While electric and hybrid-electric vehicles (EVs and HEVs) will play significant roles in this emerging mix, start-stop hybrids (also known as micro-hybrids) are projected to claim over 40% of the market in the next five years. In a start-stop hybrid, energy from the battery is not used to propel the vehicle. Instead, the start-stop technology saves energy by shutting off the engine while the vehicle is at rest, automatically restarting it when the driver steps on the gas pedal.
Start-stop is the simplest kind of hybrid technology and is expected to become the most common because it is the lowest-cost hybrid alternative. At a cost as low as USD300 per system, it is one of the most cost-effective means of improving efficiency for vehicles using in internal-combustion engine. Current micro-hybrid technology can improve fuel economy by 5-10%, and future systems may achieve savings as great as 15%. If widely adopted, it could substantially reduce fuel consumption and air pollution from idling vehicles.
To date, the main barrier to the widespread adoption of micro-hybrids is the limited suitability of current battery technologies. Lead-acid batteries are currently the most common choice for micro-hybrids because of their low cost, but they have critical performance limitations. Studies have shown that even the most advanced lead-acid batteries may take up to 10 min to recover from a single start-stop event, meaning that the battery’s ability to recharge after a stop declines rapidly from first use. With such a handicap, the battery is unable to support the next stop event, resulting in a corresponding decrease in fuel economy.
In addition, micro-hybrids typically use oversized starter motors to accomplish quicker engine crank-ups, creating electrical loads that lead-acid batteries are unable to support. The batteries begin to deteriorate within weeks, rendering them unable to support minimum system voltage during engine start. To compensate, automakers have been forced to develop workarounds that further diminish the cost and fuel-economy advantages that lead-acid batteries might offer. In some modifications, the start-stop function can be momentarily disengaged, although that decreases fuel economy. In others, supplementary systems are added to compensate for voltage depression, increasing system cost and complexity.
Lithium-ion (Li-ion), another alternative vehicle battery technology, also has limitations. Although Li-ion batteries offer high performance, they are also costly due to complex manufacturing processes. Li-ion batteries also suffer from safety problems: thermal issues have on occasion caused them to combust spontaneously.
