Lead-Acid Battery Testing Methods
The state of charge
of a storage battery depends upon the condition of its active materials,
primarily the plates. However, the state of charge of a battery is indicated by
the density of the electrolyte and is checked by a hydrometer, an instrument
that measures the specific gravity (weight as compared with water) of liquids.
The most commonly used hydrometer consists of a small sealed glass tube
weighted at its lower end so it floats upright.
Within the narrow stem of the tube is a paper scale with a
range of 1.100 to 1.300. When a hydrometer is used, a quantity of electrolyte
sufficient to float the hydrometer is drawn up into the syringe.
The depth to which
the hydrometer sinks into the electrolyte is determined by the density of the
electrolyte, and the scale value indicated at the level of the electrolyte is
its specific gravity. The more dense the electrolyte, the higher the hydrometer
floats; therefore, the highest number on the scale (1.300) is at the lower end
of the hydrometer scale.
In a new,
fully-charged aircraft storage battery, the electrolyte is approximately 30
percent acid and 70 percent water (by volume) and is 1.300 times as heavy as
pure water. During discharge, the solution (electrolyte) becomes less dense and
its specific gravity drops below 1.300. A specific gravity reading between
1.300 and 1.275 indicates a high state of charge; between 1.275 and 1.240, a
medium state of charge; and between 1.240 and 1.200, a low state of charge.
Aircraft batteries are generally of small capacity but are subject to heavy
loads.
The values specified for state of charge are therefore
rather high. Hydrometer tests are made periodically on all storage batteries
installed in aircraft. An aircraft battery in a low state of charge may have
perhaps 50 percent charge remaining, but is nevertheless considered low in the
face of heavy demands that would soon exhaust it. A battery in such a state of
charge is considered in need of immediate recharging.
When a battery is tested using a hydrometer, the temperature
of the electrolyte must be taken into consideration. The specific gravity
readings on the hydrometer vary from the actual specific gravity as the
temperature changes. No correction is necessary when the temperature is between
70 °F and 90 °F, since the variation is not great enough to consider.
When temperatures are greater than 90 °F or less than 70 °F,
it is necessary to apply a correction factor. Some hydrometers are equipped
with a correction scale inside the tube. With other hydrometers, it is
necessary to refer to a chart provided by the manufacturer.
In both cases, the corrections should be added to, or
subtracted from the reading shown on the hydrometer. The specific gravity of a
cell is reliable only if nothing has been added to the electrolyte except
occasional small amounts of distilled water to replace that lost as a result of
normal evaporation. Always take hydrometer readings before adding distilled
water, never after.
This is necessary to allow time for the water to mix
thoroughly with the electrolyte and to avoid drawing up into the hydrometer
syringe a sample that does not represent the true strength of the solution.