de-ionized water. Never add water to partially discharged batteries unless the battery plates are exposed. In that instance just add enough water to barely cover the plates. Wait until the batteries are fully charged and check the liquid level then as electrolyte will rise with increasing charge level.  

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