Electric engineering challenges: battery charging

It’s a temptation many publications simply can’t resist: using some variant of “charging” to headline any article about electric cars. (And yes – we’re guilty, too.) But the easy turn of phrase hides some very serious technical difficulties with battery recharging for the new Formula E racing series.

Let’s not get overly bogged down by the chemistry here. The essentials are this: batteries deplete with use. In the case of Formula E racing, the batteries are expected to last just 25 minutes with today’s technology. Therefore, in an hour-long race, each driver will use car A for the first 25 minutes, swap into car B for 25 minutes while car A is recharged, and then swap back into car A for a final 10 minute sprint.

If that sounds less than ideal and a bit of a faff, it’s because it is. The promoter, FEH, and the FIA believe that Formula E will give engineers a platform to push battery innovation further and faster than mainstream car makers are doing, which commonly happens in racing. It is hoped that the speed of development will be so swift that the clumsy car-swap format will disappear relatively quickly.

In the meantime, there’s not much alternative. Battery swapping isn’t permitted, mostly for safety reasons. The batteries need to be sealed to prevent any risk of electrocution or fire, whether in pit stops or in crashes. That’s not too dissimilar to modern Formula 1, which bans in-race refuelling to keep drivers and pit crews safe.

Developing the batteries and then working out how to recharging spent batteries during the inaugural 2014 season is going to be quite a challenge in itself, one with which Williams Advanced Engineering is tasked. That’s the sister organisation of the famous F1 team, set up to find commercial applications for technology developed for racing.

The discharge challenge

So what are engineers up against? ProEV’s Cliff Rassweiler – racing driver, analyst and designer – explains it well in a recent technical article. “The Formula E prototype uses SAFT lithium ion VL41m cells, which are fairly representative of the technology available. SAFT specifies that the cells are capable of a 3C continuous discharge rate. That means they can be fully discharged in one third of an hour. That’s right at the limit for a 20 minute race.”

Batteries are restricted in their discharge rate due to heat. Draw too much juice from the battery and it will overheat. Overheating is not helpful – the batteries can fail, catch fire or even explode.

“The prototype’s battery pack consists of 200 tightly packed cells, which will all generate heat as they discharge,” Rassweiler goes on. “To overcome the heat, the pack will need to be actively cooled. Battery pack cooling is a challenging part of the electric race car design.”

That’s going to require close collaboration then between Williams, responsible for the batteries, and Dallara, which is designing the chassis. If the radiator intakes are too small, for example, or the aero doesn’t supply those intakes with sufficient cool air, or the race track temperature is too hot, those batteries are going to get very warm. This is a similar problem for F1 designers, who have to keep cars in an optimum operating window, so it is by no means insurmountable.

The recharge challenge

Far more difficult is how to recharge exhausted batteries after the first stint of the race.

“SAFT specifies that the maximum continuous recharge rate is 1C,” says Rassweiler. “That means the batteries can be charged at a rate that will recharge them in an hour (even though they cannot actually be fully charged in an hour).”

Formula E racers will need the batteries to be charged to around 50% capacity in 20 minutes. That’s a big ask. For a start, a unit that can meet that requirement doesn’t yet exist.

Electric car maker Tesla is currently rolling out a portfolio of “superchargers” in the US and Europe. These are effectively electric filling points, powered by 120kW chargers embedded in permanent infrastructure and plumbed into the grid. Each supercharger is estimated to cost around $175,000 and can accommodate up to 10 cars.  

Such a beast would be able to fill Formula E batteries to around 50% charge in less than 30 minutes – with some provisos. Analysing the technology, Rassweiler explains to Current E: “The sweet spot for charging is between 20% and 80% charge. When the battery charge is in that range, it will take as fast a charge as you can give it, as long as you manage the heat. When the battery gets to 80%, it gets harder to charge.”

Rassweiler goes on: “So if the car is not run down to zero charge and if there is a very effective battery cooling system, slamming in a quick charge might do the job. That’s a lot of ‘ifs’.”

And the Tesla system is not an option – it uses a proprietary plug and is not removable after the race.

What about an even bigger charger? The Silex hypercharger, claimed to offer 1.5mW recharge capacity, is aiming to make it to market next year. Even by the most optimistic standards, that’s unlikely. And even if such a charger was made available, today’s batteries simply couldn’t handle that charge rate. The charge rate is limited by the physical and chemical composition of the battery: using such a charger would be like trying to fill a drinking glass from a fire engine hose.

The infrastructure challenge

Then there’s the issue of power draw. “When someone develops and builds a suitable fast charger, where will the teams plug it in?” asks Rassweiler. If Formula E attracts a full field of 20 cars, there will be a sudden load of 2,400kW. That’s 2.4MW, roughly equivalent to adding 4,500 UK households to the grid.

And that’s not just once. Teams will need to recharge cars between practice and qualifying, between qualifying and the race, and during the race. “The local power utility will need to add this capacity for use one day a year or risk the race blacking out the city,” explains Rassweiler. That includes generating the power to meet peak demand and installing sufficient infrastructure to transmit and distribute the power. “This will need to be built and tested before the race, at every race destination.” None of that comes cheap.

What’s more, recharging will take place during the day, when demand is already high. Shutting down a city’s power grid as well as its transport network for each street circuit is not a fast-track route to endearing the new series to local residents.

The elephant in the room smells of diesel

There is another way to produce local electricity without stressing the grid: diesel generators. While using on-site fossil-fuelled powerplants flies in the face of the championship’s environmental and noise aims, it might be the only realistic short-term solution that can provide enough capacity to get the batteries topped up.   

There’s a bright side though. Rassweiler says: “One third of the way into the race, when 20 generators lined up in the pit lane go to full power, the earth will shake. And nobody will miss the roar of the internal combustion engine.”

One thing is certain: Formula E engineers are going to have to be every bit as resourceful as their Formula 1 counterparts – if not more so – to assure technical success in the new series.