Electronic Design

Stack Monitor Chips Without Isolation Concerns To Give Your Electric Car Some Zip

Okay, you want to design an electric car. Whatever kind of motor you decide on, you’re going to want to run it at a pretty high voltage. That means stacking many batteries in series to get to that voltage, which introduces interesting challenges in monitoring and charging circuits as potentials at the negative electrodes rise above system ground.

This is not a new problem. But as long as it’s been confined to products like golf carts and nuclear submarines, engineers have had to attack it on an ad-hoc basis The potential product volumes of the electric car have changed all that. Ever alert to new business opportunities, Linear Technology has staked a claim to the market with the LTC6802. This highly integrated multicell battery monitoring IC can handle up to 12 individual cells all by itself. Multiple LTC6802s can be stacked in series without optocouplers or isolators for precision voltage monitoring of every cell in longer strings of series-connected batteries (see the figure). Ultimately, that lets you run your motor at more than 1000 V. The LTC6802 was designed with an eye on fussy lithium-ion (Li-ion) batteries, but the chips really don’t care what kind of batteries you’re using.

There are three modes of operation. Standby mode is a power-saving state where everything except the serial interface is turned off. In measure mode, the chip records cell voltages and stores the results in memory. Measurements are based on an internal precision voltage reference. In measurement mode, the chip also monitors each cell overvoltage (OV) and undervoltage (UV) conditions. In monitor mode, it just checks the cells for UV and OV conditions. (UV and OV are reported on a status pin. The registers are read out ondemand over the SPI bus.)

Maximum total measurement error is guaranteed at less than 0.25% from –40°C to 85°C, and all cell voltages in a battery stack can be measured within 13 ms. There is a control pin for an external MOSFET switch, which you’d need in a practical system to discharge overcharged cells. The LTC6802 comes in an 8- by 12-mm surface-mount package.

Pricing starts at $9.95 each in 1000-piece lots. Samples and demonstration boards are available.



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