Nickel metal hydride (NiMH) battery systems are emerging as a competitive technology in the power electronics and tool market and for electric and hybrid vehicle applications. The ability to rapid charging the NiMH batteries can greatly enhance their market acceptance and penetration. We have demonstrated the possibility of rapid charging a commercial NiMH traction battery system at high rates. This paper will discuss the mechanism that limits the rapid charging capability of the system. We will further discuss how to control the charging process.