no de baterias de coche no, de moviles cámaras etc...... Finding the right battery charger for your cellphone, digital camera, camcorder or PDA among the sea of anonymous black boxes that clutter our homes is a complete pain - as is finding a free socket when they all need charging at the same time. But this modern hassle could be short-lived. Instead of each device needing its own charger, it may soon be possible to recharge phones and cameras by placing them on a plastic pad the size of a mouse mat. A dense array of coils buried in the pad will transmit energy to the gadget to charge its batteries. And it will be possible to charge as many gadgets as can fit on the pad at once, the inventors say. The idea has been a long time in gestation. Splashpower, a spin-off company from the University of Cambridge, UK, has been promising to launch its SplashPad charger for the past three years. Now it has filed a series of patents showing how it will work. "It was a tougher challenge than we initially thought," Splashpower's co-founder James Hay admitted last week. "This is ground-breaking technology and that takes time." On the face of it, this seems like an odd statement, as the principle is not new. Rechargeable electric toothbrushes, for instance, work similarly and have been around for years. These gadgets contain a coil that sucks power from a companion coil housed in a separate unit and connected to the mains. And the idea of charging from a flat pad is not new either. While checking one of Splashpower's applications, the UK Patent Office found an electronics enthusiast's website that describes a wireless charger in which an electrical device (a wireless mouse) can be recharged by placing it in a precisely defined location on a mouse mat. Magnetic fields The Splashpad is different, as the devices being charged can be placed anywhere on the pad, and several can be charged at the same time. It has also dealt with the problem of ensuring that the magnetic fields it generates are low enough not to erase any nearby credit cards or video tapes. Splashpower's patent applications (WO 2004/038887, for example) reveal how the firm has achieved this. Inside the pad, an array of coils spread a low-power magnetic field low and wide over the pad's flat surface so that devices anywhere on the surface can intercept charging flux (see graphic). The pad has numerous flat primary coils embedded under the surface. The coils can be of different sizes and shapes: rectangular, circular or ellipsoid. Splashpower-compatible cellphones, digital cameras or camcorders will have a thin, flat receiver attached to them or inside their casing. The receiver is a sheet of magnetic alloy, the size of a stick of chewing gum, with a coil wound round it. Current induced in the coil when it is on the charging pad is then fed to the device's charging circuit. Splashpower hopes to license the technology to makers of portable devices who want to build power receivers into their products. It says six companies are evaluating its system, though it will not say which they are. The company's promotional material show photos of Nokia and Palm products. Though no firm licences have been signed, "it will definitely happen", Hay says. "We could see the first drop-and-charge devices before the end of the year."