All cars with a combustion engine are equipped with a rechargeable battery that is tasked with providing power to start the vehicle and provide electricity for vehicle lighting and ignition. About the size of a large shoe box, the familiar battery is usually referred to as an "SLI battery" (meaning starting, lighting and ignition). The typical SLI battery is a lead-acid type. Their construction makes them very heavy, prone to corrosion, and they often have a relatively short service life (not surprising, as they were invented in 1859!). While they do a fair job starting tens of millions of cars each morning, they are simply too heavy and inefficient for a larger role in an automobile.

EV Batteries
Electric vehicles (EV) and hybrid-electric vehicles (HEV) are both designed to operate under full or supplemented battery power. The battery packs must be much larger than a standard car battery, as they provide energy in lieu of the power developed by an internal combustion engine. While these batteries operate in much the same manner as the SLI battery, the electrical and service demands on a battery designed for EV and HEV is much more complicated.

Batteries designed for EV and HEV vehicle propulsion are generally described as being in a package, or battery pack. Without overly complicating things, it is easiest to think of a "battery pack" as many small batteries working together to make a large amount of energy-batteries are much more effective when they are arranged in this manner.

Some of these battery packs contain hundreds of smaller batteries, or "cells" (the Toyota Prius uses 168 individual battery cells and the Toyota Highlander Hybrid uses 240 cells, while the Tesla EV uses 6,831 cells!). As a result, they are generally located in the trunk or center of the vehicle where they minimally impede passenger and cargo space.