|Energy and Power requirements for full hybrids (two-mode), plug-in hybrids, and E-Flex range-extended EVs. Click to enlarge.|
GM held a media briefing yesterday, along with its battery partners from Johnson Controls-Saft, A123Systems, and Cobasys, to provide some perspective on the development of lithium-ion battery technology for its plug-in and E-Flex efforts.
Interest in and demand for the Chevrolet Volt, the series plug-in hybrid concept car unveiled at the Detroit auto show, has been intense and gratifying, according to the GM executives, but they also want to manage expectations as best they can around the key issue of battery development.
On the one hand, we want to be excited. On the other hand, we’re getting emails asking if [they] can buy the Volt tomorrow. It’s important to understand that a lot of work remains before the batteries are ready for mass production. It’s a balance between the promise and the hurdles that remain.—Scott Fosgard, Director of GM Powertain and Advanced Technology Communications
Joe LoGrasso, the engineering group manager for hybrid energy storage systems at GM, noted the difference in power and energy requirements between a standard two-mode hybrid, a plug-in version of such a hybrid (the Saturn VUE two-mode plug-in under development, earlier post), and an E-Flex range-extended EV such as the Volt (a series plug-in hybrid in configuration, earlier post).
In GM’s view, all-electric mode in a conventional two-mode hybrid at this point is limited to lower speeds and loads. EV mode in a plug-in two-mode hybrid such as the Saturn VUE will support a range of 10+ miles, and the E-Flex/Volt series hybrid plug-in will provide 40+ miles all-electric range.
A plug-in version of a two-mode hybrid would require about twice the discharge power and significantly more usable energy than a conventional two-mode hybrid battery pack. In turn, a range-extended, battery-dominant plug-in series hybrid such as the Volt would require more than twice the discharge power and usable energy of the plug-in version of a two-mode, according to GM’s view of vehicle requirements.
|GM’s View of Hybrid and Plug-in Hybrid Battery Requirements|
|EV Range||Only at low speeds and loads||10+ miles all-electric in city||40+ miles all-electric in city|
|Recharge||Only while driving||While driving and with plug-in||While driving and with plug-in|
|Power||EV driving in low-speed city only||EV driving in city||EV driving full vehicle performance|
|Battery technology and vehicle requirements. Click to enlarge.|
GM and its battery partners asserted that lithium-ion batteries will provide the range of solutions required to support the different vehicle applications. The chemistry offers both high energy density and power density, and as such is necessary for future hybrid performance and critical for plug-in and E-Flex applications. Ongoing developments in new materials such as lithium titanate anodes and lithium iron phosphate cathodes continue to improve performance.
|Different battery requirements for the different types of cycles. Click to enlarge.|
But GM also took pains to outline the challenges for battery design optimization given the different requirements between charge-sustaining and charge-depleting applications.
(In terms of areas of focus for GM in future energy storage, the company also noted that supercapacitors, because of their very high power density and potentially low cost, may be very well matched by mild hybrid applications.)
|Different applications require different li-ion cell and pack characteristics. Click to enlarge.|
Li-ion cells designed for a charge-sustaining hybrid application require high power but low energy, and feature ultra-thin electrodes, thicker current collectors and short pulse power.
Cells designed for plug-in and E-Flex applications require both high power and high energy, and thus thicker electrodes and thinner current collectors. The batteries must be able to support longer power draws.
While GM is working with a battery pack built from large format cells for the two-mode plug-in, and an array of two or three such large-format packs for the E-Flex. However, GM is also exploring a single battery pack made of very large-format cells for the E-Flex as well.
The key challenges to making lithium ion successful are robustness, cost and life. While today it may not be ready for prime time, it’s not a revolutionary requirement but evolutionary advance that will help us meet the requirements.
GM is using a multi-phased approach, starting with qualifying the cell, proving out the cycle life and calendar life at the cell level, then developing the pack and testing it on the lab bench. All this is necessary as a precursor to declaring a solution ready. Vehicle integration is the final step before the production program. The challenge is how do we develop battery solutions and vehicles in parallel?—Joe LoGrasso
Mary Ann Wright, the former chief engineer for the Escape Hybrid and new CEO of JCS (earlier post), provided a brief overview of lithium-ion chemistry and cell construction, noting that manufacturers can balance the power and energy based on system requirements in part by making the lithium-ion electrode coating thicker for energy requirements and thinner for power and acceleration.
Theoretical voltage and capacity is a function of the anode and cathode material. The practical capacity brings in the effect of the separator, the electrolyte, the connector, the temperature and the rate.—Mary Ann Wright
Ed Bednarcik, vice president and general manager of the pack and system group from A123Systems, provided an overview of manufacturing processes. Producing high-power li-ion batteries requires the use of nanomaterials that need proprietary mixing, handling and coating processes and systems, he noted.
The use of very large currents (100 times grater than conventional li-ion) require new termination and current collection schemes. To prevent contamination, the electrodes are assembled in clean room environments.
The additional burden of a 10-15 year battery life (compared to a 2-3 year life for consumer applications) also requires different assembly processes to seal the cells such as the use of laser welds to provide superior hermetic packaging.
|The major battery pack system components. Click to enlarge.|
Scott Lindholm, the vice president of systems engineering from Cobasys, focused on the development of the battery systems, rather than the individual cells. The three major components of the battery system are the batteries, the thermal management system, and the electronics bay. An initial challenge faced is the mechanical packaging, especially in vehicles not originally designed to hold large battery packs.
Batteries like to live where we live. They like to live at the same temperature. If we can’t control the battery temperature, we lose control, and the system will fail. Things you may not consider—salt-mist testing, vibration, thermal shock, mechanical shock—are all wrapped into the system design.
From the consumer perspective, we want to ensure there is no exposure to high voltage. What happens when a kid dumps a Slurpee down the back seat?—Scott Lindholm
The focus on the 10-15 year, 150,000-mile battery life appears iron-clad, despite suggestions from some, such as Felix Kramer of CalCars, that there might be the possibility of negotiating a lower regulatory requirement and/or supplementing that with a warranty package in favor of getting more cars into production more quickly.