Many of the subsystems needed for the E-Rex were available off the shelf, but not a dc/dc converter or a battery management system, the latter considered a critical element for squeezing the most out of a multi-cell lithium ion battery.
"These systems didn’t exist so we built our own," said Worry.
"There are a lot of technologies that only handle one cell at a time, but having a circuit board for every cell is expensive so we went with a solution we think scales better and is more cost effective," said Worry.
The OptaMotive team decided to use nine Maxim 11068 battery management chips daisy chained together on standard I2C and SMBus interconnects. Each chip can measure voltages and temperatures on 12 separate battery cells within a 10 microsecond window.
The design "can make a big stack of batteries look like one battery," said Steve Lajeunesse, who manages the automotive business for Maxim.
"The single largest problem is the system integration," said Worry. "You can have a couple hundred amps drawn at times on a very noisy electronic system, and data buses like to pick up a lot of radiated and conducted noise," he said.
To get accurate voltage and temperature information, engineers are refining the design of shielded cables and algorithms to reject noise. They are also putting in additional circuitry to ruggedize gate structures.
"Noise was coupling on to gate lines of FETs causing them to turn on when we didn’t want them to," Worry said. "Poor battery management design can introduce imbalances like that," he added.
Worry hopes the subsystem attracts business for custom designs from other EV makers.
"This will not be a standard module, but more of an engineering contract to build versions for specific vehicles," said Worry. "It’s a system integration problem because if you sell the chip to someone who doesn’t know what they are doing it won't work and can even create battery imbalance problems," he said.
A growing number of companies are gearing up to provide battery management modules or other subsystems for electronic vehicles. They including EV maker Zap that licensed battery management technology from a national lab in April.
"Big car makers such as Ford and Toyota build or specify battery management systems that can cost hundreds of dollars and are based on discrete components," said Lajeunesse of Maxim. "They buy a mux, level shifters, amps and a/d converters and tune them in their own production line, isolating the output from each battery cell," he said.
General Motors and Nissan are unique in designing battery subsystems in house from the ground up for their upcoming Volt and Leaf EVs, Lajeunesse said. Most of the designs are done by an emerging group of subsystem makers under contract to the big car companies.
"The whole industry is in transition," said Lajeunesse.
"We've seen at least three or four [battery management subsystem makers] who came in and existed this area," he said. "Our chips are designed into four battery management systems now and we expect to be in another two or three by the end of this year," he added.