Testing Battery Specific Gravity with Hydrometer

Temperature’s Impact on Charging Deep-Cycle Batteries

The chemistry of flooded lead-acid deep-cycle batteries makes them one of the most cost-effective methods of energy storage. The composition of the battery’s design, however, makes it sensitive to temperature, which can affect its charging and discharging rate, something that should be addressed in regular maintenance routines.

Cold temperatures slow the rate of charging and discharge, while warmer temperatures increase the rates. This means that it may take longer for your batteries to fully charge in the winter than they will in the summer. Additionally, in the warmer summer months, batteries may discharge more quickly. Battery manufacturers use 80-degrees F (27 C) as the baseline temperature for optimum operation and calculating charge and discharge rates. Obviously that doesn’t work for everyone, so it’s important to take specific gravity readings with a hydrometer to know if and when your batteries are properly charged in all temperature conditions.

Specific gravity is the ratio of the weight of a solution to the weight of an equal volume of water at a specified temperature. A hydrometer can give you an indication of the state of charge of the battery’s electrolyte. A higher number indicates a higher concentration of acid in the electrolyte, indicating the battery is charged. A lower number indicates that the concentration of acid in the battery is less, showing the amount of discharge of the battery.

Battery manufacturers recommend using a simple correction factor to your hydrometer’s readings. Using 80-degrees as your baseline, subtract (.004) from your hydrometer reading for every 10-degrees below 80 °F (5.6-degrees below 27 °C). For example, if the temperature of the electrolyte is 50 °F and your battery specific gravity reading is 1.200, you must subtract .012 from your reading. In this case .004 for every 10-degrees equals .012. Subtract this from 1.200 and your corrected specific gravity reading is 1.188.

Specific gravity readings must be done on every cell of each battery in the pack. Compare the readings to the battery manufacturer’s specifications to indicate the state of charge of your batteries. While it’s not necessary to calculate your hydrometer’s readings for slight variations above or below 80 °F, it should be done in extreme weather conditions or seasonally to ensure that your battery-powered vehicles or equipment are performing at their best.