The kilograms of equipment that soldiers need on their missions can quickly add up: On a 5-day-operation, disposable and rechargeable batteries alone account for about ten kilograms, not to mention the various charging devices for cell phones, PDAs and visual systems. A hybrid power supply will soon be able to lighten the load. “It consists of the power management system, a heavy-duty rechargeable battery, a methanol fuel cell, and the fuel itself,” says Dr. Jens Tübke, who led the development at the Fraunhofer Institute for Chemical Technology ICT in Pfinztal – under contract to the German ministry of defense and its military test center for land-based vehicles. When the rechargeable battery is empty, it can be recharged from the fuel cell. And that’s not all: “The special features of this power supply are its variable input and output voltages,” explains Tübke. “As an alternative to using the fuel cell, the rechargeable battery can also be recharged in other ways – for instance from a solar panel, a 1.5 volt battery or a conventional wall socket.” To make that possible, the researchers have equipped the power management system with three different inputs – one for low voltages between 1 and 10 volts such as mignon or mono batteries, one for the medium range from 5.5 to 30 volts, such as fuel cells or solar panels, and a third one for high currents and voltages between 9 and 45 volts, for example a wall socket. The solution is based on a battery management system and DC-DC converters that scale the voltage up or down – from the 1.5 volts of a disposable battery to the 12 volts needed for the rechargeable battery, for example. To eliminate the need for different charging devices, a charging connection is integrated in the power supply. There are two outputs to ensure that small devices such as cell phones and PDAs can be recharged while laptop computers and radio equipment are also in operation. The researchers have already built a prototype. “The system could go into service with the German army in about a year’s time,” Tübke estimates. By then the scientists will also have installed an SMBus controller. This automatically switches off the fuel cell as soon as the battery is fully charged, instead of merely putting it in standby mode. The controller also switches off the fuel cell when the power supply is additionally connected to a wall socket.
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