September 2, 2022, ©. Leeham News: This is a summary of the article Part 35P, Lilium battery cells.
It discusses the requirement the Lilium jet principle puts on its battery cells and how this is solved, both with a slightly different cell type and with some operational adaptations.
The Lilium architecture, with its 30 jets, puts extra strain on the battery system during hover and transition, as discussed in Part 30.
Figure 1 is from the Lilium battery blog, where the typical power consumption during different phases of flight and the use of the battery’s State Of Charge (SOC) is shown.
The high hover power need of 2,500kW peak, 2,200kW average, leads Lilium to adopt an NMC cell with a silicon based anode developed by Zenlabs with an energy density of 319 Wh/kg at a 1C discharge rate.
Due to the high power demand from the battery at transition and hover, a Lilium pilot flying VFR (IFR certification is a later step for the jet) must decide at around 30% battery SOC to land at the heliport or to divert to an airfield for a normal landing. The reason is, with a power demand of 2,500kW, the hover must be finished before 20% SOC, or the battery cannot guarantee a 2,500kW power level.
The range given in Figure 1 is 94nm with a new battery. A battery typically degrades to a charge level of 80% SOC after about 1000 flights. How fast this degradation happens depends on the charge C rate. The 1000 cycles are for a charge rate of 1 C, i.e., it takes about 45 minutes to charge the battery after a flight that has consumed 75% of the battery. With a mission that has 80% SOC at the start and that must be finished before 20% SOC, the range is 55nm.
If you decide to divert at 30% SOC, you have energy for an alternate airfield that lies within 25nm/45km from your decision point. This assumes the VFR landing procedures (any vectoring followed by landing) take five minutes.
The reserve strategy from Lilium with a normal landing after a decision point before transition and hover is a consequence of the design with 10 times higher hover power consumption than for normal flight. It will work if airports are available within 10 minutes flight of the decision point.
Will the above diversion strategy be accepted by the regulator, or will the rules contain extra reserves like a diversion airfield holding time? The reserve rules for EASA SC-VTOL, which are the certification rules Lilium follows, are not set yet. We will have to wait and see.