Toyota Prius Battery Pack Thermal Evaluation PDF Manual

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 [1]. 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. [2]. 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 [4]. The Pack. The Prius battery stack consists of 38 prismatic NiMH modules connected in series. It delivers a nominal 273.6 Volts and has a 6.5 Ah capacity. The modules are stacked side by side and then compressed together in a rigid, non-expandable structure that prevents expansion from internal pressures. The complete battery pack consists of the battery stack, enclosure for structural support and airflow, battery electronic control unit/monitor (left end of pack), relays and safety switch (also left end of pack enclosure).

The weight of the complete battery pack is 53.3 kg. The pack, shown in Fig. 2, is horizontally positioned in the trunk of the vehicle partially under the back seat. Power electronics (inverter, DC-DC converter) are under the hood and a blower for moving air and associated air ducts are in the trunk. We conducted hybrid pulse power characterization (HPPC) tests per the PNGV Battery Test Manual [3] to obtain power performance of a module at various states of charge (SOC) and temperatures (0°C, 25°C, and 40° C). The discharge and regenerative power performance of the pack (extrapolated from module tests) is shown in Fig. 3. Discharge power capability of the Prius pack is around 20 kW at 50% SOC with regenerative capability of 14.5 kW at 25°C. The power capability increases with higher temperatures and decreases at lower temperatures. Active thermal management can improve power capability at lower temperatures. PACK THERMAL MANAGEMENT Generally, the purpose of a battery thermal management system is to keep the batteries operating at a desirable temperature range; prevent the batteries from exceeding a high temperature limit that can damage the batteries and/or reduce life; and maintain battery temperature variations to low levels to prevent highly imbalanced batteries. Pack imbalances can reduce performance and can also damage the battery and/or reduce life. Thermal management of the battery pack is typically accomplished with the combination of two approaches.

First, a cooling/heating system is designed to extract/ supply heat to the battery pack. Second, the battery controller adjusts the vehicle’s use of the battery pack based on the conditions in the batteries. Forced Air System. The Prius supplies conditioned air from the cabin as thermal management for cooling the batteries. The pack’s forced air system consists of two vents located in the cabin under the middle brake light (exhaust from the cabin or inlet to the pack); ducting to the battery pack enclosure; the enclosure manifold; air gaps between modules; ducting out of the pack to a blower that pulls the air through the system; and two exhausts (one to the trunk and the other to the outside). A hydrogen vent from each module is connected in series with tubing. Any gases released are exhausted from the vehicle through the gas manifold to avoid any increased hydrogen concentration and, thus, potential for explosion. Outside air is conditioned (heated or cooled) by the vehicle’s thermal comfort system to a level comfortable for the driver.

This approach has the advantage of providing air that is not only comfortable to the passenger(s), but also ideal for use in heating or cooling the NiMH batteries. However, recent in-car dynamometer tests show that the Prius does not use the forced air for heating the batteries. Moreover, the concerns with this approach are twofold: relatively slow transient time to heat or cool outside air and thus the battery pack; and the ducting between the battery and cabin, which in the event of an accident or catastrophic failure, provides hydrogen and other gases a path to the cabin. To achieve a relatively uniform temperature distribution across the modules, a parallel airflow scheme is used, as suggested by Pesaran et al. [5], rather than a series configuration. In a parallel configuration, each module is set up to receive the same amount of airflow and thus the same cooling. To achieve this in the Prius, cabin air enters the pack through a plenum that runs beneath the battery stack horizontally from passenger side to driver side. The cross-sectional area of the plenum is largest at its entrance and linearly decreases as it goes under the modules. Then the air flows vertically through the gaps between each module (formed from dimples and protrusions on side walls). Finally, the air enters into the top plenum. This plenum’s cross-section increases linearly in the direction of the flow. With this design, the pressure drop across each module is expected to be uniform and, consequently, should lead to a uniform flow rate around each module. The air is drawn by a 12 V blower installed above the driver’s side rear tire well.

The air is either exhausted to the trunk or through a vent on the driver’s side C-pillar. The fan has four settings, depending on the maximum temperature of four monitored batteries. Toyota also monitors the inlet and outlet air temperatures. The fan settings are off, low, medium, and high speed. We conducted airflow tests with the pack in vehicle, out of vehicle, with flow meter, and without flow meter and obtained correlations between pressure drop, flow rate, blower power, and blower settings. The blower setting depended on temperature, as shown in Fig. 4, and transitions with hysteresis depending on whether temperature is increasing or decreasing. Table 1 summarizes the results of the flow tests for Denver elevation (1 mile above sea level, equal to 0.81 atmosphere). At sea level, the pressure drop across the pack and the required power will increase compared with tests at Denver elevation.

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