By simply using a hydrometer to measure a battery’s state of charge, most fleet managers can determine the condition of the flooded lead-acid batteries powering their vehicles. The process can determine if a battery is at the end of its life, if it has a bad cell, or if the charging methods aren’t properly keeping the batteries fully charged.

For those that include this as part of a maintenance routine, you must also make sure to factor the temperature of the electrolyte. Heat and cold will affect the outcome of any hydrometer reading and without adding or subtracting a correction factor, hydrometer measurements can be off.  This is one of the most common questions that fleet and maintenance managers ask when trying to keep a log of battery performance across their entire fleet.

The basic temperature factors to add or subtract from hydrometer readings is simple, if you start from a baseline of 80⁰F or 27⁰C.  The first important point to note is that this baseline is for the temperature of the electrolyte, not ambient temperature. So it’s important to have a quality thermometer to use with your hydrometer when taking state of charge measurements.

The correction factor is simple:

In Fahrenheit-
Add 0.004 for each 10⁰F the electrolyte temperature is above 80⁰F
Subtract 0.004 for each 10⁰F the electrolyte temperature is below 80⁰F

In Celsius-
Add 0.005 for each 7⁰C the electrolyte temperature is above 27⁰C
Subtract 0.005 for each 7⁰C the electrolyte temperature is below 27⁰C

Make sure to compare your readings to the manufacturer’s specifications for battery open circuit voltage (OCV) vs state of charge (SOC). Most are listed by battery voltage and also show specific gravity (SG) vs state of charge. It’s also important to use the proper safety precautions when performing any maintenance to flooded lead-acid batteries.

By applying the temperature correction factor, you’ll be certain that your hydrometer readings are correct, and you can take the appropriate action to get the most performance and longevity from your batteries. To find additional resources on battery maintenance and ways to increase battery efficiency and service life, visit U.S. Battery’s website at