Lithium ion batteries are truly fair weather friends – just like people, they fare best in a comfortable climate. Lately, we at Pike Research have been delving deep into how environmental factors, such as temperature, affect battery performance and the rates at which vehicles are charged or discharged. Our discussions with automotive companies and battery pack assembly companies have revealed numerous approaches for optimizing performance and extending a battery’s life – comparable to the many ways people dress to beat extreme heat.
According to our research, lithium ion batteries perform optimally, and will last longer, if they are kept at temperatures between -10°C and +30°C. This range is consistent with findings by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE).
In very cold temperatures, batteries don’t achieve their full rated power until the battery cells warm up. According to Ford engineers V. Anand Sankaran and Bob Taenaka, this short-term effect has greater implications for battery electric vehicles (BEVs) than for plug-in hybrids (PHEVs). A PHEV can rely on its gas engine for power during warm up, but BEVs don’t have that other power source.
As the accompanying EERE graphic shows, batteries exposed to hotter average temperatures lose their ability to store energy; the hotter the temperature the faster they lose their storing ability. So BEV owners in Phoenix will likely be looking to replace their batteries faster than owners living where the thermometer doesn’t often reach 110°F.
To combat the extreme temperature effect and keep batteries within their optimal temperature range, automakers use thermal management systems relying on either air or liquid cooling. As the EERE data shows, liquid cooling is generally more likely to preserve a battery’s capacity than air cooling, though performance variations will occur depending on how well a battery management system was designed to control temperature. According to Ford, the liquids used in cooling systems can retain a temperature for a long time, which contributed to Ford’s decision to use liquid cooling on the Ford Focus EV. Ford has also used air cooling on its hybrid Escape and Fusion, as have Nissan and other BEV manufacturers on their vehicles.
In addition to external heat potentially shortening the usable life of a battery, operating batteries at high charge and discharge rates can have another negative impact. That is particularly true for fast DC charging a battery pack at a rate of 50 kW for as little as 30 minutes (the expected time to charge a BEV 80%). If done every day, that would generate enough heat to reduce the battery’s capacity. BEVs that offer fast charging were designed with this fact in mind, so their battery management systems can force an EV charging system to slow down, thus protecting the batteries well before the pack is fully charged.
The interaction of batteries and fast charging is one of the many EV topics that Pike Research will explore at the Plug-In 2012 conference, the premier North American EV industry event, on July 23, 2012, in San Antonio. I’ll be representing Pike Research at the conference where Ford and many of the leading companies will be discussing business models, technology challenges, and EV rollout strategies.