Since Sony introduced them in 1991, lithium-ion (Li-ion) batteries have transformed portable electronic products. Now, carmakers and would-be automotive battery suppliers are looking for a similar transformation to redefine automotive propulsion.
Currently, well over 2 billion Li-ion battery cells are sold annually for consumer electronic applications, most notably cell phones and laptop computers. If cars become a significant market for Li-ion batteries, this number will rise dramatically. However, any similarities to Li-ion batteries residing in consumer products could be in name only.
The Li-ion technology (lithium cobalt oxide) used in portable electronics, which requires precise control, has encountered some well-publicized problems, including explosions. Much higher current and voltage levels and the rigorous requirements of the harsh automotive environment dictate unique and more complex battery management and control systems.
The competing nickel-metal-hydride (NiMH) battery chemistry, successfully used in hybrid vehicles for more than a decade, has several shortcomings compared to Li-ion technology. For example, Li-ion batteries are 20% to 30% smaller and 50% lighter than NiMH. Li-ion technology also promises:
- Two to three times the power for the same mass for greater acceleration and fuel efficiency
- Three to four times the energy for the same mass for greater electric-only vehicle range and fuel efficiency
- Two to three times faster recharge
- Enhanced cycle life for longer battery operating life
DRIVING LI-ION TECHNOLOGY
Essentially all carmakers in the established industry, as well as several new players such as Aptera, Fisker Automotive, Tesla Motors, and Think, are vying for a piece of the electric propulsion business. Hybrids, plug-in hybrids, and extended-range vehicles like the Chevy Volt with various engine combinations and degrees of hybridicity (such as mild hybrids), in addition to full electric vehicles, also use battery-powered propulsion.
Though vehicles with Li-ion batteries are just starting to emerge, carmakers have explored their capabilities for many years. In fact, Nissan has been on the case since 1992, a year after the technology was first applied to cell phones.
Initially investigating a cylindrical cobalt-based technology, Nissan switched to a manganese-based laminated design. The company will implement this technology in the Leaf EV, slated for production in 2010. Through the Automotive Energy Supply Corp. established in 2007 by Nissan, NEC, and NEC Tokin, Nissan and Renault will obtain battery-cell module and batterymanagement system technology.
While research has been ongoing for years, the first company to supply a production vehicle with a Li-ion battery was startup vehicle manufacturer Tesla Motors. Tesla introduced its Roadster electric vehicle in 2008. Out of necessity, when it started to develop the Roadster earlier this decade, Tesla engineers chose existing commodity Li-ion cells with the 18650 form factor commonly used in portable computers.
As other carmakers joined the hunt for an improved chemistry, the cell quantities became significant enough to attract volume suppliers of batteries as well as several startups. Companies developing Li-ion batteries and battery systems include LG Chem and its subsidiary Compact Power Inc. (CPI), A123Systems, Atieva, Altairnano, Continental, EnerDel, JCI-Saft, Sakti3, and Valence.
In the dynamic world of Li-ion batteries, the close working relationship between carmaker and battery supplier doesn’t preclude exploring alternatives. General Motors chose LG Chem to supply Li-ion batteries for its Chevy Volt extended-range vehicle, which will be launched in 2010. However, GM is keeping its options open.
“The guy that seems to be the best battery supplier today may not be the best one tomorrow,” says Ronn Jamieson, director of global battery systems engineering at General Motors. “So, we are constantly monitoring the supplier base and the technologies. At any particular time, we could have some number of different battery suppliers’ cells in our test lab to characterize and understand.”
With its battery lab facility, the largest in North America, GM can perform extensive evaluations both at the pack level for validating in-process production programs such as the Chevy Volt and at the cell level and groupings of cells level (Fig. 1). The lab can characterize and qualify cells for production.
In some cases, carmakers have established their own battery technology. Toyota took this approach for NiMH batteries for the Prius, and Nissan did the same for its Leaf EV.
While Li-ion describes the nominal electrochemistry of the cell, variations on the Li-ion theme continue to grow. As Mohamed Alamgir of Compact Power and Ann Marie Sastry of the University of Michigan concluded in “Efficient Batteries for Transportation Applications,” which won the best paper award at the Convergence 2008 biennial conference on automotive electronics, “Not all Li-ion battery systems are equal. Choice of chemistry, separator, and packaging can provide significant advantages to one system compared to another.”
A few of the competing approaches demonstrate the available variations. For example, JCI-Saft employs a spiral-wound, cylindrical design for its cell. The cathode uses nickel cobalt aluminum (NCA). Daimler approved JCI-Saft’s cell design, which also will be used in the 2010 Mercedes S 400 hybrid.
“What’s important is to clarify a lot of the noise out there about the chemistries,” says Michael Andrew, director of government affairs and external communications, hybrid battery systems, JCI-Saft. “When an OEM customer approves a technology, there has been a tremendous amount of very focused testing that has been done to validate the technology as being appropriate for that application.”
LG Chem takes advantage of a proprietary manganese-based cathode chemistry, often called spinel, in a flat, laminated package. Reducing the amount of cobalt in the cell design improves the abuse tolerance and increases safety to the cell. Specially developed high-temperature, safety-reinforced separators minimize potential thermal runaway due to internal shorts.
Altairnano, with its unique nano Li-ion chemistry, claims higher levels of operational abuse tolerance than existing batteries. Units have a recommended cutoff voltage in the –40°C to 30°C range of 1.5 V and charge cutoff voltage at –40°C to 20°C of 2.9 V. The flat design uses a high-surface-area nano lithium-titanateoxide- based anode material. Phoenix Motorcars Inc. selected the battery technology for an electric sport utility truck.
EnerDel, a supplier to the Think City electric vehicle, says it is the first and only U.S. domestic manufacturer of commercialscale, automotive-grade lithium-ion battery systems. The company uses a lithium-manganese-oxide (spinel structure) cathode and lithium-titanate-oxide (spinel structure) anode in two of its cell designs. The cell structure is a laminated construction (Fig. 2).
To obtain a complete battery system, suppliers start with a specific cell technology, assembling several cells into modules and several modules in parallel and series configurations into battery packs (Fig. 2, again). In addition to the cell technology, a vehicle battery system consists of battery-management system software and hardware, electronics, a mechanical subsystem, an electrical subsystem, and a thermal-management subsystem.
Whatever the chemistry, hybrids, plug-in hybrids, and electric vehicles need advanced battery-management technology. “Beyond the chemistry and beyond the separators, there is a lot of work that goes into the electronics and the software for safety,” says Martin Klein, engineering director for Compact Power, which designs the battery management for its battery packs. This involves distinct functions that include protection, performance management, diagnostics, and an external interface (Fig. 3).
CPI identified about 80 different discrete functions that the battery needs to perform, including estimating the state of charge, estimating the state of health, monitoring the voltage and current in the battery and temperature, and making decisions about charging. Bringing these factors together involves cell integration and an integrated circuit.
“Our approach has been to do a master- slave layout, where the slaves are tied to modules or groups of modules,” says Klein. “Their responsibility is to measure the cell voltages, to measure temperatures at the module, and to perform the cell balancing act.” CPI developed an ASIC to make those measurements and reduce the number of discrete components in the battery- management system (Fig. 4).
Continental’s battery system in the Mercedes S 400 Hybrid features a cell supervisory circuit (CSC). “It’s not entirely an ASIC. There is some more complexity there,” says Mark Gunderson, engineering manager of battery electronic controls with Continental’s Hybrid Electric Vehicles Business Unit. “Our first generation was a discrete design.” Continental has explored options for communication within the battery that include simple dualwire communications, the serial peripheral interface (SPI), and the controller area network (CAN).
Communication between cells is certainly one of the variations in approaches for IC suppliers and battery-system manufacturers. One of the common problems for all IC suppliers is high voltage.
“The common-mode voltages in these batteries is the thing that is problematic because you can have a typical automotive battery now around 300 V,” says Bob Shoemaker, systems engineering manager for battery management systems, electric transportation, at Texas Instruments. “Conquering the common-mode problem is challenging.” TI developed the BQL76P536 to provide a high-voltage solution that addresses the functional requirements for battery management and monitoring.
One of the critical aspects in the battery fuel-gauging application is very accurate measurements. “The 536 actually has a very high-performance analog-to-digital converter in it and a very high-performance bandgap reference so that we can maintain very, very tight accuracy over a huge temperature range,” says Shoemaker. “For instance, in a Li-ion phosphate cell, approximately 1 mV represents 1% of the available charge, so the need for great precision is very strong.”
Linear Technology’s LTC6802 multicell battery stack monitor, which also addresses the accuracy issue, comes in two versions. A serial interface allows the serial ports of multiple LTC6802-1 devices to be daisy-chained without optocouplers or isolators. The LTC6802-2 provides individually addressable serial communications but requires optocouplers or digital isolators. Both versions offer 0.25% maximum total measurement error (Fig. 5).
Maxim Integrated Products has a pair of ICs for high-voltage interfacing in the battery- management system. The MAX11068 and MAX11080 connect to the cells. The MAX11068 provides the primary control. The MAX11080, connected in parallel, is basically a bank of comparators that supplies redundant protection.
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.
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.
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.