Formula E batteries are being “refreshed” by Williams Advanced Engineering in time for summer testing at Donington Park.

The batteries were originally designed to last for two complete seasons but changes to the sporting and technical regulations after the batteries had been manufactured called for a rethink.

“When we designed the battery two years ago, the regulations were different,” explains Okan Tur, technical lead at Williams and designer of the battery. “The power and energy values changed.”

Those changes were significant. Race mode power had been set at 133kW when the battery was designed but this was changed to 150kW shortly before the first season began. “Average power output per race lap was meant to be around 90kW,” Tur goes on. “At some races it was around 120kW. That’s a 30% increase.”

Energy was also an issue. The design brief called for a battery with 30kWh capacity, including regen, but once the sporting rules freed up the use of regen, energy levels began reaching 34kWh. “These numbers started putting pressure on the battery,” Tur explains.

Another factor was that the circuits turned out to be much longer than initially envisaged (see here for more details).

One symptom of using the battery so far outside its design envelope was the overheating issue that reared its head on numerous occasions in the first season. “It is not unexpected that the batteries will struggle with thermal load because of the changed spec,” says Tur. “It was more than double.”

Despite the demands for extra performance, Tur says the batteries held up very well in the first season. “The battery has not only matched its initial specification but it has exceeded it. The reliability of the battery was appreciated by everyone. Over 11 races with 40 cars, only stopped on track because of a battery fault. That was Daniel Abt in Putrajaya and it turned out to be a connector that hadn’t been fitted correctly. Just one fault in an entire season of racing.”

Reliability was something that Williams kept a very close eye on at every race, with a trackside team inspecting every unit. “The batteries derated from time to time, but not prematurely,” says Tur. “We could see the reasons and it was mainly to do with how teams were using the batteries and their strategies. Some teams got to grips with how to manage the batteries quite quickly but even front-running teams made mistakes.”

However, while the units behaved very well in the first season, Williams was quickly aware that operating so far outside the design specification would shorten battery life.

“As soon as the season had started, we ran tests based on the changed spec and realised that the batteries would not be performing at the same level in the second season,” says Tur. “We agreed with the teams and FEH to go through a refresh programme. We are pulling the batteries back and renewing critical components, such as cells. We will effectively be using the same batteries but with brand new cells.”

Other parts of the Williams-designed system are getting upgrades too, including the cooling system, which should help improve thermal management in season two. Further modifications or upgrades would have been difficult in the short timeframe available and with some of the components that have been used.

While Formula E’s technical roadmap calls for an increase in power output from 150kW race mode to 170kW in season two, demanding even more power from a battery already functioning well outside what it was intended to do could cause more issues. We understand that discussions remain ongoing as to how to unlock additional power without adversely affecting battery performance and that the 170kW figure is not yet set in stone.

Season two brings eight new manufacturers to the track and a whole new raft of new engineering challenges. “Testing started in April when the race season was not finished; based on those mileages we will refresh some of those batteries too before the season starts,” says Tur. “We now have eight new powertrains. Each of them have their own parameters. There were a number of things we had to calibrate and integrate. There are some differences between the solutions that affect the batteries, such as the initialisation sequence. ECU software is not standard either. We might see some systems issues at Donington Park, some minor glitches, but we’ve been testing with all of them for some time so I think we understand what we might see.”

Williams has already submitted plans for season three and beyond. Preseason testing starts on Monday at Donington Park.