Saft batteries play key role in SunShark success

Rechargeable lithium-ion batteries help propel budget conscious solar electric car to third place in the World Solar Challenge

The University of Queensland SunShark Solar Racing Team achieved a remarkable third place in last year's World Solar Challenge (a 3010 kilometre endurance race across Australia) with the help of Saft's high performance, lightweight rechargeable lithium-ion batteries.

The on-board batteries 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.

The Li-ion technology's combination of very high energy density and suitability for use with solar charging equipment makes the batteries much more efficient than the lead-acid batteries the team had used in previous races.

In contrast to many of their fellow competitors, who received multi-million dollar sponsorship from major vehicle manufacturers, the SunShark team had to operate within a limited budget of around US$125,000 which necessitated a car design based largely on commercially available solar cell and battery technology.

This approach earned the team the GM Sunraycer Award for Technical Achievement for 'achieving technical innovation that can likely be translated to commercially viable real-world applications'.

SunShark employs an eight metre square solar array comprising 15 panels with 64 commercial grade cells in each panel.

This array (rated at 16.5 per cent efficient) produces just 1.2 kW under ideal conditions, barely enough power to run a two slice electric toaster.

In contrast, a modern family car, powered by an internal combustion engine, will generate up to 100 kW.

On-board battery - requirement for high energy storage density and light weight With available power at a premium, overall efficiency became the guiding principle for the SunShark design team.

That is why selection of on-board batteries offering the optimum combination of light weight, high energy storage density and reasonable cost was identified as a crucial factor, particularly since the race rules limit the battery storage capacity.

In previous years SunShark had used sealed lead-acid batteries, primarily because they are both inexpensive and offer a greater life span than more exotic and high-cost, high performance alternatives such as silver-zinc batteries.

Their disadvantage is their weight, since, for the same energy storage capability, they weigh four times as much as their silver-zinc or Li-ion cousins.

Saft lithium-ion batteries - cost effective lightweight alternative to lead-acid After discussing their requirements with the Saft technical team in France it became clear to the SunShark designers that there was now a commercially available, cost-effective and lightweight alternative to lead-acid batteries available in the form of lithium-ion batteries.

The key advantage of Li-ion cells is their energy density, which is around four times that of lead-acid, three times nickel cadmium (NiCd) and twice nickel metal hydride (NiMH).

They also offer another key benefit for solar vehicle applications, which is their relatively narrow charging voltage window.

To explain in detail: when charging NiCd or NiMH cells it is necessary to apply some 1.6 to 1.7 V per cell at the end of charge, for a nominal voltage of 1.2 V.

With lithium-ion, a 4.1 V per cell charge is required for a nominal 3.6 V.

In the first case some 40 per cent more voltage is required, in the second just 14 per cent.

In other words, for the same battery nominal voltage and capacity, a larger number of solar cells would be needed to charge NiCd or NiMH batteries.

Li-ion cells offer a further advantage since their Faradic charge efficiency is practically 100 per cent, whilst NiCd or NiMH must be overcharged by from 10 up to 40 per cent.

This facilitates the use of a solar cell array with a smaller surface area.

MP series prismatic cells - extremely high capacity Saft recommended that SunShark should use its high performance, compact format MP series prismatic Li-ion cells which are used typically in a host of portable and standby applications from cellular phones, rugged computers, video and camcorders and medical equipment to military radio communications.

The MP cells' unique rectangular shape gives them extremely high capacities by making use of the volume lost when conventional cylindrical cells are packed together - a single prismatic cell has some 20 per cent higher volumetric energy density than an equivalent pack of cylindrical cells.

Saft's lithium-ion electrochemistry uses a graphite negative electrode and lithiated cobalt oxide positive electrode.

Operating temperatures range from -30°C to +60°C (in discharge); charge retention is 95 per cent after one month's storage; and cycling capacity is 85 per cent of initial capacity after 500 charge-discharge cycles, and 75 per cent after 1000 cycles.

The specific cell used in SunShark is the largest in the MP range, the MP 176065.

With a nominal voltage of 3.6 V and a capacity of 5.2 Ah this offers a gravimetric energy density of 125 Wh/kg and a volumetric energy density of 277 Wh/L.

The cell can be fully discharged at 12 A (or 40 W), while higher power pulses are possible.

The SunShark batteries comprise eight MP 176065 cells in parallel providing 41.6 Ah.

These were assembled into blocks, equipped with pole bolts and a fuse and shipped to Australia.

A total of 32 battery blocks were then assembled in series in a cradle located in the car, together with sophisticated electronic devices to control the charging process.

The battery provides a nominal voltage of 115.2 V (with a required charge voltage of 131.2 V) and 4793 Wh of stored energy.

It was designed to supply a current up to 40 A.

The battery weighs 48 kg, representing around a quarter of the car's 190 kg total weight.

World Solar Challenge The World Solar Challenge involves a 3,010 kilometre timed race across Australia from Darwin to Adelaide.

In 1999 it started on October 17, with the last of the 42 entrants finishing on October 26.

Racing commenced each day at 08.00 and finished at 17.00, with the teams camping by the side of the road where their vehicle had stopped.

This is where the charging efficiency of the solar cell and Saft Li-ion battery combination was especially important as the stationary car was tilted towards the setting sun in order to keep charging the batteries as long as possible.

SunShark completed the course in 41 hours 37 minutes at an average speed of 72.5 km/hour to claim a best ever third place after crossing the finishing line at 09.09 (Adelaide time) on October 22 - just 32 minutes behind the winner, Aurora 101 from Melbourne.

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