Hydrometer State of Charge Testing

What is Hydrometer Battery Testing?Hydrometer

It’s measures specific gravity of battery electrolyte.

Specific gravity measures the density of a substance.  The unit of measurement itself is generally referenced to water.   Water has a specific gravity of one.   Substances that are more dense than water will have a value that is greater than one.  Those that are less dense will have a value that is less than one.

Within a lead acid battery, the specific gravity changes with the state of charge.  The higher the charge, the greater the specific gravity.  The lower the charge, the less the specific gravity.




State of Charge based on Hydrometer Specific Gravity Measurement

Determing SOC by using a hyrdometer requires you to know a little about your battery type.  The table below shows what is typical for a lead acid car battery at about 26 degrees C ( 78 deg. F).

Battery SOC and Specific Gravity

State of ChargeSpecific Gravity

100% SOC Specific Gravity Values For Other Types of Batteries

UsageSpecific Gravity
Deep Cycle1.300
Golf Cart1.270
Cell Phone Tower1.210
Back Power1.210

Why The Difference in SOC?

Its all about the application.   Generally speaking the higher the specific gravity, the greater the energy density (capacity). However the higher density comes with a higher corrosion rate.

Conversely, the lower the specific battery, the lower the energy density, but also, the lower the corrosion rate.

In terms of the deep cycle, its intended for those who want to go longer times between charges.

With respect to back up batteries, longevity is more valued than capacity.

Lead Acid Battery Specific Gravity and Temperature

The specific gravity in a battery changes with temperature.  As it gets colder, the denser the electrolyte becomes.  This results in a higher reading.

When the temperature gets higher, the less dense the electrolyte becomes. When this happens, the specific gravity reading will be lower.