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6 V. 6 Ah offered by ordinary Liion cells. More notable is the difference in weight: where a similar Sony or Panasonic Li-ion cell might weigh 88 g or 93 g, the A123 M1 cell weighs just 70 g. A reason for this is that the casing of the cell is made from aluminium (Figure 1) rather than sheet steel. This also has the advantage of improving heat conduction from the cell. Breaking with tradition the casing forms the positive terminal of the cell, with a thin layer of ferromagnetic material (presumably nickel) forming the actual contacts.

Lithium technology is expected to be seen in hybrid vehicles such as the Toyota Prius III from 2008. Better cycle life and considerably higher energy and power densities will enable considerable progress in hybrid vehicle technology and moves towards zero-emission vehicles. A series of projects in the USA is under way to develop ‘plug-in hybrids’ whose batteries can be recharged from a supply socket. A123 is working with automotive supplier Cobasys and, together with Johnson Controls (Varta/Saft), has agreed to work with General Motors to develop lithium batteries for a plug-in SUV.

Figure 4 shows very clearly, however, that the terminal voltage of the cooled cells initially fell very sharply and then, as the internal temperature increased, returned to the same level as if the experiment had been started with the cells at room temperature. The difference in the final temperature of the cells (47 °C versus 49 °C) is surprisingly low. One explanation is the temperature dependence of the internal resistance of the cells: considerably more power is dissipated internally when the cells are at a lower temperature.