First reminder, a battery is a device containing one or more cells that convert chemical energy directly into electrical energy.

How a battery works

Batteries and cells are electrochemical systems, which store energy in chemical form and release it in electrical form. Batteries are based on a reversible electrochemical system. They consist of several voltaic cells connected in series. Each cell contains two electrodes, each made of a different material, and a conductive electrolyte. The positive electrode is called the “anode” and the negative electrode is called the “cathode”. While most cells use a single electrolyte, some have different electrolyte requirements for the anode and cathode. In these cases, two different electrolytes are used and the cells contain a separator that prevents the electrolytes from mixing but allows electron transfer.

Once these electrodes are connected via the electrolyte, this causes a specific chemical reaction called “redox” (reduction-oxidation). This reaction leads to reduction (addition of electrons) at the cathode and oxidation (removal of electrons) at the anode. It is this migration of electrons that produces the electromotive force (EMF) inside the cell. The electromotive force measured between the two electrodes while the cell is neither charging nor discharging is the open circuit voltage that the cell is capable of producing. The voltage varies depending on the materials used to manufacture the cell.

In order to obtain the desired voltage, the battery pack will consist of more or less of these cells, they can also be connected in parallel to increase the electrical capacity. There are two types of battery, the first called primary or non-rechargeable are designed so that once all the available electrons have migrated from the anode to the cathode. The battery no longer produces any current and must be replaced. The second type, called secondary or rechargeable, operates on the principle of the reversible oxidation-reduction reaction. This process recharges the battery by sending electrons back to the anode. This allows the oxidation-reduction reaction to be repeated once the charger has been removed.

Batteries in aircraft

Nowadays all types of aircraft are equipped with an electrical system. The most “primary” electrical systems are equipped with one or more batteries. Their purpose is to power the electrical systems and the auxiliary power units/engines during the pre-flight phase. Once in flight, the auxiliary power units/engines will recharge them and supply the electrical circuits. The batteries intervene in the event of problems, a breakdown or necessary isolation. In these cases, the batteries will supply the whole aircraft. Some elements such as the Emergency Locator Transmitter (ELT), the CVR, the FDR, have their own dedicated batteries. Safety devices such as flashlights, megaphones or defibrillators also have their own batteries.

Batteries in aircraft can be of either primary or secondary type. Above all, however, they must be safe but also, ideally, have a high energy density, be light, reliable, require minimal maintenance and be able to operate efficiently over a wide environmental envelope. Battery manufacturers continue to develop new technologies in an attempt to achieve these ideals, but in many cases it is necessary to compromise on these non-safety objectives and in some cases the safety implications of new designs have been overlooked, particularly with the increasing use of lithium batteries.

The crucial point is that, as batteries are a source of energy, their failure due to damage, defect, malfunction or misuse represents a critical potential hazard in terms of equipment, hazardous fumes, smoke or fire.

Where it’s located?

In general, an aircraft is equipped with Multiple rechargeable batteries, all providing critical power to the respective devices. Typically, as an example, the main power unit and the auxiliary power unit (APU).

The role of the main battery is to power the aircraft’s systems during the pre-flight phase. It is also used to support ground operations, such as refuelling and powering the braking system when the aircraft is towed. The main battery also provides backup power for critical systems during flight in the extremely unlikely event of a power failure. It is located in the Forward Electronics Equipment (EE) bay, which is located under the main cabin floor at the front of the aircraft. The APU battery provides the power to start the APU, which in turn can start the aircraft’s engines. The APU, and its battery, are also part of the multiple layers of redundancy that would provide power in the rare event of a loss of primary power sources. Located in the aft electrical compartment, which itself is in the centre of the aircraft close to the wings. As you can see on the example of a Boeing 787, shown here.

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Maintenance & Battery lifetime

Typical battery lifetime is seven to nine years for long-term aircrafts, five to seven years for regional aircraft and ten to twelve years for business jets. However, the life of lead-acid batteries is half to one third of that of Ni-Cd batteries and can be significantly reduced by operating at high temperatures. For safety reasons all batteries require regular checks. The regular maintenance of these devices therefore guarantees an optimal performance level and reduces the risk of breakdowns, which can lead to costly delays and serious safety problems.

Battery maintenance is carried out on board the aircraft in specialised battery workshops that can be found all over the world and which are usually owned by the operator or specialised distributors. They are required to be approved by local airworthiness authorities and may also apply for optional approvals from battery manufacturers. Whether for lead-acid or Ni-Cd batteries the procedures remain the same. In addition to the usual tools such as torque spanners and multimeters, equipment is required to charge and discharge the batteries. Vented batteries also require means to adjust electrolyte levels and check cell vents. Ni-Cd specific means are used to deep discharge individual cells in order to remove any cell imbalance.

Various specialist manufacturers offer systems that allow these tests to be carried out with a minimum of effort. The maintenance intervals are defined by the aircraft manufacturer based on the battery manufacturers’ recommendations in relation to the aircraft in question and its use.

These maintenance intervals depend on the energy required for emergency needs and the electrolyte reserve of the airframe. They are influenced by various factors such as the battery charging system, battery operating temperature, type of starting, number of starts, flight time, ground operation and battery technology.

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