The batteries (main batteries) in a passenger aircraft are important components of the on-board power supply system. For decades, rechargeable nickel-cadmium batteries having a nominal voltage of 24 volts have been used for this purpose, which nickel-cadmium batteries consist of twenty nickel-cadmium battery cells which are connected in series and each have a nominal voltage of 1.2 volts. The batteries are used for example to supply power in emergencies until other redundant power sources become available, to start the auxiliary power unit (APU), and to supply power to sub-systems on the ground for servicing and in preparation for flight. In these cases, strict requirements are placed on safety, reliability and availability, as a result of which servicing intervals are relatively short.
At present, the batteries are removed after a certain number of flight hours (FH) or after a defined period of time has elapsed. The batteries have to be removed from the aircraft and reconditioned in a workshop in order to determine the condition thereof and to carry out maintenance thereon.
However, in their documentation, battery manufacturers always give operational hours (OH) for the servicing intervals. However, since aircraft operators only record flight hours, a factor is defined using the following ratio: factor=OH/FH. This ratio can vary within a particular range depending on the mission profile of the aircraft. The mission profile is calculated using the FH/FC ratio, where FC is the flight cycles, and is classified as being short haul, medium haul or long haul. A relationship to the turnaround time is thus established.
A typical cycle of an aircraft battery, which is removed after the applicable time limit has elapsed, is for example storage-installation-operation in aircraft-removal-maintenance-storage. Measures taken during the maintenance stage are for example: periodic checks, including for example insulation measurements, voltage measurements and filling level measurements; regular checks including periodic checks and capacity tests; and general overhaul including regular checks and cleaning.
In an aircraft, it is conventionally only the total voltage and the charging or discharging current that is recorded. Furthermore, in some types of batteries, a temperature is also measured at a central measuring point. These data are not analysed or combined in the aircraft, it being impossible in any case to reach a satisfactory conclusion on the state of ageing of the battery on the basis of these data.
In the workshop, the state of health (SoH) of an aircraft battery, as set out by the manufacturer in the component maintenance manual, is determined using the insulation values, the cell voltage during loading, the filling level of the electrolyte and the capacity. There are doubts as to the validity of this method in respect of the state of health. If the aircraft battery passes the capacity test without any difficulty, it is deemed to be in working order irrespective of the operating condition of individual cells and can thus be installed in the aircraft. In this way, weakened cells go undetected and sustain further damage during continued operation. If the fault is later detected during maintenance operations, the battery should be disposed of if there is a low number of faulty battery cells, for example five. In any case, weak points or potential faults in a battery which is in use are not detected or remedied until maintenance is carried out in the workshop, if at all. These faults are thus dormant faults; it is not possible to detect these faults at an early stage or in a timely manner. Therefore, no preventative measures can be taken either in order to counteract irreparable damage and the associated requirement of permanently taking individual battery cells or the whole battery out of operation.
In the worst case scenario, the capacity required in an emergency, which should ensure that there is an emergency power supply for 30 minutes, cannot be drawn from a battery that has been previously damaged in this manner, and this can have an adverse effect on flight safety.
Historical analysis demonstrates that nickel-cadmium batteries used in aircraft are very reliable components. The number of times that the batteries are removed when undamaged is therefore relatively high, resulting in unnecessary effort. An analysis of a particular type of passenger aircraft carried out in 2012 showed for example that only a particular proportion of the removed batteries had a dormant fault which had not been detected during operation in the aircraft. Therefore, a fault was not found in many of the removed batteries, and therefore said batteries would not have needed to be removed.
If a battery cell weakens steadily over time, the other battery cells are loaded more heavily and may likewise become damaged. This imbalance between the individual battery cells cannot be detected from the outside. Even the tests required by the manufacturer only inadequately identify weakened battery cells. This increases the need to take whole batteries out of operation unnecessarily because a plurality of battery cells may already have a serious defect at the time when the fault is discovered.
Another drawback is that there is no universal standard for the removal of batteries. Each operator establishes, in consultation with system developers, the servicing intervals after which a battery should be removed. Empirical values and manufacturer recommendations form the basis of this decision. A solid database in the form of continuously collected measurement values does not currently exist. This poses a risk in respect of the evaluation of the state of ageing of the batteries.
The object of the invention is that of providing a system and a method for monitoring an aircraft battery which provide for cost-effective and efficient servicing and maintenance, increase the average operational life of the batteries and flight safety, and reduce the amount of irreparable damage to the batteries.
This object is achieved according to the invention by the features of the independent claims.