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Lithium-ion batteries go solar racing with Solion

Solion is using VL E high energy, lightweight rechargeable lithium-ion cells to create high performance ‘plug and play’ battery packs designed specifically for solar powered racing cars.

Solion, a spin-off company from the University of the South Bank in London, is working in close co-operation with Saft to use its VL E high energy,lightweight rechargeable lithium-ion cells to create high performance ‘plug and play’ battery packs designed specifically for solar powered racing cars.

The onboard batteries, which incorporate a sophisticated electronic management system essential to ensure the safe and reliable operation of lithium-ion cells, are used to store solar energy to power the car during low light conditions as well as providing a boost in power for climbing hills.

Solar racing cars compete in two main races, the World Solar Challenge (WSC) - a 3000km race across Australia - and the American Solar Challenge (ASC) - which this year followed Route 66 for 3680km from Chicago to Los Angeles.

The exact specification of the cars varies from race to race, however a typical car is between 4 and 6m long, weighs around 230kg, has a top speed of 160km/h and can cruise all day at 100km/h.

Dr Mike Duke, Director and founder of Solion, explains the advantages of lithium-ion technology for solar racing cars: ‘A successful solar racing car requires an on-board battery capable of storing over 4kWh of power.

A lead-acid battery with that capacity weighs over 100kg, which is around half the permitted all-up weight. Yet the same power capacity can be achieved with a lithium-ion battery weighing just 30kg. This weight saving offers much greater flexibility in vehicle design as well as a vast improvement in the crucial power to weight ratio for faster acceleration and improved hill climbing ability.

Charging efficiency from the solar panels - which range from 1 up to 2.5kW - is also very important and lithium-ion batteries have a charge/discharge efficiency of 99% compared with the 90% efficiency available from even the best lead-acid batteries. Lithium-ion batteries were first used in solar racing cars in 1999, however these had to use over 900 AA size cells to provide the needed capacity, and the construction problems and reliability issues created by so many connections prompted Dr Duke to review alternatives.

Saft was able to offer the ideal solution with its VL E lithium-ion cells as their 3.6V nominal voltage, 45Ah capacity and high energy density of 310Wh/dm3 enabled a suitable 100.8V battery to be constructed using a single string of just 28 cells in series.

Dr Duke continues: ‘It was clear that the Saft cells offered the optimum combination of high-power, safety and reliability. A battery pack using the Saft VL E cells made its debut on the University of the South Bank’s 2001 entry for the American Solar Car challenge. The cells performed exceptionally well enabling us to average 600km a day, and in fact they were never fully drained during the entire race. So many teams expressed interest in our battery pack that I was prompted to start Solion to manufacture the battery packs on a commercial basis’. John Taylor of Saft UK said: ‘The increased complexity of lithium-ion cells dictates that they need to be managed more carefully than conventional batteries, especially when charging. That is why it is Saft’s normal practice to supply them only as part of a complete modular battery system. This enables us to guarantee that they have been correctly assembled and wired and will be used only in conjunction with the correct electronic management circuitry to ensure safe and reliable operation’. ‘Saft has formed a strong working relationship with Solion in which they have spent extended time with our engineers to be trained and instructed in the correct handling and assembly of the VL E cells.

In addition, we have also vetted their electronic management system. So in this particular case we have agreed to supply Solion with individual cells’. The Solion battery packs are made to order for each car as a ‘plug and play’ solution fitted with a Lemo connector which plugs directly into the car wiring loom.

Ctedit to: Saft Industrial Batteries of Bagnolet France.

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