Aircraft batteries are used to start the engines just like car batteries. However, the comparison stops there as aircraft batteries are required to do a lot more before and during the flight. An electrical failure is a very serious issue and often is an emergency that requires that the aircraft batteries take up power load to run the essential functions until the power generation is restored or the aircrafts lands and the passengers and crew are evacuated. If there is an engine shutdown during the flight due to flame out or any other reason, the batteries are required to restart the engines.
The aircraft batteries are also very important in acting as a buffer to regulate the DC bus voltage on the aircraft. This ensures that the quality of power received by the electronic and electrical equipment is of good quality and acceptable within the tolerances. These functions clearly show the importance of batteries on the aircraft and how crucial is it that they are to the safety and airworthiness of the aircraft. Therefore the batteries have to be of high quality and they should be maintained and kept in perfect working condition at all times.
Two types of batteries used in modern aircraft: Nickel-cadmium (Ni-Cd) and Lead-Acid batteries. The Ni-Cd batteries are of the vented type and the Lead-acid batteries are both vented or Valve Regulated (VRLA) types.
Mechanism of Failure
All batteries eventually lose their ability to hold a charge over time and go to scrap or recycling. The Ni-Cd batteries can fail in three ways (i) Failure of the Oxygen barrier, (ii) failure of the separator, and (iii) permanent loss of capacity loss due to the damage of its active elements.
The lead-acid batteries fail due to (i) deterioration of its active elements, (ii) high internal resistance due to material degradation, and (iii) corrosion of the current collecting plates.
All batteries suffer from thermal runaway. It occurs when the generated heat cannot be dissipated fast enough resulting in a rise in temperature which may lead to degradation of performance and even failure of the battery.
The batteries must be regularly checked to ensure safety and improve reliability. Regular maintenance ensures peak performance and reduces the chances of onboard failures that cause aircraft delays which are very costly to the aircraft operators. Battery maintenance is performed off aircraft in dedicated maintenance facilities such as e2b calibration.
Basic maintenance procedures are similar for both Lead-Acid and Ni-Cd. Apart from the usual tools such as torque wrenches and multi-meters, dedicated equipment is required to perform the charging and discharging sequences of the batteries. The electrolyte levels of the vented batteries are also to be checked and topped up regularly.
The aircraft-specific chargers are designed to ensure that your batteries are maintained in the most precise way possible, using the latest technology. As such, most of these chargers are microprocessors-based systems to make sure that the aircraft batteries are charged to their peak capacities.
Operation of aircraft batteries exceeding their ambient temperature specifications or charging voltage can result in high cell temperatures which may lead to boiling of electrolyte and damage to the cells, and eventually battery failure. Charging must be conducted in a well-ventilated area at ambient conditions.
Correct charging is very important and will affect the overall life of the battery. If the internal resistance is high then the charging will not be 100% efficient and there will be a loss of charge capacity. If the residual lead sulfate increases in the plates, the battery will lose its capacity to hold sufficient charge.
Overcharging happens if constant current charging is done without adequate controls. That is why constant voltage charging is preferred over constant current charging. Overcharging a battery will corrode the positive grids and vaporize the electrolyte.
Constant Voltage Charging
In the aircraft, the battery charging system is of constant voltage type. An electric generator is connected via the electrical bus to the battery. When using a constant-voltage system in a maintenance shop, a voltage regulator automatically maintains a constant voltage. This is the preferred method to charge lead-acid batteries.
Constant Current Charging
In the maintenance shop, a constant current charging method is the most convenient as many batteries of varying voltages can be changed simultaneously. A constant current charging system consists of a transformer and a rectifier to convert the AC supply to DC at the correct voltage. If a constant current charging system is used, several batteries are connected in series, and the charging current is controlled so that the batteries are not overheated.
Testing an aircraft battery is a complex process, because of several essential tests that must be performed. Also because several testing parameters have to be met exactly. Proper equipment and procedures are essential to ensure that battery testing is efficient and optimum. It must meet the specifications given by the manufacturer of the batteries.
Batteries must be removed from the aircraft and tested to determine if they meet the specifications of the manufacturer. Batteries are part of the emergency system of the aircraft. In case of a power failure, the batteries are needed to start the APU (Auxiliary Power Unit) or to provide power to the 28V bus.
In case of such an emergency, it is expected that the battery will supply power until aircraft power generation is restored or it should be able to power the essential electrical and electronic systems until the aircraft safely lands on the ground. A poorly serviced battery can be very costly as it may result in an aircraft accident if the failure occurs in the air or cause delays if the aircraft is on the ground, as it will not be allowed to take off with a problematic battery onboard.
In this test, the battery is discharged at the rated current for one hour. The battery passes the test if each cell remains above 1 volt. In a 20 cell, 40A-Hr battery, this will mean that a battery discharging at a rate of 40A, must run for 1 hour with a minimum of 20 volts.
Deep Cycle Test
In this test, all cells are discharged to zero, then recharged fully, and then tested for capacity. If the cells do not charge properly, then the battery will fail the capacity test.
Charge Acceptance Test
It is to test if the battery plates can retain the required charge. Cell voltages are expected to increase continuously and may appear to remain flat once the cell is well charged. Cells must achieve a minimum end charge voltage.
e2b calibration is ISO 17025: 2017 accredited facility. We test, analyze, and charge the aircraft batteries to the highest aviation standards. Our services are unmatched in the industry. We ensure that your battery is delivered in top condition so that the aircraft can fly without any battery issues. e2b calibration can also provide on-site services. Please contact us for all issues related to aircraft batteries.