Recharged
Mar 5th 2009
From The Economist print edition
Energy: There is life in a 150-year-old battery technology yet, it seems, as
venerable lead-acid batteries are given a modern overhaul
MCC
The Tesla of its day ran on lead-acid batteries
LEAD-ACID batteries seem to have been around for ever. They were invented in
1859 by Gaston Planté, a French physicist, and have done sterling work over the
decades starting car engines and powering slow-moving vehicles such as fork-lift
trucks and milk floats. Compared with the newer energy technologies that are now
sweeping the world, however, they look old-fashioned and a bit frumpy. These days
the catwalk is crowded with nickel-metal hydride and lithium-ion batteries, showing
off their ability to pack lots of energy into a small space and deliver a steady current
over a long period. The fact that these modern batteries are also lighter (lead is,
after all, one of the densest elements in the periodic table) has made them the
obvious choices for powering truly serious electric vehicles, as opposed to the ones
that potter about in warehouses and on suburban streets.
But it is a mistake to dismiss something just because it is old. Another way of looking
at things is that lead-acid batteries are tried and trusted; they may just need a bit of
pepping up. Axion Power, a firm based near Pittsburgh, Pennsylvania, has found that
the ideal tonic is carbon.
A conventional lead-acid battery is made up of a series of cells, each of which
contains a positive electrode made of lead dioxide and a negative electrode of
metallic lead. These are immersed in an electrolyte of dilute sulphuric acid. Car
batteries tend to have thin electrode plates, which allows a lot of energy to be
discharged quickly, but only for a short period of time. That is fine for turning a
starter motor, but it is not so good for powering an electric motor intended to move
a car any distance. Moreover, lead-acid batteries can be ruined if they are discharged
completely, as many motorists have discovered to their cost when trying to start
their cars on icy mornings. Lead-acid batteries with thicker electrodes can tolerate
such “deep” discharges better than those with thin ones, but only at the expense of
making a heavy battery even heavier.
In Axion’s new battery the negative electrode is replaced with one made from
activated carbon, a material used in supercapacitors. Normal capacitors—those that
power the flashguns in cameras for instance—can be charged and discharged rapidly,
but cannot store much energy. Supercapacitors are meatier versions that are able to
hold a reasonable amount of energy, and can take it in and release it quickly. Some,
indeed, are already used in tandem with the lithium-ion batteries in electric cars to
boost acceleration and recapture energy during so-called “regenerative” braking.
Axion’s plan, therefore, is to have the best of both worlds by building a hybrid
battery that is based on lead-acid/carbon (PbC) chemistry.
The carbon, which is protected within a sandwich of other materials, is more
effective than metallic lead at exchanging protons with the acid during charging and
discharging. In tests, Axion says, its PbCs withstood more than 1,600 charges and
deep discharges before failure, which is three times better than standard lead-acid
batteries that are specifically designed for such deep cycles.
True, the new batteries are still heavy compared with lithium-ion ones. “But not
everyone needs or can afford an electric car that accelerates like a Tesla,” says Ed
Buiel, Axion’s chief technical officer, referring to the fastest electric car yet to be put
into production, which uses a huge number of lithium-ion cells. And for those who do
not require Tesla-like performance, this makes sense. PbC batteries are durable and
cheap to make because, according to Dr Buiel, they can be produced on existing
lead-acid production lines. A Tesla costs $109,000. Axion, by contrast, has converted
a pickup truck to run on a pack of its batteries for around $8,000. (It has a range of
70km, or about 45 miles.) The company is also working with a number of other small
engineering firms to convert other sorts of vehicles.
Nor is transport the only application. In another project, called Power Cube, Axion is
putting banks of its batteries into a shipping container, which can then be used as a
mobile energy-storage system that can supply up to 1MW of power for 30 minutes,
or 100KW for ten hours. Power Cubes could help deliver power to local electricity
grids that might otherwise suffer “brownouts” as a consequence of demand
temporarily exceeding supply. They might also provide an answer to the problem of
matching the supply of solar and wind energy to the demand for electricity, by
storing electricity during the day, or during particularly windy periods, so that it is
available at night, or during calm periods when the wind drops.
Other companies are also looking at ways to give the lead-acid battery a new lease
of life. Furukawa Battery, a Japanese firm, has developed the UltraBattery. This, too,
combines the benefits of a lead-acid battery with those of a supercapacitor, but in a
different way: it splits the negative electrode in two, with one part made of lead and
the other of carbon. In September 2008 East Penn Manufacturing, a big American
battery-maker, struck an agreement with Furukawa to put the UltraBattery into
production.
Meanwhile, America’s marines are testing a special version of Axion’s battery in
some of their assault vehicles. If they can satisfy such demanding users, lead-acid
batteries with a dash of carbon could be ready to lead a new charge.