Up to this point, we have examined the various types of lead-acid batteries used in truck, bus, and heavy equipment applications. Alternative battery technology refers to all categories of batteries that do not use a lead-acid principle. These alternative battery technologies have become increasingly important because of the increasing popularity of electric hybrid drive systems. Probably the most limiting factor to the growth of hybrid electric vehicles (HEVs) is the fact that current battery technology falls short of industry expectations of reliability and longevity. Nevertheless, it is becoming increasingly important for truck technicians working on pickup-and-delivery units and transit bus technicians to know something about the batteries used in electric hybrid drivetrains. Some of the first-generation commercial HEVs used banks of lead-acid batteries but these have proven to be costly. Today HEVs are usually spec’d with banks of one of the following battery categories.