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Li-ion battery tech to make or break Chevy Volt

Posted: 22 May 2009 ?? ?Print Version ?Bookmark and Share

Keywords:GM Chevy Volt? Li-ion battery? electric car?

T-shaped battery
The uniqueness of the Volt battery can be seen right away in the T-shaped design of the battery pack. The pack contains a number of modules connected in series, as well as components used for the control and monitoring of discharge or charge energy from each module. Also included within the T-shaped battery back are temperature sensors to control cooling, flexible interconnects between each battery module, and manifolds and coolant lines that allow for heat exchange with the cell surfaces in each module.

The power comes from the individual battery modules within the battery pack. Each module consists of numerous Li-ion cells, with each cell delivering its electric current as the result of an electrochemical reaction. As the lithium ions move from the positive electrode (cathode) to the negative electrode (anode) during charging and vice versa during discharging, the electrical current is generated.

The Volt's battery consists of more than 200 cells (288, according to some estimates) with each cell weighing less than a pound and measuring 5-inch x 7-inch and less than a quarter-inch thick. Each cell contains a carbon anode, a manganese-based cathode (with additives to improve its life span under high-temperature conditions) and a reinforced separator that is the medium for the transfer of charge ions between the anode and the cathode.

LG Chem's proprietary Safety Reinforced Separator (SRS), coated with ceramics, serves as the semi-permeable insulating membrane between the electrodes in the cells. LG Chem says the ceramic coatings differentiate its separator from competitors, as the SRS enhances the abuse tolerance already provided by the manganese base used in cells. The design is said to offer superior mechanical and thermal protection from abuse conditions such as internal shorts and extreme overcharge.

The end result is a battery with an energy capacity of 16kWh, or an effective capacity of 8.8 kWh when you take into account a maximum charge of 85 percent and the fuel-based engine's kicking in at 30 percent. The battery at this capacity meets GM's requirement of operating up to 40 miles on pure electrical power from the battery. After 40 miles, the battery reaches a 30 percent discharge mode (a number chosen to maintain optimal performance of the Li-ion battery for at least 10 years of use) and the fuel-based engine begins to operate, driving a generator to allow the battery to maintain a 30 percent state of charge until the driver can get to an electrical outlet and fully recharge. The approximate charging time for the battery is six to seven hours.

Inside the Chevrolet Volt battery

With the battery design complete, the largest obstacle for the Volt's development has been cleared. "It's almost scary that we aren't seeing any problems with them," Lutz said of the batteries.

Before he stepped down as CEO, Rick Wagoner made the Volt a focal point in the revitalization of GM and, in his testimony to Congress, a key argument for a government bailout of the beleaguered automaker.

Only time and further rigorous testing will determine whether the battery will deliver on LG Chem's promised electrical output and durability over a 10-year span. For now, however, the Chevrolet Volt continues to roll toward its November 2010 launch date.

- Allan Yogasingam

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