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What is Specific Gravity and why do some batteries have higher or lower values?

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Specific gravity is the ratio of the weight of a solution to the weight of an equal volume of water at a specified temperature.  Specific gravity is used as an indicator of the state of charge (SOC) of a cell or battery.  It should be noted that although the state of charge can be determined the measurement of specific gravity cannot determine a battery’s capacity.  

The specific gravity decreases during the discharging of a battery to a value near that of pure water and it increases during a recharge.  The battery is considered fully charged when specific gravity reaches its highest possible value.  The value of specific gravity does vary with temperature and the quantity of electrolyte in a cell.  When the electrolyte is near the low-level mark, the specific gravity is higher than nominal and drops as water is added to the cell to bring the electrolyte to the full level.  The volume of electrolyte expands as temperature rises and contracts as temperature decreases.  As the volume of electrolyte expands, specific gravity readings are lowered and, conversely, specific gravity increases with colder temperatures.

The specific gravity for a given battery is determined by the application it will be used in, taking into account operating temperature and battery life. 

Specific Gravities



Heavily cycled batteries such as for electric vehicles (traction)


Automotive (SLI)


UPS – Standby with high momentary current discharge requirement.


General applications such as power utility and telephone applications.

In the selection of a battery for a given application, some of the effects of high or low specific gravity to be considered are:

Higher Gravity

Lower Gravity

  •  More capacity
  •  Less capacity
  •  Shorter life
  •  Longer life
  •  Less space required
  •  More space required
  •  Higher momentary discharge rates
  •  Lower momentary discharge rates
  •  Less adaptable to “floating”
  •  More adaptable to “floating”
  •  More standing loss
  •  Less standing loss

If you need battery test equipment from reliable manufacturers like Alber, Megger or Storage Battery Systems, Protec Equipment Resources is ready to supply your needs with dependable test sets like the Alber Cellcorder Battery Analyzer, Megger BITE Battery Impedance Testers, Megger Torkel Battery Load Unit or the SBS-2500 Hydrometer. We have a wide selection of tools to cater to your testing needs. Give us a call to discuss available plans for equipment rentals and sales.