The Key Things to Know about eVTOL Batteries

The move to electrification might have been popularized by the automotive industry, but the energy, and therefore the size of the battery, needed to power a car is far lower than the power need for an aircraft to fly. And whilst the physical size and weight of a battery that powers a car is important, it is more so in an aircraft, as it affects its payload / range capabilities, especially in the first stage of flight – if a battery is too heavy, the aircraft might not even be able to lift itself off from the ground. Therefore, improving the battery performance per unit weight will be more crucial than equipping heavier batteries on aircraft.

Batteries for electric VTOLs face critical challenges in achieving high specific energy, high specific power, charging speeds, and high cycle life. Despite the challenges that the battery industry faces, improving battery technology is crucial to the future development of electric aerial vehicles.

This section will introduce current battery technology, as well as its direction for the future.

1. Basic concepts

Before moving on to the difficulties of battery technology, this subsection will introduce the four fundamental concepts of batteries. These four concepts are the primary indicators to evaluate battery performance and form the foundation of discussion on a vehicle’s range, charging time, cost, and other characteristics.

a. Specific power (SP)

SP describes how much power is in a unit weight of a battery, usually defined as W/kg. SP depends on battery chemistry and packaging. SP determines the capacity of the aircraft and the required battery weight.

b. Specific energy (SE)

SE describes how much energy is in a unit weight of a battery and presents the performance of the battery, usually defined as Wh/kg. SE also depends on battery chemistry and packaging. SE determines the range of the aircraft and the required battery weight.

c. C-rate

C-rate is a parameter used to measure how fast a battery can be fully charged or discharged, where a higher C-rate means a shorter charging time. For example, a C-rate of 1C means the battery can be fully charged or discharged in an hour. The upper limit of the C-rate depends on the type of electrolyte and electrode material used in the battery. C-rate also determines the driving performance of a vehicle. In cases where larger lift or thrust is required, the C-rate is subsequently larger. For example, during vertical takeoff or landing, the gravity to be overcome is greater than in a horizontal cruise, resulting in a higher C-rate; faster acceleration and cruise requires higher discharging power, also resulting in a higher C-rate. The chart shows the relationship between each component.

d. Cycle life

Cycle life is the number of discharge-charge cycles the battery will experience before it fails to meet the lowest power demand of the aircraft. The cycle life depends on the C-rate, depth of cycles, temperature, and battery type. The depth of cycles refers to the percentage of electricity discharged in a cycle. A greater percentage means a deeper depth of discharge. Cycle life determines the life of the battery.

Interrelationships between specific power, specific energy, C-rate, and cycle life

The SP, SE, C-rate, and cycle life all correlate to each other. For the same cell (in the figure above, the bands in different colors represent different electrical devices), SE and SP changed reciprocally (although they can only do so in the corresponding band). The SP cannot be infinitely large, as the corresponding SE will drop rapidly after reaching a certain level; hence, there is a balance point between SP and SE that needs to be met to meet power demands. A discharge rate of 5C (as shown by the diagonal dotted line in the figure below) is an appropriate reference standard, as it balances maximum specific power and specific energy, with a relatively fast discharging rate.

However, whilst a rate of 5C would seem to be ideal, it would mean that the battery’s cycle life would be fairly short. This is because as the cycle of charges and discharges increases, the battery will age, and its performance, such as capacity and specific energy, will decrease. A higher C-rate causes the battery to age more quickly. Therefore, eVTOL batteries need to balance SP, SE, C-rate, and cycle life.

2. Impact on eVTOLs

Improving specific energy and specific power whilst achieving fast charging and a long battery life are the four main challenges in developing eVTOL batteries. The four concepts mentioned in the previous section are particularly important for evaluating the performance of batteries and aircraft, and unfortunately, they can be mutually restrictive.

This section will further discuss the specific impact of battery performance indicators on eVTOLs, based on existing technical levels and actual needs, and introduce ways to improve battery performance.

a. The specific power limits the vehicle’s load capacity.

The different power requirements of the aircraft in each flight phase are more evident in the eVTOL. A typical eVTOL trip has five stages: takeoff, climb, cruise, descent, and landing, where the power output required by the battery at distinct states of the vehicle’s flight is different. Most eVTOLs use the most power whilst taking off and landing.

In addition, the need for specific power varies by thrust design. Different power layouts will change the minimum power requirements in the takeoff and landing, and cruise phases.

At a discharge rate of 5C (a typical discharge rate needed for hovering), battery technology can currently achieve specific power of 1 kW/kg. However, this will hopefully reach its target performance of 2.5 kW/kg for eVTOLs by 2040. This current barrier might be overcome by using new materials and redesigning battery cells.

b. The specific energy limits the vehicle’s range.

The battery’s specific energy determines the upper limit of the vehicle’s range.

According to the Fast-Forwarding to a Future of On-Demand Urban Air Transportation report published by Uber in 2016, eVTOL vehicles should have a minimum effective range of more than 100 miles (about 160 kilometers). Using this potential minimum range, the required minimum available specific energy of the battery would be around 230Wh/kg. However, considering system efficiency, backup energy, and battery pack design, only 50% to 60% of the specific energy is available during the flight. Therefore, the specific energy of the whole battery pack should be about 380~460Wh/kg to fulfill the minimal SE demand. Taking this into consideration, current generation batteries would only be able to power eVTOLs on short-distance flights of less than 50 kilometers.

Specific energy can be increased by:

• Using more advanced electrode material in design
• Applying a more compact battery pack design

c. Battery recharging speeds limit operational intervals

As stated previously, the goal is to reach a 5C charging speed. This rate is theoretically feasible, yet one needs to consider battery life in practical applications. An excessive charging speed can lead to a drastic reduction in battery life; thus, manufacturers did not widely adopt this rate, but limited the charging rate to under 1C to prolong it.

Only a limited amount of energy remains in the aircraft after landing, and the vehicle requires recharging before the next takeoff. The aircraft can be recharged before the next group of passengers board the aircraft, or the battery could be swapped out directly for a fully charged one, however, purchasing multiple batteries for replacement is likely to be expensive –  battery cost currently ranges from 20% to 50% of the overall manufacturing cost of aerial vehicles.

To increase the operational efficiency of eVTOL flights, charging times should be as short as possible, especially during busy times. If the eVTOL needs to take off 12 minutes after landing, it requires a 5C charge rate. If the operator needs to reduce the changeover time by half, the charge rate must be doubled.

d. The aging of the battery at a high C-rate limits the lifespan of eVTOL.

At a 1C discharge rate, the number of cycles in a battery’s cycle life is around 1500. Batteries for eVTOLs need to operate under high charge-discharge current which reduces the battery’s life if it is charged quickly at the current level of technology. If charged at 5C speed, the battery’s cycle life will be around 1,000 charge / discharge cycles. Thus, a major challenge for batteries is the need to ensure their lifespan to reduce operating costs. Even if the aircraft only performs three flights a day and charges three times, the battery needs to be replaced once a year, which is likely to be expensive.

An asymmetric temperature modulation (ATM) method that charges the battery at a specific temperature and voltage can significantly slow the rate of battery aging. It was reported that a battery charged under 6C with ATM method can retain 92.3% capacity after 2000 cycles.

3. Next-generation battery technology

The impact of new battery technology on the future development of eVTOLs should not be underestimated. Lithium-ion batteries are no longer the only choice, solid-state batteries, sodium-ion batteries, and fuel cell technology are also emerging. These new technologies meet different needs such as long range, large payloads, and fast charging, offer unique battery performance characteristics, and provide a solution for future large-scale applications of eVTOLs.

a. Solid-state batteries

A solid-state battery is a battery technology that uses solid electrodes and a solid electrolyte, which theoretically means that the capacity and power of these batteries will be higher than lithium batteries. Semi-solid-state batteries are already in mass production, whilst full-solid-state batteries are expected to enter mass production in 2025.

b. Sodium-ion battery

Sodium-ion batteries are similar to lithium-ion batteries but use sodium ions as the charge carrier. Compared to lithium-ion batteries, current sodium-ion batteries have somewhat higher costs, slightly lower energy density, better safety characteristics, and similar power delivery characteristics.

c. Hydrogen Fuel Cell

A hydrogen fuel cell is an electrochemical cell that converts the chemical energy of hydrogen and uses an oxidizing agent to run electricity through a pair of redox reactions. The most significant feature of hydrogen fuel cells is their high specific energy and their replacement of the hydrogen bottle, which cuts down the time to charge when compared to lithium batteries.

4. Key Take Aways

Battery technology impacts the development of eVTOL profoundly, as it affects the range, carrying capacity, charging time, and maintenance costs. Compared to batteries for electric cars, eVTOL batteries require a higher level of performance, such as having higher specific energy and power, shorter charging times, and the ability to work continuously at a high discharge rate while still having a suitable life span. Traditional battery technology cannot fulfill all of these requirements at once. Due to the intricacy of battery requirements, batteries for passenger-grade aerial vehicles are more complicated to develop and manufacture. Solid-state batteries, hydrogen fuel cells, and other next-generation batteries, already being adopted on a small scale, may provide the solution to meet the desired performance level.