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No charge? No problem

FreePlays diminutive dynamo charges mobile phones with just a few turns of a crank

By Joseph Ogando, Senior Editor -- Design News, September 9, 2002

Hardcore mobile phone users fill with dread at the thought of wandering too far from a wall outlet or car charger. These people don't relish silence. No, without a place to charge their phones, they feel marooned, stranded. This attitude may explain the upswing in new portable energy sources for phones and other mobile devices. The Consumer Electronics Show in Las Vegas this year saw the introduction of more than a half dozen portable energy sources, including solar models, disposable zinc-air fuel cells, and hand-powered chargers. "The market for all types of alternate energy sources is sure to expand as consumers demand power anytime and anywhere," says Gregg Locher, business manager for companion products and accessories at Motorola (Schaumburg, IL). While some portable energy sources promise just a few minutes of emergency use or need a sunny day, the hand-cranked FreeCharge from the FreePlay Energy Group Ltd. (London, England) offers as much energy as your arm can provide.

The FreeCharge can provide on-th-go charge to a variety of Motorola phones. It accommodates differing physical connections and charging requirements with a set of male connectors, and each connector contains power electronics that tailor the charger to the individual phone's charging needs.

Far from the kind of cheesy, hand-squeeze device you might find in an airline catalog, the FreeCharge packs a carefully designed a crank-driven alternator, power train components, a high-capacity battery, and power-conditioning electronics into a small housing made from ABS overmolded with a thermoplastic urethane. Weighing in at 230g, the whole unit measures just 54 × 145 × 60 mm. The alternator itself, which is driven by the crank via a three-stage 25:1 gearbox, occupies an even tinier envelope of 54 mm in diameter and 11 mm high. Depending on the motivation of the person cranking it, FreeCharge yields about three to four minutes of talk time, or a few hours of standby time, for 45 seconds of cranking. Harder cranking puts in more energy, so a desperate sales executive might charge it even faster. The unit's 3.6V, 1,300-mA/hr battery system takes on a full charge after about 35 minutes of winding. And thanks to an ac adaptor that lets it charge in wall outlets, the FreeCharge can also serve as a back-up battery for those averse to manual labor.

FreePlay already has plenty of experience with this kind of mechanical energy storage system. "All of our know-how is in extracting energy from human beings," says John Hutchinson, the company's engineering director. More than seven years ago, FreePlay started out with hand-cranked radios that employed a wound spring as the energy storage device. These rugged radios, which were developed to serve remote areas of Africa, later on got rid of the springs in favor of alternators and batteries. These more sophisticated hand-cranked radios, with built-in flashlights, sold like hot cakes in 1999, driven by all the millennial doomsday fears. Hutchinson reports that the company has now sold about six million radios with the more recent sales coming from the outdoor-equipment market. With the FreeCharge, the company has adapted its proven energy generation technology to phones and other consumer electronics.

At the heart of the FreeCharge is a crank-driven alternator that occupies a space of just 54 mm in diameter and 11 mm high.

In coming up with this pint-sized electric machine, FreePlay engineers had some tough challenges to overcome. Not surprisingly, size and weight constraints drove many of the engineering decisions. And the FreeCharge's intended use also magnifies the usual concerns about the ruggedness of any portable electronics device. "Humans have a tendency to break things—especially those things designed to absorb human energy," Hutchinson says. "We had to design the FreeCharge so even the hardest cranking will not break it." Yet ruggedness couldn't come at the expense of a lightweight, efficient package, which lead to extensive use of engineering thermoplastics throughout the device. Finally, FreePlay engineers worked closely with their counterparts at Motorola to come up with a generic charger that works with the differing physical connections and charging protocols of individual phones. "Every phone has to think that it's hooked up to the wall charger that came with it," Locher says.

Making power. As with most modern alternators, efficiency and power density emerged as the two most important factors in coming up with a small yet durable generator design for the FreeCharge. An efficient system, by making the most of input torque from the user, generates more current from less winding, which makes it easier for even puny users to produce useful amount of energy and also limits the long-term wear and tear on the cranking mechanism by beefier users, Hutchinson explains. A high-power density, meanwhile, keeps the alternator's size in check without sacrificing the amount of current put out by the tiny dynamo. "We needed a powerful generator in order to charge phones quickly, but we had a very small space to work with," he says.

FreeCharge's alternator achieves these two design goals mostly through conventional design strategies. It makes uses of powerful neodymium-iron-boride (NdFeB) permanent magnets to maximize power density. And the design features small air gaps and tightly packed windings in the stator to boost efficiency, Hutchinson reports. Finally, the alternator has a multi-pole configuration to reduce cogging torque—which users otherwise experience as a jerkiness in the winding handle. Hutchinson readily acknowledges that the alternator starts with some well-known principles, but he adds this patented design has a few twists related to the reduction of cogging torque and efficient packaging. He wouldn't go into much detail, citing concerns about trade secrecy. But as one example, he says the design team had to strike a balance between the number of poles and the size of the air gap. "More poles reduces the cogging torque but additional poles create problems with the small air gaps that we wanted for efficiency's sake, "he says.

In keeping with FreePlay's weight and durability goals, all of the main power train components are injection molded from tough engineering thermoplastics—the winding handle from glass-filled nylon and the spur gears from acetal and glass-filled nylon since like materials could have caused the gears to chatter and wear badly. Hutchinson reports that the plastic gears needed small teeth to combat high tooth stresses. "The whole powertrain had to be highly torque resistant," he says. "Cranking stresses the powertrain enormously."

All of FreePlay's close attention to alternator and power train design paid off. Hutchinson puts the alternator efficiency at about 75%, or about 20-25% better than an average automotive alternator. A variety of mechanical losses in the power train and electrical losses when the ac current is rectified do push the overall system efficiency down to about 50%, he adds. Still, that efficiency is plenty for this application, which requires lots of current quickly in order to replenish a dead battery. According to Hutchinson, an average person inputs about 16-20W mechanically and the system captures about 8-10W of that input power, enough to provide somewhere between 2-3A of current. "That's plenty to charge the battery and do it quickly," Locher adds.

In the tight confines of the FreeCharge housing, though, these acceptable losses and the relatively high input current did pose one threat. "Heat dissipation is a considerable issue in such small package," Hutchinson says. In practice, even an aggressive user won't crank hard or long enough to generate a damaging amount of heat. "The phone would be fully charged before the device would be damaged," Hutchinson says. But the thermal issues did manifest themselves during Motorola's lifecycle testing—which heap on enough abuse to put the toughest winder to shame. Hutchinson recalls that early prototypes of the FreeCharge generated enough heat to soften the plastic, causing the stator to shift and forcing plastic gears out of mesh. So in order to pass the lifecycle tests, FreePlay engineers added a heat sink. "It's not for the user, but for the life cycle tests this market requires," he says. FreePlay engineers managed to craft this heat sink from the stator's steel base and metal mounting posts. Before the addition of the heat sink, they had planned to use plastic bosses to mount the stator.

Storage and delivery. Rather than trying to directly charge phones from the alternator, the FreeCharge stores energy from its alternator in a 3.6V battery system—really three 1.2V NiMH batteries. To satisfy the need for small size and to ease energy generation, the FreeCharge makes use of a 1,300 mA/hr battery that fits within the space of three AA batteries. While not the least expensive choice, this relatively high capacity battery system charges quickly when the FreeCharge operates in its crank mode. "Batteries absorb current in proportion to their capacity," Hutchinson points out.

Using batteries at all might seem to add unnecessary cost compared to a direct charging design. Yet Hutchinson argues that it's actually more efficient from an electronics standpoint and less costly to rectify the ac current and store the charge in a battery than to design the bulky, expensive electronics that could provide a direct charge to the phone within Motorola's tight specs. Plus, the battery makes the unit more functional by enabling it work without winding.

Once the battery has taken on its charge, the unit has to interface to the phone—or make that "phones." As Motorola's Locher points out, the FreeCharge had to work with more than a half dozen models with differing physical connections and power requirements. "Each phone has a different charging protocol," he says. Both Motorola and FreePlay understandably declined to reveal the specs, but Hutchinson describes them as "very precise" in regard to current, voltage, and time. "They have very tight control on current ripple," he adds. The FreeCharge meets the diverse charging requirements with the help of custom male connector system called "Hot Pod." These connectors, three in all, snap into the FreeCharge body and can make physical connection to most of Motorola's phones. The connectors also contain power electronics that regulate voltage and current according to each phone's charging requirements.

Early in the design process, FreePlay also rejected the idea of using a dc generator, which would have done away with the need to rectify the current. Though the company has used dc generators in the past and found them less subject to cogging torque, "they would have been cost prohibitive to manufacture," Hutchinson says. "For this application, alternating current fit the bill."

Cranking up the future. A small size, ruggedness, and the flexibility to address different types of phones could serve FreePlay well as it goes to the next step. According to Hutchinson, they could easily power other brands of phones, as well as other portable consumer electronics like games, MP3 players, and PDAs. Motorola started out with the exclusive rights to market the device and at first sold them only for its own phones. "But we're taking a hard look at making the next generation available for other types of phones and mobile electronic devices, too," Locher says.

There's another opportunity for the technology beyond its use as a stand-alone energy source. Hutchinson reports that the FreePlay can deliver the FreeCharge technology as a module suitable for integration into a variety of OEM designs. Hutchinson envisions military and medical diagnostic equipment for remote areas as two future users of these self-sufficient energy sources.

And FreePlay's crank-driven energy sources may get even easier to carry or integrate in the future. According to Hutchinson, the second generation of the product will be about 40% smaller than the existing model—largely from the use of planetary gears rather than the spur gears in the current model. "Of course, we can't make our products much smaller," Hutchinson says. "Or people won't be able to crank them comfortably."

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