To minimize conduction losses in the propulsion system, battery packs for hybrid and all-electric vehicles stack low-voltage cells and battery modules to achieve high voltages. For example, total voltage across the battery stack ranges from 375 V in the Tesla Roadster to 330 V in the Ford Escape, 288 V in the Toyota Highlander, 201 V in my Gen-III Prius from Toyota, and 144 V in the Honda Insight.
To borrow a meme from GM, they aren’t your father’s Die Hards. They cost a lot of money and are fussy about things like temperature. Diagnostics and engine-control systems require continual, precise, and fine-grained data about their health.
One problem with fulfilling those needs lies in measuring and reporting all the voltages in a series stack, where the negative terminal of each successive battery module is at a potential higher and higher above system ground. The crude solution would be opto-isolators (or inductive or capacitive isolators), but car-makers are sensitive about bill-of-materials costs.
Recognizing this, Linear Technology introduced the LTC6802 in 2008 (see “Stack Monitor Chips Without Isolation Concerns To Give Your Electric Car Some Zip” at www.electronicdesign.com). Basically, the chip comprises a 12-input multiplexer and a serial-output analog-to-digital converter (ADC). What made it unique was that multiple battery-monitor ICs could be connected in series to handle battery packs with output voltages greater than 1000 V—no isolators needed.
In February, Linear introduced the next generation of the product at the same price and the same pinout, but with additional features that evolved from 24 months of dealing with the auto industry. For example, some of the new features came about from working with the automobile industry on a new standard, ISO 26262.
“Because ISO 26262 is a system-level specification, no IC can be designed to be fully compliant with the standard by itself. However, Linear has been working directly with OEMs and tier one suppliers to simplify the development of ISO 26262-compliant systems,” says Erik Soule, vice president of signal conditioning products at Linear. “To us, that meant the battery-stack monitor IC needed to have comprehensive diagnostic capabilities of both internal (on-chip) and external (system level) faults.”
Like the earlier LTC6802, Linear’s LTC6803 is intended to be designed into battery packs. Each chip can monitor up to 12 individual battery cells in series. The input channels on both chips can handle common-mode voltages up to 60 V, and both offer the stackable architecture, plus built-in FETs that allow overcharged cells to be discharged.
The really special feature is the serial bus, which can be daisy-chained to provide full reporting on the entire battery bank via a single port. The precision of those measurements is guaranteed to be 0.25% or better, and guaranteed maximum measurement time for all cells in the system is 13 ms. Other standard features include an on-chip temperature sensor plus two external temperature-sensor inputs and on-chip passive cell-balancing.
The LTC6803 also adds a wider cell measurement range (0.3 to 5 V) to support nickel-metal-hydride (NiMH) batteries and supercapacitors in addition to the original support for lithium chemistries. Furthermore, unlike the LTC6802, the power input of the LTC6803 is isolated from the stack, allowing the chip to draw current from an independent source. When powering from this input, the current draw on the pack is reduced to less than 1 µA. Essentially, within the pack, the monitoring chip doesn’t cause any self-discharge.
The LTC6803 is designed to surpass the environmental, reliability, and safety demands of automotive and industrial applications. It’s fully specified from –40°C to 125°C, which is a new feature and essential for all-weather performance.
Packaging is the same 44-lead shrink small-outline package (SSOP) as the LTC6802, with the same pinout, and 1000-unit pricing is still $9.95. Production volumes are available now.