TOKYO -- Suppliers and carmakers are trying out competing energy-storage systems to power the fuel-saving engine stop-start systems that are increasingly common on vehicles.
In Japan, automakers are working with lead-acid, lithium ion and nickel-metal hydride batteries, as well as capacitors.
Each has pluses and minuses. The industry is still juggling cost against safety and performance. But the technology is already delivering better fuel economy.
Toyota Motor Corp., Honda Motor Co., Mitsubishi Motors Corp., Mazda Motor Corp. and Suzuki Motor Corp. are introducing vehicles that shut down the engine when the car comes to a stop. The move saves gasoline but requires beefed up batteries to run the radio, navigation, air conditioning and other electronics while the engine is disengaged.
When the driver lifts up on the brake pedal, the engine fires up again and is ready to go.
The technology is popular in hybrid vehicles, which already have big batteries to store power and have regenerative braking systems to recharge those batteries when the car slows down. But the stop-start systems are migrating to traditional gasoline-powered cars.
Those cars typically need more electricity storage than the standard 12-volt battery.
Here's a look at some of the storage technologies Japan-ese automakers and suppliers are trying.
Capacitors. Mazda is among the first to equip its cars with capacitors to store energy for use when the engine shuts down. The technology debuted as a standard feature, called i-Eloop, on the redesigned Mazda6 and will be deployed in other models.
It improves fuel economy by 5 to 10 percent, Mazda says.
Mazda's capacitors are supplied by Nihon Chemicon Corp. The carmaker chose capacitors over standard batteries because they can charge and discharge more quickly, says Tatsuro Takahashi, a vehicle system development engineer at Mazda.
The capacitors can fully charge in 10 seconds, absorbing a higher percentage of the energy generated as the car decelerates. Batteries, by contrast, take longer to charge and have a shorter lifespan of charge-discharge cycles.
Also, capacitors are not as sensitive to heat as lithium ion batteries can be, which limits options for positioning batteries in the engine compartment. In the Mazda6, the capacitor is located below the headlamp.
The downside: Capacitors have limited capacity. Indeed, they can't rival lithium ion batteries in such tasks as powering the wheels to move a car under electric power.