Where do you put the batteries in an electric aircraft?

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A full-scale wing mockup supplied by GKN Aerospace which the CDO2 team has used as a framework to integrate its battery technology. (credit: CDO2)

Few passengers are aware that an aircraft’s wings are home to the fuel tanks, CDO2, a UK-based novel battery sensor technology developer, thinks the wings can also be where batteries are housed too. But ensuring battery cell packs are easily accessible, located for peak operational efficiency in the airframe and meet regulations is not a simple equation for OEMs.

Regulations on putting batteries in aircraft are tough, in part due to the problems Boeing had with thermal runaway in lithium-ion batteries onboard the Boeing 787-800 back in 2013. Crews from Japan Airlines, United and All Nippon Airlines 787-800s discovered smoke coming from auxiliary power units shortly after landing. The whole fleet was grounded. A National Transportation Safety Board  (NTSB) investigation found a cell within the lithium-ion battery was believed to have short-circuited, causing thermal runaway. Boeing made modifications, including stainless steel casing and a duct for ventilation to the aircraft’s exterior, proving the safety case to the FAA.

These are events the industry does not want to repeat. However, it has made battery-electric aircraft design more of a challenge than it already is. “You need batteries to be light, you need batteries to be safe – all are pinch points that make it the hardest area to be in,” says Gary Kendall, director, CDO2. “If you’re going to put a lithium-ion battery in an airframe you need to put it into a fireproof box. The problem is it makes the battery heavy. So that is the problem we have, there are people like us who have new ways of anticipating failures and measuring battery performance, but the regulator is still wedded to the need for a fireproof box. 

“So, what we have done is try to meet that regulation with as smaller weight overhead as possible by using composite boxes rather than metal for example,” adds Kendall.

According to Tine Tomažič, director of Engineering and Programs at Pipistrel, aircraft design is as much about converting great ideas into reality as it is about making tradeoffs. “Building the batteries into an aircraft is not more difficult than building-in a fuel tank or elements of cargo space,” he tells Revolution.Aero. “The challenging part is understanding the nature of batteries’ behaviour, both in the nominal and off-nominal case. What matters is not only the safety of batteries, but the predictability of their performance and ageing. Only these three elements together constitute a commercially viable and predictable product.”

Pipistrel can now determine all aspects of battery performance, including its mass and volumetric impact to the aircraft’s design, and “bake-in” battery system permutations from its in-house conceptual design suite. “In addition to this, we have processes that validate certifiability of a battery system, and we carry EASA level approvals for battery manufacturing and servicing/overhauls, says  Tomažič. The firm also scouts for emerging battery technologies, including solid-state and structural systems.

Regulatory authorities are currently working on special conditions for the certification of the batteries in eVTOL aircraft. EASA became the first to award a type certificate for an electric aircraft when it certified Pipistrel’s Velis Electro (Virus SW 121) back in 2020 under SC-ELA.2015-01 (CRI F-101). Kendall says: “Forming new regulations makes sense, regulators are not in a position to turn around and say the new rules are less stringent than anything previous, if anything they have to be tighter. Also, many eVTOL aircraft do not necessarily glide well so some motive power is required even to perform a forced landing.”

Critics of the battery-electric aircraft sector claim that technology which offers meaningful distances does not exist. Commercial viability is going to be everything post-certification for OEMs and getting the design right now is key. Take Heart Aerospace, a battery-focused Y Combinator graduate, which upped its 19-seat plans to a 30-seat regional aircraft with hybrid-electric power. Or BETA Technologies concurrent development of an eCTOL version of its ALIA-250 eVTOL aircraft.

Joshua Ng, director, Alton Aviation Consultancy tells Revolution.Aero: “Improving specific energy and power revolves around getting more out of the existing lithium-graphite battery in the near-term, to developing and commercialising other lithium-based battery technologies such as Li-Sulphur or Li-Silicon or even novel battery chemistries in the mid to long term. With each successive generation of battery technology, energy densities are expected to improve – enhancing eVTOL vehicle performance. With some eVTOLs having the ability to swap batteries, upgrading to the latest battery technology could be as simple as a battery swap.”

There is a coefficient between good aircraft design and simplified maintenance. It is more complicated to work out what makes maintenance less of a challenge. For Tomažič, this is an elemental discussion that not only touches on the aspects of a good aircraft design, but also, when it comes to vehicles with electrified (battery electric, hybrid electric or hydrogen fuel-cell electric) propulsion. “Its not straightforward to assess the state of the eVTOL industry relative to the ease-of-MRO, given that theres yet to be a design fielded, but its fair to say that the first priority in the design, because of technological constraints (e.g. battery performance), is the overall packaging for light-weight and performance, and elements of cost effectiveness and ease-of-MRO come secondary.” This is due to the natural flow of the engineering process, says Tomažič. “To optimise for anything, one first needs to gain profound knowledge of that topic. Indeed, the a-priori optimisations for MRO activity on eVTOLs are present, but the test of time will show whether focus was actually put on correct elements.”

Simplified maintenance might be an oversimplification of what operators need out of a vehicle, according to Ng. “We prefer to use the concept of maintainability of the aircraft instead,” he says . “Maintainability refers to the balance of aircraft and system-level reliability, as well as the amount of maintenance required to keep the aircraft flying.” Ng estimates eVTOL maintenance costs could amount to only 40% of those for  helicopters. “Vehicles are also designed with predictive maintenance in mind. Onboard sensors and data communications equipment are present to support real-time or near-real-time data feeds to aircraft monitoring systems on the ground with ability to identify aircraft issues before they happen.”

When considering the maintenance cycles, the airframe should outlast the batteries. So, in the same way an aircraft requires an engine overhaul, it will require a battery overhaul instead. The Velis Electro’s maintenance is not based on cycles, but on a state of health (SOH) computed index, which goes from 100% to 0%, according to the OEM. 100% means new and 0% means it is due for replacement. In-between, there are about 1,400 cycles, depending on the usage style and environmental circumstances. “This is a real issue that needs addressing,” says Kendall. “It is absolutely crucial to have a maintainable battery, easily accessible in the wing. Being able to monitor each individual cell allows us to see how each is performing rather than an aggregated total. This means we can get more energy safely out the cells capable and perform preemptive maintenance.”

Whilst it might be hard to work out where efficiencies might be best made in MRO, the location of the batteries in the airframe is simpler to work out. In an eVTOL aircraft this means in the wing (if there is one) or as near to the source of vertical lift as possible. In a helicopter-inspired design this could be in the centre point of the fuselage. “It is all to do with the mechanical strain on the airframe,” CDO2’s’s Kendall explains. “If the batteries are in the fuselage, then the wings have to carry all of that weight, which puts quite an extreme bending force on the wing. If you’re able to move those batteries into the wings you are basically putting the weight next to the lift – reducing the forces endured and making the wing structure lighter.” Kendall and the CDO2 team demonstrated their technology inside a mockup of an eVTOL wing designed by GKN Aerospace.

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