Precise monitoring maximizes HEV-lithium-ion-battery performance
By Graham Prophet, Europe Editor -- EDN, January 8, 2009
Linear Technology has been working closely with a major HEV (hybrid-electric-vehicle) manufacturer in developing the LTC6802 high-voltage, precision battery-stack-monitor chip. The vehicle builder is making the transition from nickel-metal-hydride to lithium-ion-battery technology, which, according to Linear, is rapidly overcoming the shortcomings that have prevented its use in HEVs. For example, changes in the cell chemistry eliminate the thermal-runaway problem, and new cell designs remove any ignition points that can lead to fires in the event of electrical failure or mechanical damage.
The need for precision cell-by-cell monitoring of battery voltage remains, however, and, in an HEV—or a purely electric—vehicle, battery stacks may deliver hundreds of volts: Linear mentions arrays of 96 cells. The lithium-ion cells have a flat discharge curve, and, to maximize their life, you ideally use 30 to 70%—or, in some cases, 20 to 90%—of their total capacity. Therefore, you need to measure to millivolt precision to assess the state of charge of each cell; the so-called gas-gauging approach, which consists of counting charge into and out of each cell, is not viable. The 6802 therefore measures the voltage of as many as 12 cells in a series-connected stack. You can stack multiple 6802s in series for larger battery packs with a voltage as high as 1000V; they have a level-shifting interface that permits such a connection without requiring optocouplers or isolators. With a precision-trimmed internal 10-ppm/°C voltage reference, the chip is accurate to 0.25% over –40 to +85°C. It has built-in diagnostics and fault detection, communicating through an SPI (serial-peripheral interface), and Linear has qualified it to AEC (Automotive Electronics Council)-Q100 for automotive use.
The chip must work in an electrically noisy environment, with acceleration and regenerative-braking currents of hundreds of amperes. Linear designed it using a delta-sigma ADC with integral FIR (finite-impulse-response) filtering to cope with such a signal environment. It completes measurement of all the cells in a stack in 13 msec.
The chip also implements cell balancing by resistively discharging cells that are at higher voltages than neighboring cells by a current of as much as 50 mA. You might use the 6802 in applications such as robotics, portable medical equipment, high-power electric tools, or uninterruptible-power supplies, as well as in vehicles. It is available for sampling now and costs $9.95 (1000).
