Bjorn’s Corner: Sustainable Air Transport. Part 34P. eVTOL battery cells. The deeper discussion.

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August 26, 2022, ©. Leeham News: This is a complementary article to Part 34, eVTOL battery cells. It discusses the trickiest system on an eVTOL, the battery system, and its key component, the battery cells.

The Lithium-Ion cells in an eVTOL battery system are the core of the energy supply system. The cells are of a high power and energy type and must be used and managed correctly to last long and be safe.

The eVTOL battery cell

The battery cells used in eVTOL projects are Lithium cells of the NMC type, Figure 1.

Figure 1. Different Lithium cell chemistries. Source: Elithion

There’s a lot of discussion about different new and ground-breaking cell chemistries. In reality, I know of no eVTOL project batteries that do not use NMC cells. There are variations within the NMC types, with recent developments focused on silicon-based anodes for higher power with retained energy capacity.

We will look at such a cell type in the next Corner when we discuss how Lilium solved its power demand problem. Here we look at a state-of-the-art NMC cell, the Molicell INR-21700-P45B, chosen for the Vertical VX4 VTOL.

The INR-21700-P45B is a cylindrical cell of 21mm diameter and 70mm length (therefore 21700 in the name). The 21700 dimension is a later, larger variation of the 18650 cell form factor used in many electric aircraft/VTOL battery systems.

Figure 2 shows the main data of the cell. First, we have the typical capacity, 16.2Wh, 3.6V Voltage, and the maximum discharge current, 45A.

Figure 2. Main data from the INR-21700-P45B datasheet. Source: Molicell

We also have a maximum cell temperature of 60°C, a Gravimetric energy density of 242Wh/kg, meaning each cell weighs 70g, and it has an internal DC Impedance of 0.015 Ohm. We will see how this internal Impedance will cut into the capacity and cause heating of the cell when high currents are demanded.

With a 16.2 Wh typical capacity, we need 9,260 cells to make an eVTOL battery of 150kWh.

The C-rate parameter

The charge and discharge of a cell depend on the power drawn from the cell. The remaining energy of a cell or battery is called its State of Charge, SOC, and is measured in percent of total capacity.

As Power = Voltage * Ampere and Energy is Voltage * Amperehours, the energy from the cell is the area under the Voltage and the mAh curves in Figure 2. The curves vary by C-rate. In the diagram, Molicell has put Amps for higher demands instead of higher C rates.

Figure 2. Discharge characteristics for INR-21700-P45B. Source: Molicell.

C-rate is a normalized way to measure how fast we charge and discharge a cell or a battery system. C-rate 1 means a 16.2 Wh cell is charged to this capacity in one hour. If we charge with 2 C, it takes half an hour, and 3 C cuts it to 1/3 hour.

A discharge with 1 C empties the battery in one hour and 10 C in six minutes. So C-rate is the way to say how hard we charge or discharge a cell or a battery.

A cell for the automotive industry is optimized toward energy capacity and reacts badly to high C-rates, both for charging and discharging. Typical C-rates are below two for such cells, and the overnight charge is done at C rates below one.

As we will see below, we need C-rates up to eight for VTOLs and for electric aircraft, up to four. It’s the reason the aeronautical industry can’t use high-volume automotive cells. The demanding VTOL industry must, consequently, use power-optimized cells made in small volumes.

VTOLs power demand

As we can see from the discharge rate diagram, the cell has different energy content dependent on how fast we discharge the cell. The nominal 16,2 Wh is only valid when the discharge rate is 1 C. The average Voltage is then 3.6V over a 4,500mAh discharge cycle (3.6V*4.5Ah=16.2Wh). We see Voltage is 4.1 at the start of takeoff and declines to 2.5V for a 1 C rate, where the cell shall not be discharged anymore (it can damage the cell).

If we discharge with the maximum continuous 45A, the capacity (the area under the discharge curve) shrinks to about 65% of 16.2Wh.

The shape of the curves and how these vary with different demanded currents is important. It shows we have plenty of power from a newly charged cell but that its capacity (Volts*Ah) drops at the end of State of Charge.

Let’s see what this means:

  • The Vertical VX4 needs about 800kW continuous power in hover and about 1,000kW if we include control margins and that a motor or power electronics unit or part of the battery can fail in the final hover (depends on the VTOL redundancy concept).
  • A 1,000kW power level means 108W per cell. At the takeoff hover, we have ~3.7V from a newly charged cell when we draw 30A from the cells for the 20 seconds of takeoff hover. We are well below a maximum power draw of 45A (Figure 1).
  • The landing hover is more critical. It lasts 45 seconds, and now the cells are at their last part of the capacity. Let’s say there was congestion at the heliport, and we need to use a 20 minutes VFR reserve (we will discuss the VTOL reserve issue in a later Corner). This means our 45-second landing hover is done on the last bit of energy in the cell.
  • We are now close to the cutoff Voltage of 2.5V. At 2.7V, our current draw is 40A, close to the 45A maximum from the cell. We can see we can use the capacity to the 4,000mAh line but not further. Our hover must be finished before we hit this line or the control of the VTOL in a redundancy case is at risk.
  • The above assumes we have a current limit of 45A. But there is also a temperature discharge limit of 80°C for the cell and a general specification limit of 60°C. The cell is combined with 9,260 other cells in our battery, and there can be tougher discharge cell limits imposed by this packaging. This is the reality of VTOL batteries; at the landing, we are running against the limits of the cells and the battery.

The consequence is that VTOLs will not be able to run their batteries to 0% SOC. A cutoff of use at 10% to 20% will be necessary, depending on the VTOL concept.

The cell cycle life and the AND problem

NMC cells age with use cycles. They are sensitive to how fast they are charged, to what SOC, how fast they are discharged, and to what final SOC. A cell is deemed to last until the SOC at a full charge does not go beyond 80% of a new cell.

If you want the cell to last long (over 1,000 flights), you shall charge it at a maximum of 1C to a maximum of 80% SOC, discharge it at high C rates only for short durations and leave as much SOC as possible in the cell at the end of the flight, preferably 30% to 40%.

The fact that you must obey all these rules to get cells to last longer than the typical 800 to 1,000 cycles is called the AND problem. As cells are the highest cost item in the operation of a VTOL (we will analyze the operational cost in a future Corner), these are important facts.

VTOL investor presentations talk about 15 minutes charge cycles and 30 flights daily. This necessitates fast charges, which compromise cell life. Here’s why:

  • With 18 active hours (no flights from midnight to six o’clock), the flight plus turnaround time is 36 minutes. If the flight takes 20 minutes, we have 16 minutes for charging.
  • If we save the battery, this shall be a 1C charge, i.e., we can only charge 27% of SOC, or 40kWh, during this stop. Our flight is thus only allowed to use 27% of our battery.
  • As the takeoff hover, transition, climb, descent, transition, and final hover energy consumption stays the same (assuming the same cruise altitude), our range is now minimal. It all depends on our total battery capacity, but with the example VTOL Vertical VX4 and 150kWh battery, the range would be close to zero.

The above is the essence of the AND problem. The battery can do all these things but not combined. Luckily Vertical has not made such claims (to my knowledge), but others have.

Conclusion

We can see that the battery and its cells are complicated, yet we have not talked about how these cells must be managed to achieve these performance values when combined in the thousands. It’s the job of the Battery Management System (BMS), and we will talk about the BMS in a future Corner.

Before discussing the BMS, we will look at how Lilium solves its battery performance problem by using a cell with silicon anodes and a different reserve strategy.

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