BATTERIES - ATA 24

A battery is an electrical power energy storage device consisting of electrochemical cells relying on electrochemical reactions of the cells to deliver the energy.

AIRCRAFT BATTERIES

Aircraft batteries are used for many functions (e.g., ground power, emergency power, improving DC bus stability, and fault clearing). Most small private aircraft use lead-acid batteries.

Most commercial and corporate aircraft use nickel-cadmium (Ni-Cd) batteries.

However, other lead acid types of batteries are becoming available, such as the Valve-Regulated Lead-Acid (VRLA) batteries. The battery best suited for a particular application depends on the relative importance of several characteristics, such as weight, cost, volume, service or shelf life, discharge rate, maintenance, and charging rate.

Any change of battery type may be considered a major alteration.

PRIMARY CELL BATTERIES

The dry cell is the most common type of primary-cell battery and is similar in its characteristics to that of an electrolytic cell.

This type of a battery is basically designed with a metal electrode or graphite rod acting as the cathode (+) terminal, immersed in an electrolytic paste. This electrode/electrolytic build-up is then encased in a metal container, usually made of zinc, which itself acts as the anode (-) terminal. When the battery is in a discharge condition an electrochemical reaction takes place resulting in one of the metals being consumed.

Because of this consumption, the charging process is not reversible. Attempting to reverse the chemical reaction in a primary cell by way of recharging is usually dangerous and can lead to a battery explosion.

These batteries are commonly used to power items such as flashlights. The most common primary cells today are found in alkaline batteries, silver-oxide and lithium batteries. The earlier carbon-zinc cells, with a carbon post as cathode and a zinc shell as anode were once prevalent but are not as common.

SECONDARY CELL BATTERIES

A secondary cell is any kind of electrolytic cell in which the electrochemical reaction that releases energy is reversible.

The lead-acid car battery is a secondary-cell battery as are some aircraft batteries. The electrolyte is sulphuric acid (battery acid), the positive electrode is lead peroxide, and the negative electrode is lead.

A typical lead-acid battery consists of six lead-acid cells in a case. Each cell produces 2 volts, so the whole battery produces a total of 12 volts.

Other commonly used secondary cell chemistry types are nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Liion), and Lithium ion polymer (Li-ion polymet). Lead-acid batteries used in aircraft are similar to automobile batteries. The lead acid battery is made up of a series of identical cells each containing sets of positive and negative plates.

A practical cell is constructed with many more plates than just two in order to get the required current output. All positive plates are connected together as well as all the negatives.

Because each positive plate is always positioned between two negative plates, there are always one or more negative plates than positive plates. Between the plates are porous separators that keep the positive and negative plates from touching each other and shorting out the cell. The separators have vertical ribs on the side facing the positive plate.

This construction permits the electrolyte to circulate freely around the plates. In addition, it provides a path for sediment to settle to the bottom of the cell. Each cell is seated in a hard rubber casing through the top of which are terminal posts and a hole into which is screwed a non-spill vent cap. The hole provides access for testing the strength of the electrolyte and adding water.

The vent plug permits gases to escape from the cell with a minimum of leakage of electrolyte, regardless of the position the airplane might assume. In level flight, the lead weight permits venting of gases through a small hole. In inverted flight, this hole is covered by the lead weight.