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GM cars use plastics in power storage

By David Vink
Posted 19 November 2012

The batteries in General Motors' Opel Ampera and Chevrolet Volt electric vehicles contain more than 40kg of polymers. Matthew Carroll, engineering group manager at General Motors in Michigan, the US, revealed details of the EVs' rechargeable energy storage system (RESS) at the 2012 VDI plastics in automotive conference in Germany in March.

The RESS is made up of nine automatically assembled lithium ion 18- and 9-cell plastic battery modules, which account for 22% of battery system weight. These hold the battery cells in position, transfer pressure to the cells, provide heating/cooling, electric connection, cell tension sensors and integration of temperature sensors.

In each RESS, 27kg of polymers is used in the end frames and 135 repeater frames (made in BASF's polyamide 66 grade, Ultramid 1503-2F NAT, which is 33% glass filled and hydrolysis stabilised), the nine interconnect boards (ICBs) and the housing. Soft alignment with pins, tabs and slots are used together with brass compression limiting inserts and EPDM seals to assemble the frames into battery blocks, in preference to full snap-fit engagement.

The total amount of glass fibre reinforced PA66 used in the frames and the ICBs is 18kg.

Yazaki North America supplies the ICB. Its manufacture involves insert moulding a metal cell connector (1.4kg PA6 per RESS) on a vertical 300-tonne Nissei machine. The company uses DuPont's Zytel 7335F grade of PA6 for the base plate and connector housing, as well as Zytel 70G33 HS1L (35% glass filled PA66) for overmoulding the ICB.

Mann+Hummel uses a 420-tonne KraussMaffei electric drive injection moulding machine to mould repeater and end frames in a four-cavity mould. Each cavity contains a Kistler 6157 pressure sensor to optimise control of injection pressure. Statistical analysis of injection pressures in the range 48.5-49.7 MPa over the four cavities has shown standard deviations for individual cavities of 0.38-0.40 Mpa.

M+H monitors coordinate measurement machine (CMM) values on 35 consecutive samples from each mould cavity, ensuring a maximum 0.125mm dimensional tolerance window is respected. Some parts are made on an Engel moulding machine and others on a machine from a Japanese company not named in Carroll's presentation.

Carroll said the 9.7kg watertight RESS housing is the heaviest part, accounting for 17% of battery system weight. It is moulded in lightweight BD 840V vinyl ester resin based sheet moulding compound (SMC) from Ashland Chemical. This contains 7% hadite nanoclay filler and 40% glass fibres that are 25mm in length.

The housing is covered in a reflective metal foil for electromagnetic shielding. The use of nanoclay filler has reduced density from 2.0 to 1.5 g/cm3 and cut 1.13kg from the part weight reduction. The coefficient of linear thermal expansion is equivalent to that of steel.

The SMC contains a styrene-based low profile additive that is acceptable so far. However, Californian AT-PZEV (Advanced Technology Partial Zero Emission Vehicle) regulations may require a switch to a styrene-free formulation in future.

Continental Structural Plastics, of Huntington in the US, supplies the silicone-sealed watertight SMC housing. The housing, which has 73 openings, is moulded in an open-to-close time of 180s and an overall cycle time of 4min, allowing for cooling and flash removal. Ashland Chemical designed the mould tooling, which was made by Century Tool & Gauge in the US.

Another part, a 150 µm thick pouch cell, is supplied by LG Chem. Plastics account for 10% of its weight, with PET on the outside, polyamide coating of the gastight aluminium enclosure, a ceramic coated polyolefin SRS (safety reinforced separator) and two tie layers. There is a polyurethane foam separator between each pouch to deflect compressive forces (around 50% compression at 20 kPa) and their sheet metal heat exchangers are coated with a Mylar bioriented PET film layer.

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