Automotive > Powertrain, Body, Chassis & Safety Blog

The 500 Mile Battery: Miracle or Myth?

by Ian Riches | 2月 17, 2011


As electric vehicles attempt to move into the mainstream, as I identified in my recent Insight (Electric Vehicles: What Is Different This Time?) the one key limitation remaining for EVs is range.  Sixty to a hundred miles may be more than most people need on most days – but it is not enough for everyone, every day.  I wouldn't want to trust most current EVs to get me to Heathrow airport and back, and a weekend trip to visit my parents is well into the realms of the impossible.

IBM's research into lithium-air batteries - with the specific and stated aim of developing a 500 mile (800 km) range battery - is thus of huge potential significance.  

As IBM itself points out, this is "a very high risk/very high reward, long horizon project".  There's little or no chance of seeing a lithium-air powered EV before 2020 at the earliest. Although the technology looks promising, there are still real issues in recharging the batteries.  

I had the opportunity recently to speak with Dr. Winfried Wilcke, Senior Manager, Nanoscale Science & Technology and Program Director, Silicon Valley Projects at IBM's Almaden Research Center in California.  He was clearly enthusiastic about the research - but also realistic about its limitations.  The whole project, he told me, is about reducing oil dependence.  There is, he claims, enough electrical generating capacity in the USA for all light vehicles to be electric - assuming they are charged off-peak at night.  He shares my scepticism that sufficient infrastructure can be put in place to overcome EV range anxiety for the mass-market consumer, and thus the only answer to mass EV adoption is to radically extend the range of EVs.

The key attraction of lithium-air over lithium ion - at least in terms of making a 500 mile battery - is the energy density that the technology offers.  I've seen estimates that it is up to ten times better in terms of mass density (Wh/kg) and twice as good as lithium ion in terms of volume density (Wh/l).  Fitting a 500 mile battery into the 300 litre volume limit the car makers will allow for such a battery is not seen as an insurmountable problem.  The air handling system may need additional packaging space however: a lithium-air battery is as hungry for air as a gasoline engine is for comparable power outputs.

So far, so good.

However - one key limitation of lithium-air is likely to remain.  It is inherently slow to charge and discharge.  Peak energy flows of the order of 30 kW were mentioned. This means that to have competitive acceleration, a smaller lithium ion battery and/or ultra-capacitor will need to be used.  This will enable much higher peak power outputs/inputs for acceleration and regenerative braking, in much the same way as high-speed cache RAM enhances your computer's performance.  It will also, of course, make the final system a bit more expensive.

The relatively low peak power output will also mean that high-speed sustained autobahn-style (> 90 mph / 145 km/h) cruising will not be possible.  Sorry to any Germans reading this!

The limited power output of lithium-air is not to my mind the biggest issue, however. It's the limited charge rates possible.  A 500 mile battery may well prove achievable using lithium-air.  What looks unlikely, however, is a 500 mile battery that you can use to drive 500 miles, recharge easily overnight, and then drive another 500 miles the next morning.  It can do it once, but asking for a repeat performance in short measure is likely to leave you disappointed.  Even with a fast charger, an eight hour full recharge is still a target, not a certainty: it’s not easy to put energy into a lithium-air battery that quickly.  

On a domestic 4kW supply, recovering the full 500 miles of range could take in the region of 30 hours or more.  That’s not a limitation of the lithium air battery – just a simple matter of mathematics and limited domestic supplies.  Electric vehicles face a huge challenge when compared to gasoline vehicles in this regard.  A level III 50kW fast charge may sound impressive – but whenever you fill your gas tank the equivalent energy flow is in the order of 6 MW, more than 100 times greater.  

As Dr. Wilcke admitted, even if the technology works well, the lithium-air powered EV will not be the vehicle for your 2000 mile vacation road trip.

All sorts of questions fire off on this realisation.  Will consumers accept such a limitation?  And, perhaps more fundamentally, will car makers ever manufacture such a vehicle? Given that even the 500 mile EV still cannot match what a conventional gasoline or diesel car can do, is there any point?

Lithium-air appears to offer a bright promise for smaller, lighter batteries that store more energy.  Whether they'll ever be used to make a 500 mile EV is another question altogether.  I can't help wondering if they'll end up powering EVs with, say, a 200 mile range that can be recovered on a domestic outlet in 12 hours or so overnight.  That would make sense to me, and would broaden the EV’s appeal considerably.  

For those who need a family vehicle to do more than 200 miles or so in one go, my crystal ball says that range extenders and hybrids are the future.

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