Power and performance: the Formula E battery

Williams overcame technical and time constraints to develop a battery for Formula E.

The battery is the car, according to Gary Ekerold, operations manager at Williams Advanced Engineering and head of the programme to design the battery for the Formula E electric racing series. It’s a comment that is difficult to grasp until you see the battery itself.

Formula E battery 7

It is a huge trapezoidal lump, wrapped in carbon-fibre composite and studded with hoses and anchoring points; much larger even than a Formula One engine. The driver’s monocoque safety cell bolts onto one face, and the inverter, motor pack and rear suspension assemblies onto the opposite side, making it a major structural part of the car in a way unique to electric vehicles. When we normally talk about structural batteries we’re referring to bodywork materials that can store electrical energy; we’re not talking about things that actually hold the car together and take the bending moments, stresses and forces generated by the car’s interaction with the road — especially not at racing speeds.

’The battery is the car. It is difficult to grasp this comment until you see the battery itself

Williams Advanced Engineering’s journey to becoming the customer’s car battery supplier was a somewhat more tortuous one than the other members of the car supplier consortium. ‘Another OEM was originally lined up to supply the battery, but when it saw the brief in detail it pulled out,’ explained Ekerold. ‘So we joined the consortium about six months after Dallara [which designed and built the bodywork] and McLaren [which built the motor and its management system]. We came on board in June 2013, which meant we had slightly less than a year to complete the project.’

CX-75
Williams developed the battery system for Jaguar’s CX-75 hybrid supercar concept

Tight timescales are not unusual for motorsport companies, but collaborating to such an extent with companies that are normally bitter rivals is. Moreover, the team was joining a project at a point where many of the vital parameters of its contribution had already been set by the other partners. In particular, noted Okan Tur, chief technical specialist for hybrid systems and design lead for the electronic aspects of the battery, its size had been decided so that Dallara and Spark Racing Technologies, the lead consortium member for delivering the whole car, could get on with their part of the design. ‘When we went for the tender we were given a requirement specification based on that day’s FIA regulations and Spark Racing Technologies’ existing design position,’ Tur said. ‘We were given a predefined battery box dimension, and because the battery is a structural element of the car it had to be designed based on a number of FIA rules and regulations, and as we joined six months in, the external dimensions had already been established — although the box itself hadn’t been designed.’

’Two projects — KERS for Formula One and the battery system for the Jaguar CX-75 supercar — gave us the credibility to address this challenge

Gary Ekerold

The other parameters were based on the performance the battery had to deliver. ‘At high level, the FIA stated 200kW maximum power from the battery, and at that time overall usable energy was limited to 30kWhr including the contribution from regenerative braking. There was a limit on the cell weight to 200kg, whatever chemistry we used,’ Tur said. To fit in with McLaren’s work on the motor, Williams was provided with a voltage range and two ‘golden lap’ performance profiles — one for qualification and one for racing — to provide the power for the motor’s expected performance. ‘So we knew what we’d have to do, how many cycles it would have to go through over how many race weekends.’ The team also knew what the cooling requirements of the battery would be, and that it had to recharge fully within an hour.

Formula E battery 6

Williams was no stranger to this field. Its battery experience began with work on its own KERS (kinetic energy recovery system) for Formula One, said Ekerold, which started out as a flywheel system (the basis of which it has recently sold to GKN) and then shifted to a battery. When the Advanced Engineering division was set up in 2012, providing specialist services to automotive customers, it used its experience in designing its KERS battery to build battery systems for the Jaguar CX-75 concept hybrid supercar. ‘Those two projects were the main things that gave us the credibility to address this challenge,’ Ekerold said.

The constraints of the battery performance and the limited time meant that choice of battery chemistry was limited; they couldn’t develop and test new chemistries despite Williams’ involvement with Technology Strategy Board-funded projects to do just that. Tur therefore selected off-the-shelf cells from an existing supplier with well-understood lithium-ion polymer chemistry. ‘Our first challenge was fitting it into the box,’ he said. ‘It was fine from a volumetric viewpoint: we had the volume available to fit enough cells in to give us the storage we needed. But none of the form factors of the available cells were right, so our design engineers had to put in a lot of effort to come up with a very difficult mechanical arrangement within that box.’ This involved not only getting the cells to fit, but also to distribute the liquid cooling system needed to keep them in a thermal ‘sweet spot’ to optimise their performance throughout the race time; something that makes the battery distinct from any commercial electric vehicle (EV) battery system, which are all air cooled, as well as the power-to-energy ratio being significantly higher.

The design of the battery is modular and designed in a four-way consortium with Dallara, Spark and Renault Sport F1, which is in charge of system integration. ‘That was based on the KERS battery as a starting point, because that already had the necessary electronic, thermal and electromagnetic protection layers integrated into it,’ said Tur. The box and cell modules all have structural elements that contribute to the box’s overall mechanical performance, and the live system had to pass all the FIA’s crash-test protocols. ‘I don’t know of anyone else who is crash testing EV batteries to that extent,’ Ekerold said.

From next season, Formula E opens up so that teams can develop their own cars rather than using the consortium’s customer vehicle; and for this reason many of the technical aspects of the Williams battery — the precise chemistry, for example — are being kept secret. But with the rules of the series specifically designed to encourage innovation in powertrain rather than aerodynamics, Tur and Ekerold are sure that battery performance will improve. ‘Of course we haven’t raced yet, but we know from F1 that once you get drivers involved, they test out the limits of your technology and help you identify where you need improvement,’ Ekerold said. ‘And there are many things we could have done differently if we were involved in the project from the start,’ Tur added.

‘It’s easy to see how Formula E will act as a hothouse for EV technologies, and it is set up to make it simple to transfer those technologies to conventional road vehicles,’ said Ekerold. ‘I do believe we need a step change in battery technology to help EVs towards better market penetration, but motorsport is a superlative way to develop that step change; if Formula E catches on — and I think it will — it will be a very powerful incentive to accelerate that technology to give the teams the wins they’ll want to show their supporters.’

Williams wants to continue its involvement in Formula E, although Tur and Ekerold couldn’t say in what form: commercial discussions are bound to begin once the season gets into full swing. ‘As far as the whole powertrain goes, we have expertise in all of it. We could develop any part of a whole car, if we wanted to, or even the entire thing. We’ll see how it goes,’ said Ekerold.