The Antares 20E is equipped with a battery-system utilizing Li-Ion cells of the type SAFT VL41M. The Antares 20E was worldwide the first product to utilize these cells, but meanwhile an impressive array of other users have recognized the advantages of this cell, and its field of application is constantly broadening.
Lithium is the lightest of all metals, and has the highest negative standard potential. The low weight and high voltage level of the system result in a high specific energy density. Compared to other currently available Lithium based cells (Li-Ion / Li-Po / Li-Su), SAFT VL41M exhibits very high current capability and very good cycleability. This qualifies SAFT VL41M Li-Ion cells for application as aircraft energy storage before all other available cells.
Performance
Positioned in the leading edges of both inner wings, the two battery packs consist of a total of 72 cells divided into 24 modules containing 3 cells each. Each cell is carefully and redundantly monitored and controlled to ensure that it delivers maximum performance while remaining within its safety limits.
A full charge will deliver the Antares 20E approximately 3000m above its starting point. It is off course possible to divide the available energy into one or more takeoffs and climbs. An extra takeoff costs approximately 100m climb altitude. A full charge takes approximately 9 hours to complete using the built in charger (optional 230 V or 110V AC). Needless to say, this requires the charger to be connected to the power grid. Li-Ion batteries have no memory effect, so partial charges and discharges have no negative effect.
As with all batteries, the performance of Li-ion batteries is affected by the battery temperature. To counter this effect, the battery-packs are equipped with electrical battery heating. If the battery charger is connected to the power grid, then the energy required to heat the batteries can be provided by the battery-charger. In the air, the battery modules use their own energy to maintain optimal battery temperature. This process happens automatically, and each battery module is controlled individually. It is possible to turn the heating off and on in flight in order to conserve energy during very long flights. With the battery heating turned off, the good isolating properties of the wing ensures that it takes a long time for the batteries to cool down to a temperature where the performance of the battery is affected.
Battery life
The capacity of a battery diminishes with the number of charge-discharge cycles it undergoes. SAFT recommends exchanging the cell after 1500 SAE cycles. A SAE cycle consists of a full charge and a discharge with 20% of the battery capacity left. A partial charge and discharge equals a partial full cycle. After 1500 SAE cycles, SAFT expects that the cells will retain at least 80% of their original capacity. This corresponds well with tests run by Lange Aviation.
To a costumer, this means that a battery pack will yield 3.600.000 m (11.811.024 ft) climb altitude or a 228.000 km (123.110 n mile) range in alternate climb/glide before they have to be replaced.
An effect which will probably affect costumers more than the number of battery cycles is natural ageing. SAFT recommends replacing the battery after 8-11 years due to natural ageing. An application example with relevance to this issue is the application of VL41M cells as a backup power source for so called micro base transciever stations (micro-BTS), which are a new generation of cell phone base stations for urban environments. The producer of the backup power units expects a battery change interval of 10 years when performing constantly at 30°C. External conditions have a substantial effect on the ageing of the cells, and thus, if the operating temperature is reduced to 20°C, then the expected battery change interval is increased to 20 years.
Availability
As a user of SAFT VL41M cells, Lange Aviation is in good company. SAFT VL41M are also used in the RQ-4B Global Hawk UAV, and will be built into the F35 Joint Strike Fighter in 2007. In the Global Hawk the cells provide emergency power to mission critical systems in the event of an engine failure. In the Joint Strike Fighter the cells will be a part of the 270V Electrical Power Management System. Next to being a great vote of confidence to SAFT VL41M cells, these two implementations mean that the cells which are now being built into the Antares 20E will be available at least until 2031.
Antares 20E: Silent Power