Electric-vehicle And Data-center Batteries Need Precise Measurements Using Robust Chips

.tags Battery cells, supercapacitors, and fuel cells need careful monitoring to extend range, prolong life, and ensure safety in energy-storage systems, such as those in electric and hybrid vehicles (Figure 1). The use of batteries in automobiles is developing along a range of applications. Micro hybrid vehicles use a conventional 12V lead-acid battery and have alternator-motor units that allow the engine to stop when you bring the vehicle to a halt. When you press the gas pedal, the engine smoothly starts and then operates conventionally.

Hybrid vehicles, such as the Toyota Prius, the Honda Insight, and the Chevy Volt, have much larger batteries. These batteries produce more than 200V. Cell chemistries have traditionally been NiMH (nickel-metal hydride), but various lithiumion chemistries are providing more energy for a given weight (Figure 2). Fully electric vehicles, such as the Tesla Roadster and the Nissan Leaf, have the largest batteries; their battery-stack voltages range from 300 to 400V.

The higher the voltage in a battery, the lower the current for a given power will be, reducing the gauge of expensive copper cabling. More important, the higher voltage allows the winding of higher-output motors. In 2004, Toyota added a boost converter to the Prius that raised the battery-stack voltage from 200 to 500V. This step allowed Toyota to redesign the propulsion motor and improve torque from 350 to 400 Nm and power from 33 to 50 kW

Data centers also use 300V battery strings for UPS (uninterruptible-power-supply) backup power. In this application, lithium-ion batteries are replacing lead-acid batteries. Vehicles take advantage of lithium-ions better gravimetric energy densitythat is, the energy per pound or kilogram. UPS applications instead involve the volumetric energy density of lithium-ion batteries. Datacenter floor space is expensive; although a lithium-ion-battery system may cost more, it takes up only one-fourth the space that a lead-acid-battery system requires. This fact often allows data centers to combine the battery and inverter systems into one room. Some data centers are considering removing the inverters and distributing dc voltage to data-server computers that can accept dc inputs.

Analog companies have made a considerable achievement with their battery-stack-monitor chips. The parts must be highly accurate, small, and robust because the applications involve EMI and electrical overstress. Auto makers require redundancy and fault protection. Although vendors provide evaluation boards, it is doubtful whether you can strap that board somewhere in your system and expect it to work properly. Instead, you will have to understand the measurement, noise, and interference problems in your application and then apply good design and layout techniques. With careful design and judicious shielding, you can make a monitor system that will keep a vehicles propulsion system operating for a decade or more.

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