BASIC DC THEORY 14

 <previous  BASIC DC THEORY 14   next>

BATTERY HAZARDS

Because batteries store large amounts of energy, there are certain hazards that
are associated with battery operation. These hazards must be fully understood to
ensure safe operation of batteries.

 

Shorted Cell

Cell short circuits can be caused by several conditions, which include the following: faulty
separators; lead particles or other metals forming a circuit between the positive and negative
plates; buckling of the plates; or excessive sediments in the bottom of the jar. The primary cause
of some of these occurrences is overcharging and overdischarging of the battery, which causes
sediment to build up due to flaking of active material and buckling of cell plates.

Overcharging and overdischarging should be avoided at all costs. Short circuits cause a great
reduction in battery capacity. With each shorted cell, battery capacity is reduced by a percentage
equal to one over the total number of cells.

Gas Generation

A lead-acid battery cannot absorb all the energy from the charging source when the battery is
nearing the completion of the charge. This excess energy dissociates water by way of electrolysis
into hydrogen and oxygen. Oxygen is produced by the positive plate, and hydrogen is produced
by the negative plate. This process is known as gassing.

Gassing is first noticed when cell voltage reaches 2.30-2.35 volts per cell and increases as the
charge progresses. At full charge, the amount of hydrogen produced is about one cubic foot per
cell for each 63 ampere-hours input. If gassing occurs and the gases are allowed to collect, an
explosive mixture of hydrogen and oxygen can be readily produced. It is necessary, therefore,
to ensure that the area is well ventilated and that it remains free of any open flames or spark producing
equipment.

As long as battery voltage is greater than 2.30 volts per cell, gassing will occur and cannot be
prevented entirely. To reduce the amount of gassing, charging voltages above 2.30 volts per cell
should be minimized (e.g., 13.8 volts for a 12 volt battery).

Battery Temperature

The operating temperature of a battery should preferably be maintained in the nominal band of
60-80°F. Whenever the battery is charged, the current flowing through the battery will cause
heat to be generated by the electrolysis of water. The current flowing through the battery (I) will
also cause heat to be generated (P) during charge and discharge as it passes through the internal
resistance (Ri), as illustrated using the formula for power in Equation (4-4).

 

image

 

 

 

Higher temperatures will give some additional capacity, but they will eventually reduce the life
of the battery. Very high temperatures, 125°F and higher, can actually do damage to the battery
and cause early failure.

Low temperatures will lower battery capacity but also prolong battery life under floating (i.e.,
slightly charging) operation or storage. Extremely low temperatures can freeze the electrolyte,
but only if the battery is low in specific gravity.

 

 

 <previous  BASIC DC THEORY 14   next>

Hardsteel Media © 2012-2017 THE ELECTRICIANS HANGOUT All Rights Reserved