Battery Control System Highlights Design House’s Skills

Nov. 28, 2012
Nuvation's T-REX 160-mph, 0-to-60 in 5 seconds Electric Vehicle provides a development platform for battery management using Maxim Integrated's chips and a way for the engineering services company to market its IP.

As part of a contest, engineering design-services company Nuvation took a gas-powered three-wheel Campagna Motors motorcycle and electrified it (see the figure). Today, the E-Rex continues to undergo improvements as part of a development platform representing a collaboration between Nuvation and Maxim Integrated, which provides the battery management and monitoring chips.

Nuvation’s E-Rex electrified motorcycle project continues as a development platform, teaming with the Maxim Integrated MAX11068 for state-of-the-art battery control.

The MAX11068 12-channel battery-monitoring ICs include an SMBus laddered interface that allows up to 31 chips to be connected in series to monitor as many as 372 cells. Maxim Integrated’s approach eliminates the need for isolation components between devices, which considerably simplifies Nuvation’s system design (see “E-Rex Battery Management System From Nuvation”).

Measured at the Laguna Seca raceway in California, the E-Rex demonstrated 0- to 60-mph acceleration in 5 seconds and a top speed of 160 mph. Its city mileage is 200 miles on a charge, with 100 miles on the highway (see “E-Rex Three-Wheel Electric Vehicle From Nuvation”).

When Nuvation started building the electric vehicle, its engineers tried to find-off-the shelf components for a number of systems and found that many were not available. So as an electronic design services company, it decided to build those parts with more basic components.

Components Old And New

The propulsion system is simple. Nuvation chose a 90-pound, 125-kW electric motor to run the stock Campagna chain drive. But since the vehicle is electric, out went Campagna’s transmission, oil pan, clutch, and ignition system.

The motor gets its power from the centrally located motor controller, which takes in 350 V dc and converts it into three-phase ac. The high-voltage section also includes fuel gauging, soft start, main start, ground-fault interrupter (GFCI) checking, and other functions, as well as the dc-dc converter that generates the 12-V rail that runs the traditional automotive electronics. Beneath the controller, the battery pack includes 104 ThunderSky 100-A-hr lithium-ferrous batteries.

“When you have that amount of lithium, the batteries have to be carefully managed and controlled,” says Nuvation president Michael Worry. So that’s where a great deal of coordination between Nuvation’s engineers and Maxim Integrated took place.

Nine battery-pack management boards surround the batteries. They are designed to monitor and report the voltage and temperature of each individual cell to keep the entire pack in balance.

“If you didn’t keep the pack balanced, for some chemistries, such as lithium poly, you’d actually run the risk of fire,” Worry says. “And even with other lithium and nickel chemistries, you need to maximize range and maximize the life of the battery by keeping the battery balanced at all times.”

The nine pack-management boards are connected in a daisy chain, using that Maxim Integrated SMBus arrangement. They communicate back up to what’s called the tank controller. Then, the tank controller reports back to the system controller, which is a Linux-based computer that interfaces to the display board that tells the driver what’s going on.

Nuvation’s engineers looked at a lot of off-the-shelf candidate systems, but they had never been proven in electric vehicles. The biggest challenge was that these systems couldn’t handle the communication that was required in the noisy electrical environment inside the vehicle.

“There are huge amounts of noise in this system. You have a vehicle that’s running 200-A surge currents at 350 V dc. You can’t run multiple serial communications buses alongside that without some special design know-how,” Worry says.

“We had to take a systems-engineering approach. We had to use shielded cables, we had to minimize noise level to each level of the system, and we had to create original software algorithms that could reject noise through CRC (cyclical redundancy checking) and other techniques,” he says.

Nuvation used its third-generation system. Communications in the generation-one prototype were so challenging that the company was not allowed to drive its system on the road. “This is now extremely reliable,” Worry notes. “We don’t even need the software checking anymore because the hardware in the base system has become so reliable.”

Opportunities For Other Houses

Nuvation’s battery management boards are licensable. The company’s expertise ranges beyond electric vehicles. For example, it has built a robot that’s unbeatable at air hockey (see “Air Hockey Bot 1000 At Nuvation”)

The Maxim Integrated MAX11068 that Nuvation uses is a programmable, high-voltage, 12-channel, battery-monitoring smart data-acquisition interface IC optimized for use with batteries used in hybrid electric battery packs, electric cars, and any system that stacks long series strings of secondary metal batteries. It integrates a simple state machine with a high-speed I²C bus for SMBus laddered serial communication.

Full-scale measurement ranges from 0 to 5.0 V, with accuracy guaranteed from 0.5 to 4.7 V. All 12 cells can be measured in less than 107 µs. The MAX11068 uses a two-scan approach for collecting cell measurements and correcting them for errors. The first phase of the scan is the acquisition phase, where the voltages of all 12 cells are acquired. The second phase is an error-cancellation phase in which the analog-to-digital converter (ADC) input is chopped to remove errors.

About the Author

Don Tuite

Don Tuite writes about Analog and Power issues for Electronic Design’s magazine and website. He has a BSEE and an M.S in Technical Communication, and has worked for companies in aerospace, broadcasting, test equipment, semiconductors, publishing, and media relations, focusing on developing insights that link technology, business, and communications. Don is also a ham radio operator (NR7X), private pilot, and motorcycle rider, and he’s not half bad on the 5-string banjo.

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