Collaborative investigation on charging electic-vehicle battery systems for night-time load levelling by Japanese electric power companies

Abstract

In 1995, ten Japanese electric power companies and CRIEPI started a three-year collaborative investigation of battery systems for electric vehicles (EVs). In the first year, the charging procedure for valve-regulated lead/acid (VRLA) batteries connected in series in EVs has been evaluated for both night-time load levelling and prolonging the cycle life. An EV battery system with VRLA batteries can be charged by both constant current and constant current (CC-CC) and constant current and constant voltage (CC-CV) in less than 8 h under night-time conditions. The charging method with CC-CC prolongs the cycle life further than that with CC-CV. Excess charging capacity on the part of CV in CC-CV charging degrades the positive electrode through softening of the active material. A higher rate of the first current in CC-CC charging prolongs the cycle life by suppressing the softening of the positive electrode due to active-material particle growth. On the other hand, control of the uniform environment in battery boxes during the charging with CC-CC and CC-CV is very important in order to prolong the cycle life. This is because cooled batteries tend to have insufficient charge capacity due to their greater energy consumption for the electrolysis of water than uncooled ones.