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Precise monitoring maximises HEV Li-ion performance

by Graham Prophet

 

In developing its LTC6802 high-voltage precision battery- stack monitor chip, Linear Technology says it has been working closely with a major hybrid electric-vehicle manufacturer. The vehicle builder is making the transition from NiMH- to lithium-ionbattery technology, which— according to Linear—is rapidly overcoming the shortcomings that have prevented its use in HEVs. For example, detail changes in the cell chemistry eliminate the thermalrunaway 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 cellby- cell monitoring of battery voltage remains, however, and in an HEV—or purely electric— vehicle, battery stacks may deliver hundreds of volts: Linear mentions arrays of 96 cells. The Li-ion cells have a very fl at discharge curve, and to maximise their life, you ideally use the range from 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 up to 12 cells in a series-connected stack. You can, in turn, stack multiple 6802s in series for larger battery packs, up to 1000V; they have a level-shifting interface that permits such a connection without requiring opto-couplers or isolators. With a precision trimmed internal 10-ppm/C voltage reference, the chip is accurate to 0.25% over 40 to 65°C. It has built-in diagnostics and fault detection, communicating via an SPI interface, and Linear has qualifi ed it to AEC-Q100 for automotive use. The chip must work in an extremely—electrically— noisy environment, with acceleration and regenerativebraking currents of hundreds of amperes; Linear designed it using a delta-sigma analogue/ digital converter with integral FIR fi ltering to cope with such a signal environment. It completes measurement of all the cells in a stack in 13 msec.

In addition to its monitoring capabilities, the chip also implements cell balancing; it carries this out by resistively discharging cells that are over-voltage, compared to their neighbours, by a current of up to 50 mA. Recovering that energy is not practical in today’s architectures, the company says. 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 sampling now and costs $9.95 (1000).

Linear Technology, www.linear.com.