Worse, some early data appears to show that the beefier lead-acid starter batteries that enable the system to restart the engine dozens of times a day may need to be replaced more frequently.
That has led some analysts and electronics companies to suggest that start-stop systems will soon include not only the advanced-glass mat (AGM) batteries but also a small ultracapacitor to ease the power draw on those batteries.
In heavy traffic or frequent use, such systems may be required to restart the engine several dozen times a day--increasing the battery's duty cycle substantially.
A recent article in trade journal Ward's Auto looks at the potential for ultracapacitor use in future. It notes that the electronic devices are still expensive, but that costs are coming down.
At the moment, only a few vehicles include ultracapacitors--a dozen or so models sold in Europe by PSA Peugeot Citroen, with sales now totaling more than 1 million units.
In the U.S., ultracapacitors are used on the Lamborghini Aventador, not a car most U.S. drivers will ever have a chance to drive, and two Mazda models.
A slightly different approach is used for Mazda's start-stop system, known as i-ELOOP. It stops the engine at the precise point in its rotation that it can be restarted simply by firing the correct spark plug.
In that case, the ultracapacitor is used to store energy that runs the accessories--climate control, lights, audio, and other systems--while the engine is off.