Study Release, Summary of Findings – Feb 26, 2009

More is Not Always Better for Plug-in Vehicle Batteries

            PITTSBURGHCarnegie Mellon University professor Jeremy J. Michalek and researchers Dr. Constantine Samaras and C.-S. Norman Shiau report in a new study that plug-in hybrid electric vehicles with small battery packs may be the best bet for saving drivers money while addressing U.S. dependency on foreign oil and global warming.

            In an article to appear in the journal Energy Policy, the authors find that urban drivers who can charge their vehicles frequently (every 20 miles or less) can simultaneously reduce petroleum consumption, greenhouse gas emissions, and expenses with a plug-in hybrid vehicle whose battery pack is sized for about 7 miles of electric travel per charge. In contrast, plug-in hybrid vehicles with large battery packs – sized for 40 or more miles of electric travel – are too expensive for fuel savings to compensate, even in optimistic scenarios.

            Plug-in hybrid vehicles use charged batteries to propel the vehicle partly using electricity instead of gasoline, which gives them potential to reduce petroleum consumption and greenhouse gas emissions. “Larger battery packs allow drivers to go longer distances on electric power. But batteries are heavy and expensive,” says Michalek. “We accounted for the effects of additional batteries on vehicle cost, weight and efficiency in order to understand the net implications on petroleum consumption, cost, and greenhouse gas emissions. Over a range of scenarios -- including fluctuating gas prices, new battery technologies or high taxes on carbon dioxide emissions -- plug-ins with small battery packs are economically competitive with ordinary hybrid and conventional vehicles for drivers who charge frequently.”

            The Obama administration has set a target of putting 1 million plug-in hybrid cars on the road by 2015, and the American Recovery and Reinvestment Act of 2009 includes a range of incentives for plug-in vehicles as well as funds for research. “Plug-in vehicles can reduce greenhouse gas emissions in general, but getting the most bang for the taxpayer buck means targeting the right vehicles to the right drivers,” says Michalek. “In fact, for drivers who charge their vehicles frequently, plug-in vehicles with small battery packs create fewer greenhouse gas emissions than plug-ins with large battery packs because carrying extra weight makes vehicles less efficient. Plug-ins with large battery packs can still reduce greenhouse gas emissions for drivers who charge less frequently as well as help shift air pollution away from population centers, but they won’t save drivers money unless batteries get very cheap.”

            The study, which was funded by the National Science Foundation and the Teresa Heinz Scholars for Environmental Research Program, points out that targeting drivers with the potential to charge frequently would not limit plug-ins to a boutique market: nearly 50% of U.S. passenger vehicle miles are traveled by vehicles driving less than 20 miles per day. “If gas prices go up or battery prices come down, plug-in vehicles will be more competitive across the board, but the small battery packs remain best on cost, and new charging infrastructure could increase the number of drivers who can benefit.”



Shiau, C.-S., C. Samaras, R. Hauffe and J.J. Michalek (2009) “Impact of battery weight and charging patterns on the economic and environmental benefits of plug-in hybrid vehicles,” to appear, Energy Policy.

Last Updated: February 09