Hydrometer testing

Measuring the gravity of the electrolyte with a hydrometer is a method for determining the charge level and condition of the battery. The degree of battery charge depends on the specific gravity. The electrolyte of a fully charged battery consists of about 64% water and 36% sulfuric acid. It corresponds to a specific gravity of 1,270 kg/l. As the battery discharges, the specific gravity decreases. By measuring the specific gravity of the electrolyte, we can determine the degree of battery charge. We use a hydrometer to measure the specific gravity of the electrolyte in the battery.

A classic battery hydrometer uses a glass float (hydrometer) in a glass tube to measure the specific gravity of the electrolyte. Squeezing and releasing the rubber ball draws the electrolyte into the hydrometer tube. When filled with test electrolyte, the float floats in the electrolyte. The depth to which the float sinks in the electrolyte indicates its relative weight to water. The value on the float scale in the electrolyte level is gravity. If the hydrometer floats deeply in the electrolyte, the specific gravity is low (A). If the hydrometer floats high in the electrolyte, the specific gravity is high (B).

Electrolyte gravity measurement

Measurements are only possible on batteries that are not sealed and have openings above each cell. These measurements give a good indication of the condition of the battery. Hydrometer readings should not vary by more than a 0.05 kg/l difference between cells. A larger deviation is an indication that the battery is defective.

Since temperature affects the specific gravity of the matter, the reading should be corrected by ±0.004 for every ±10°C above or below 27°C. Most hydrometers have a built-in thermometer to measure the temperature of the electrolyte. The hydrometer reading may be incorrect if not corrected.

When the gravity of the electrolyte falls below 1,230 kg/l, it is necessary to charge the battery.

The relationship of the state of charge of a battery to the gravity of the electrolyte

To perform gravity of the electrolyte measurement with a hydrometer, follow these steps:

Preparation for measurement

Clean the upper housing of the battery. Blow out and dry the area around the lid. Remove the battery cover and check the electrolyte level in the battery cells. If necessary, top up the cells with distilled water. If water is added, charge the battery before any testing.

Gravity measurement with a hydrometer

Immerse the hose in the electrolyte of the first cell. Squeeze and release the rubber ball. Read the gravity of the electrolyte on the float of the hydrometer. Record the measurement result. Repeat the measurement procedure for all battery cells.

Measurement analysis

Compare the measured values of electrolyte gravity. When the difference between the cells is greater than 0.05 kg/l, then the battery is defective. If all the gravities are approximately equal, take the mean value and determine the state of charge of a battery based on the graph (table). Determine further actions.