According to Stephen LaJeunesse, strategic business manager of automotive and industrial battery products at Maxim Integrated Products, the MAX11068 performs cell balancing, measuring the voltage of each cell and reporting that information for comparison to programmed overvoltage or undervoltage value. All cells communicate using a special technique called System Management Bus (SMBus) laddering.
In this technique, commands are relayed up and down a stack of up to 31 MAX11068 ICs. The two-wire SMBus, a standard bus used in battery systems for consumer equipment, has several advantages for vehicle batteries. “We pay a 1-µs penalty in relaying the command forward,” says LaJeunesse.
STMicroelectronics used its BCD (bipolar- CMOS-DMOS) technology to design a battery-management chip for a Li-ion battery pack from LG Chem. The L9763 is a full ASIC with intellectual property shared by LG Chem that’s used on the Chevy Volt’s battery.
“To communicate between sub-modules and manage several battery cell stacks, the ICs use a customized, current-based vertical communication channel,” says Joseph Nataro, director of marketing and applications, Automotive BU Europe, STMicroelectronics.
BATTERY-BASED DIFFERENTIATION
Carmakers consider their engine technology their crown jewel. When the battery is responsible for the vehicle’s propulsion, the rules change and a new vehicle differentiator is anointed. As a result, automakers are intimately involved in the battery system. Unlike the commodity lead-acid batteries that provide starting and energy storage for the charging system in vehicles powered by internal combustion engines, each hybrid electric vehicle, plug-in hybrid electric vehicle, and electric vehicle battery system is unique to every carmaker and, in most cases, to every vehicle.
With several NiMH hybrid vehicles in production, General Motors is now focusing on Li-ion technology for the 2010 Chevy Volt extended-range vehicle and other plug-in hybrids. “We have defined some performance requirements that are appropriate for some [specific] applications,” says GM’s Jamieson. “There are some different requirements that you want for a pure battery electric vehicle, versus a plug-in, versus a hybrid, at least in the lithium space.”
GM’s new battery lab went into full operation in the summer of 2009. When the lab opened, GM had evaluated more than 155 different chemistries on paper from more than 100 suppliers. So far, over 60 chemistries from 20 suppliers have been tested in GM’s labs. “We have a standardized assessment qualification process that starts with a very detailed paper study,” says Jamieson. If a cell passes this initial phase, it graduates to the testing phase.
Ford engineers have explored various aspects of Li-ion battery design, such as reducing the system operating window and simplifying the control algorithm and determining state of charge. Other considerations unique to the vehicle manufacturer include design for abuse tolerance, recycling, and design validation implications. “We actually put a lot of effort into life prediction modeling,” says Ted Miller, senior manager of energy storage strategy and research at Ford Motor Company.
TOUGH LOVE
Safety is the overriding concern of carmakers and battery designers for the initial electric-powered vehicles. With adequate performance, including range and quick charge capability, mass adoption of Li-ion technology will depend on the ability of carmakers to reduce cost.
Even though Li-ion batteries are more expensive at low volumes, Ford engineers expect the technology to be more cost-effective at volumes in the few hundred-thousand range. When annual volumes of hybrid vehicles reach the 3 million plateau, the cost advantage could be as high as 30% in favor of Li-ion.
Other possible cost reductions can come from increased integration, such as that provided by the battery-management ICs, as well as vehicle-level integration. Rather than thinking about adding controls, Ford wants to have a more holistic approach to the battery system as part of the powertrain.
“We are looking for ways to further integrate these subsystems into the vehicle’s architecture itself and into the vehicle system,” says Ford’s Miller. An example could be using a single controller for both the engine and batteries, rather than adding another controller for the battery system.
With initial designs focusing on safety, the ultraconservative design windows could expand once the technology proves itself, allowing smaller batteries to work harder. The first generation of Li-ion battery-powered vehicles is on the road and will increase within the next few years. This dynamic area is poised for next-generation improvements for lower cost, higher performance, and, of course, uncompromised safety.