Hybrid electric vehicles of the power assist design have been on the market for 13 years now. Their reliability and functionality have attracted a high level of consumer satisfaction. Their only disadvantage is their cost compared to vehicles with an internal combustion engine (ICE)—the difference between a midsize hybrid and an ICE vehicle at volumes of at least 100,000 units per year is estimated at $4000–$5000. About 35% of the differential is attributed to the 1.3–2 kW-hr hybrid battery.
The technical and commercial success of what are now being called “conventional” hybrids has contributed to the premature promotion of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs). PHEVs employ a larger battery that is used extensively and is designed to be charged from the electric grid. There are two main PHEV types: the parallel configuration, which has the electric motor and the ICE in parallel and power is extracted from either according to vehicle requirements and battery state of charge, and the series configuration (also called “extended range”), in which the electrical motor powers the vehicle and the ICE generates electricity when the battery state of charge can’t meet the vehicle’s power demand.
The parallel PHEV configuration is an extension of the conventional HEV configuration, the battery size and external charger being the main differences. The merit of this configuration compared to the conventional hybrid largely depends on the battery parameters—predominantly cost, reliability, and durability. A series PHEV has the advantage of being able to support extended electric-only drive, but its need for dual powertrains and for a motor and battery with higher power is a substantial disadvantage, making packaging and cost considerably more challenging.
The battery is responsible for 30–75% of the weight, volume, and cost increase associated with the various configurations. Even more critical are battery life, reliability, and behavior under abuse as they present the largest threat to the commercial success of electrified vehicles.
Currently, over 96% of hybrids with moderate to significant powertrain hybridization employ a NiMH battery. Such batteries are a reliable power source for hybrid cars but with limited potential for enhancement.
Lithium-ion batteries offer more power and energy per unit of weight and volume, and better charge efficiency. These attributes have allowed them to capture a major part of the portable rechargeable battery market, which requires a battery life of only 2–3 years. The reliability of the Li-Ion technology for automotive applications, however, is not entirely proven. Unfriendly failure modes, for example, are a concern. Also, battery life for EVs and PHEVs is uncertain and is likely to be shorter than the 10–15-year requirement.
Safety is challenging, although it can ultimately be engineered into the battery pack. The best solutions—and related sacrifices in cost and performance—have yet to be determined. Large-scale deployment of unproven technology in the meantime would probably delay rather than accelerate the use of Li-Ion batteries in automotive applications.
The current cost of a typical 24-kW-hr EV battery, on the order of $16k–$20k, is expected to fall with increased production and technological evolution. Yet it is expected to remain on the order of $12k–$15k through 2015 and remain over $10k through 2020 and probably beyond. Those values are too high to support mass commercialization. Unfortunately, since safety, reliability, and durability are more critical than cost, and energy density is nearly as critical (due to its impact on vehicle range), and all are challenging, there is little room for cost-reducing innovation if it negatively impacts any of the other four parameters.
For a mid-size PHEV with 20–25 miles (32–40 km) of all-electric range, an 8–10 kW-hr battery is required. This is due to the fact that to guarantee sufficient power and life, only 50–65% of the PHEV battery can be used effectively. Such a PHEV battery is expected to cost $7k–$10k in 2015 and, depending on battery design and duty cycle, to have a life expectancy of 6–10 years. This implies one or more battery replacement during the life of the car. Unfortunately, high cost and insufficient durability compound each other, making the cost of replacing the battery prohibitive.
The current government-initiated trend to promote PHEVs and EVs may result in the establishment of some niche markets for these vehicles, but as market forces win it is likely that the industry will refocus on the more realistic HEV solution with the battery choice migrating from NiMH to Li-Ion over the next 10 years.
Written by Menahem Anderman