Denver Switches to NiMH Batteries
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THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS |
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ONE OF THIRTY-SIX hybrid buses operated by the Denver Regional Transportation District. All are
being fitted with nickel metal hydride batteries. |
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THESE 288-VOLT NiMH batteries, manufactured by Cobasys, come with a 3-year warranty. |
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日本語
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Less maintenance and longer life than lead acid batteries By Carroll McCormick
Nickel Magazine, November 2005 -- Switching from lead acid batteries to nickel metal hydride (NiMH) batteries in hybrid electric/compressed natural gas (CNG) buses will mean less maintenance and longer life between battery replacements for the Denver Regional Transportation District (RTD) in Colorado, USA.
RTD bought 36 hybrid buses between 1998 and 2000. Last year Cobasys, a manufacturer of NiMH battery systems in Orion, Michigan, installed its 288-volt, 120-kilowatt systems in two RTD buses. Ten more buses will be converted this year, and another 10 in 2006.
When the bus accelerates, the wheels receive power from the batteries as well as from an electrical generator driven by a 2.5-litre CNG engine operating at 2,100 revolutions per minute. Otherwise the engine runs the generator that charges the batteries.
The NiMH batteries offer several advantages to the maintenance team at RTD, says Lou Ha, the bus company’s head of engineering: "We’ve been running the two trial buses almost two years and so far have done little maintenance on the system. The batteries have a two-to-three-year manufacturer’s warranty, and based on related applications, we estimate an operating life of five years." (By comparison, the lead acid batteries have to be replaced about every six months.)
Maintenance on lead acid battery systems is intensive and time-consuming. "The 26 batteries in the system have to be in really good balance with each other," says Ha. "They require monthly maintenance, and the life-cycle cost is high." What’s more, the batteries have to be recharged on a cyclical basis, as they can go into failure mode rapidly if the difference in the voltage between them exceeds 0.2 volts.
Another advantage of the NiMH batteries is that they have battery and thermal management systems. It was not feasible to retrofit either of these to the lead acid batteries therefore this battery replacement program solved the problem.
NiMH batteries have more than twice the energy and power density of lead acid batteries. Also, they can be cycled in shallow discharges hundreds of thousands of times and last more than 320,000 kilometres under a partial charge/discharge regime, and so are well-suited to hybrid electric vehicles. They also weigh less and take up less space: each NiMH battery system being retrofitted in the RTD buses weighs about 227 kilograms less than the lead acid system it replaces.
In NiMH batteries the positive electrode is made of nickel-oxyhydroxide and proprietary elements. The negative electrode is a nickel alloy containing lanthanum or certain transition metals. The electrolyte is normally aqueous potassium hydroxide. Hydrogen is absorbed at the negative electrode to form the nickel metal hydride and generate electricity.
Carroll McCormick is a Montreal-based freelance writer.
PHOTOS: Denver Regional Transportation District and Cobasys
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Scott Reed Ray Wagner |
