Thermal Evaluation of Toyota Prius Battery Pack

The Toyota Prius is a five-passenger compact sedan powered by a 52 kW gasoline engine and a 33 kW electric motor. It has a curb weight of 1254 kg. The Prius has a complex dual-mode hybrid configuration in which energy to and from the vehicle wheels can travel along several different pathways. Mechanical energy to the wheels passes through a planetary gear set that couples to the engine, electric motor, generator, and to the final drive. Power to the wheels can be provided solely by a (273.6 Volt Nickel Metal Hydride [NiMH]) battery pack through the electric motor, directly from the gasoline engine, or from a combination of both the motor and the engine. The battery pack can be recharged directly by energy from the wheels powering the motor (regenerative braking) or from excess energy from the gasoline engine – which turns the generator.
The following sections explain how the Prius battery pack is constructed, how it is thermally managed, and how the NiMH batteries perform at different temperatures under controlled laboratory conditions that simulate various drive cycles. More in-depth analyses can be found in the DOE report written by Zolot et al.
BATTERY PACK DESCRIPTION
The Module. Toyota uses prismatic NiMH modules from Panasonic. Each module, shown in Fig. 1, consists of six 1.2 V cells connected in series. The module has a nominal voltage of 7.2 V, capacity of 6.5 Ah, weighs 1.04 kg, and has dimensions of 19.6 mm (W) X 106 mm (H) X 275 mm (L). Further notable features are as follows:
A thermal well on top of the cell allows measurement of an approximate internal temperature of the electrolyte,
A hydrogen vent provides for release of hydrogen through a manifold under gassing conditions,
Terminals on each side provide clean connections,
Tie down bolts secure the modules to structural supports,
A plastic case lowers mass, and
The side surfaces of the module provide air gaps for airflow created by dimples and protrusions when two modules are stacked.
This Panasonic design has improved in specific energy and power capabilities over the first generation cylindrical cells that are in the 1999 Japanese Prius and the 2000 Honda Insight.

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