HEV applications require a very unique performance.  The power density (Charging power and discharging power – or ability to charge and discharge the battery quickly) is much more important than energy density (storing a lot of energy) in HEV applications. In engine power assist, starting engine mode and acceleration, power assist while engine is shut down, etc. the battery will be discharged. In addition, HEV has a regenerative power brake system and that allows the kinetic energy from braking the car to charge the power to the batteries. (Links to Animation). As a result, 100% DOD (Depth of Discharge) cycle, complete discharge of the battery will not be used for HEV applications. A high number of pulse charging & discharging high power cycles are required. The battery technology for HEV application is therefore quite different from those of most of other applications like cell phone, power tool or PC applications, where the operation run time is the most important. For those applications, the competition of having higher energy density is the main race in each company. Table 1

For HEV applications the battery is typically used in the range where it is 20% to 80% full (DOD 20% to 80% range). When DOD level is at the higher end of the range, a higher charging capability, but lower charging capability will be achieved. On the other hand, when DOD level is in the lower end of the range, a higher charging capability, but lower discharging capability is achieved. Chart 1

The most optimal area of HEV use is usually around the 50% DOD level, so the power density number that is most interesting, both from an operational level and for comparison purposes is at the 50% DOD level. For HEV applications, power density at 100% DOD has little relevance as the battery will likely never be operating at that state of charge.