A Battery-Capacitor Hybrid--for Hybrids
Engineers give lead-acid batteries a makeover by crossing them with ultracapacitors
Photo: Advanced Lead-Acid Battery Consortium/CSIRO
A hybrid-electric vehicle is put through its paces using a combination ultracapacitor/lead-acid battery.
Lead-acid batteries are relics that haven’t changed much since their invention nearly 150 years ago. Heavy and unable to withstand rapid charge-discharge cycles, they are unsuitable for the automotive world’s killer app, hybrid-electric vehicles. Hybrids instead use expensive nickel-metal hydride (NiMH) batteries or, experimentally, lithium batteries. But a new, souped-up version of lead-acid batteries could change that, cutting the cost of hybrids and also improving the function of power grids and a range of other applications.
The new design combines lead-acid chemistry with ultracapacitors, energy-storage devices that can quickly absorb and release a lot of charge, which they store along the roughened surface of their electrodes. Unlike ordinary lead-acid batteries, which are slowed by the movement of chemicals within them, these could provide quick bursts of power for acceleration and then recharge during braking, a must for hybrid-electric and electric vehicles. A hybrid’s rapid recharging cycles and high currents would destroy the lead electrodes in standard batteries, because lead sulfate would build up on them. The new batteries can go through at least four times as many charging cycles as lead-acid batteries, and, crucially, would cost about a quarter of NiMH batteries.
At least two lead-acid/ultracapacitor technologies are now poised for market release. Battery giant East Penn Manufacturing Co., in Lyon Station, Pa., licensed the technology for the UltraBattery in September from Furukawa Battery, in Yokohama, Japan, which has already begun manufacturing the devices. Researchers at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), who invented the UltraBattery, tested it early this year in a Honda Insight hybrid, which ran for 160 000 kilometers.
Meanwhile, Axion Power International, based in New Castle, Pa., has developed a slightly different design, which it will test for U.S. Marine Corps assault vehicles; the company got US $1.2 million from the Department of Homeland Security in October for the tests. A bank of 1000 of Axion’s batteries will also soon be tested as a utility-grid buffer in upstate New York. Axion CEO Thomas Granville says that the new technology ”lets us get into markets that have been in the past closed to lead-acid batteries.”
The new batteries’ advantage over standard leadâ''acid batteries comes from simple tweaks of the negative electrode. Instead of a lead plate, Axion makes the electrode from activated carbon, the highly porous, spongelike material used in ultracapacitor electrodes. When a regular battery discharges, the lead electrode reacts with sulfate ions, forming lead sulfate and creating protons and electrons. Axion’s activated carbon electrode directly releases and adsorbs protons from the sulfuric acid electrolyte during discharging and charging. The batteries recharge four times as fast as conventional ones, Granville says.
The UltraBattery is slightly different, says Lan Lam, project manager of the battery work at CSIRO. The negative electrode is split into two, one half made of lead and the other half of activated carbon. The two halves are connected in parallel so that their currents combine. This split-electrode design gives the battery the best of both technologies, according to Lam. While activated carbon provides quick energy bursts, it cannot store as much energy as the lead-acid chemistry. The combination gives the UltraBattery an energy capacity closer to that of a lead-acid battery than an ultracapacitor could get alone, Lam says.
Both designs have a big cost advantage. ”Nickel-metal hydride, depending on the application, is as much as $800 to $1200 per kilowatt-hour,” Granville says. ”Axion’s battery costs $200 per kilowatt-hour.”
These battery/ultracapacitor combinations will have to compete with lithium-ion batteries as the successor to NiMH for hybrid vehicles. Cost and safety, however, are still a concern for lithium. Lithium-ion batteries can overheat, ignite, and even explode if mistreated.
The lead-acid/ultracapacitor batteries have other advantages. They are easier to recycle than NiMH or lithium, according to East Penn. Lithium-ion batteries don’t have much usable metal, so they are usually incinerated, while the nickel from NiMH batteries is consumed in the steel industry. The military, meanwhile, is interested in Axion’s batteries, not so much for hybrids but because they work at temperatures as low as 50° C and weigh less than standard lead-acid batteries, Granville says.