24-cell battery systems
Bar winding approach
Honda’s Eco-Assist System
At the SAE 2010 Hybrid Vehicle Technologies Symposium, held February 10-11 in San Diego, batteries continue to be the number one discussion topic for electric vehicle (EV), hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) success. However, other components provide cost savings as well as improved performance and fuel efficiency. Suppliers and vehicle manufacturers continue to make significant strides in a variety of areas from integrated circuits (ICs) to ultracapacitors. Here is a cross section of topics discussed at this year’s symposium.
HEV BATTERY MANAGEMENT ICs
Although safety is the overriding concern, battery management ICs must address several other important factors for effective HEV and EV battery management. These include accurate gauging through state of charge (SOC) estimation, impedance for state of health (SOH), cell balancing especially as cells age, and optimized cycle life and temperature. To solve the problems in multiple chemistries, multiple voltage and varied temperature situations, Texas Instruments has developed the bq76PL536 and bq76PL537 automotive (Li-Ion) battery management products. The two ICs are very similar: both address up to six channels and have a high-speed SPI communications interface and other common features.
One of the major differences between the two products is active balancing for the 537 versus passive balancing for the 536. According to Bob Shoemaker, systems engineering manager for Electric Transportation Battery Management Solutions at TI, traditional cell balancing approaches dissipate excess energy in a battery as heat. “Even at small energy levels, this adds about 30 W to the average automotive battery,” he said. The power pump cell balancing technique in the bq76PL537 puts a dc-dc circuit on every cell to move charge between batteries at about 85% efficiency. This minimizes the challenge of removing the excess heat.
Shoemaker noted that by controlling six cells instead of eight or twelve, in a 13-cell system that is common for lead acid battery replacement, fewer cells are left idle providing reduced system cost. The six-cell architecture also fits nicely into a 24 cell system (Fig. 1).
INTEGRATED STOP-START SYSTEM
Derek de Bono, marketing director, Engine & Electrical Systems at Valeo had an update to its StARS belt-driven stop/start system for automakers with micro hybrid applications. Launching this summer, the 2nd generation i-StARS uses integrated electronics to reduce the parts count and make the unit easier to package. Rather than a separate electronic control unit, the electronics are fully integrated into the alternator’s housing. Valeo claims the new design is the first stand-alone machine to use synchronous rectification in the alternator. In the alternator mode, the efficiency of the new unit is 77% to 78% or about 10% above today's average. The system is also about 40% less expensive than the first generation that was launched in 2004.
By taking a total systems approach and pursing the efficiency improvements to second-generation drivetrain, thermal management, regenerative braking, thermal accumulation, LED lighting and smart key and pre-conditioning technology, Valeo engineers predict that carmakers could reduce the battery capacity to save cost or increase the vehicle’s range up to 20%.
REPLACING BATTERIES WITH ULTRACAPACITORS
Working with General Motors on a belt alternator starter (BAS) system equipped Saturn, the National Renewable Energy Laboratory (NREL) determined that an HEV with an ultracapacitor performed equal to or better than the vehicle’s stock nickel-metal hybrid (Ni-MH) battery. The project involved replacing the battery on the mild hybrid with a Maxwell Technologies 48 V, 165 F ultracapacitor. The ultracapacitor weighed 14.8 kg instead of the 24.7 kg of the Ni-MH battery.
Tests included on-road initial shakedown, calibration and acceleration performance as well as ambient and cold (-20C) dynamometer testing. The benefits of using ultracapacitors in the mild hybrid application include excellent life and low temperature performance as well as low long-term projected costs. Since mild hybrids have demonstrated fuel savings of approximately 25% over a comparable internal-combustion-engine-only vehicle, a considerable amount of fuel could be saved if these vehicles are accepted by car buyers. Jeff Gonder, senior engineer, NREL Center for Transportation Technologies and Systems said, “This opens the door for an alternative energy storage technology that can satisfy the needs of a hybrid vehicle.”
CARMAKERS FOCUS ON COMPONENTS
Carmakers reported on a variety of products that they implemented for improvements in hybrid electric vehicles. In addition to engine modifications including increasing the displacement of the Atkinson cycle engine from 1.5 to 1.8 Liters, Toyota redesigned the electric water pump (EWP) in the third generation Prius. One of the factors contributing to improved efficiency is integration of the permanent magnet with the impellor. The improvements reduced the loss from the conventional water pump resulting in a 1% to 4% reduction in fuel consumption.
An area that General Motors has focused on improving is electric motors. Using a bar winding (Fig. 2) or segmented conductor design instead of wire-wound stator in a motor to achieve a higher copper fill in the slot, GM engineers were able to lower the winding resistance by 30% or more (from 14.08 mΩ to 10.7 mΩ) to reduce the overall system losses. In addition, bar wound motors have 50% or more increased heat dissipation area than the stranded wound types This increased heat dissipation can reduce the temperature by as much as 20°C and add considerably to the motor’s life, making the bar winding design quite attractive. Peter Savagian, engineering director, Hybrid and Electric Architecture and Electric Motors for General Motors said, “We really see some merits for it, especially in high duty applications.”
Perhaps one of the more significant hardware/software combinations are the tools developed to provide feedback and educate drivers to get the optimum efficiency from the carmakers’ electric propulsion systems. Ford cited a smart gauge to educate customers to the new driving requirements. Honda also brought out the significance of its Eco-Assist System to help drivers and reduce the variations in real world fuel economy between a fuel efficient driver and one who is less concerned with fuel efficiency. The company’s ECON for Effective Control Mode (Fig. 3), provides a game challenge to drivers to achieve the highest green driving score and a guide with feedback from colors and MPG readings in a multi-information display. Keiji Enomoto, who works in the automotive components department at Honda R&D noted that with the feedback and increased driving, drivers averaged about 10% better fuel economy.