Nixon on ‘Electric Dreams’ and Moore’s Law for Batteries
There is no Moore’s law for batteries – so says Dr. Fred Schlachter of the American Physical Society, Washington, DC and he’s right.
If I’d read Fred’s paper before I started looking closely at battery technology I might have never discovered that there actually is a Moore’s law at work here.
Hold on, I hear you say, you can’t both be right.
Bear with me…
I was writing a book on the future of transport fuels  and looking at how batteries and conventional fuels compare. Surprisingly this was not an easy question to answer with a standard Google search. Questions for which there is no immediate answer invite my curiosity, this indicates there is something interesting to be discovered.
I had to go deep into the literature to uncover reliable data on battery energy density trends over the past 100 years . When I plotted a graph of energy density over time, an exponential line rather like Moore’s law appeared.
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It seems that the amount of energy a battery can store is doubling every 22 years. The big rises in performance are driven by a change in the underlying technology, starting with lead-acid batteries 100 years ago to the current high performing Lithium ion batteries we use today.
So how can we both be right? It turns out that Fred and I are looking at battery technology over different timescales. Looking at a short timescale produces a view within one technology, which tends to be linear. Looking at a longer timescale shows the performance leaps with different technologies that create the exponential rise.
Both of us have come to the same conclusion though. However you look at battery technology, it will be some time yet before we see the small containers of energy that will power the electric cars of the future for the same distance as conventional fuels such as gasoline and diesel.
“So what?” I hear you say
I do admire electric cars; companies such as Tesla make extraordinarily desirable machines. There is even a world motorsport championship called Formula-e to race state of the art machines. And yet all electric cars have an Achilles heel. They can only go so far before they need a recharge, it is called range anxiety. This is why the rules of Formula-e state that the drivers can change cars during the race.
Engineers have worked hard to address range anxiety and can now claim 300 – 500 km on one battery charge. Impressive.
Yet when I look at a European mid range diesel car, one tank of fuel could take me over 1200 km before I need to refuel. Cars need to carry their fuel around with them so the amount of power you can pack into a given volume is important. A measure of this is called the energy density of the fuel, the bigger the number the further you can travel before refuelling.
As we have seen, the best battery technology is based on Lithium-ion designs. These have an energy density of approximately 0.8MJ/kg. This compares with an energy density for diesel of 46.2MJ/kg. This context makes the battery development trend less impressive now.
Graphene is currently in the news as a new wonder material for battery and super-capacitor energy storage technology. Graphene may well prove to be a leap forward. However bear in mind the scale of the challenge: Even though battery energy density is doubling every 22 years – at this rate of development it will take over 100 years to reach the equivalent energy density of diesel.
Much as I admire the developments in electric cars, dear reader, be mindful that they will have to carry rather large and heavy battery packs around for some time to come. The next time you hear claims of dramatic improvements in battery technology, just remember that conventional fuels will take some beating because Moore’s law for batteries operates on a much longer timescale than computers.
 Nixon, Adrian. Diesal? and other future transport fuels. 1st ed. Halifax, UK: Nixor Limited, 2013.
 Chen-Xi Zu and Hong Li. (2011) Thermodynamic analysis on energy densities of batteries. doi: 10.1039/C0EE00777C
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>