The immediate question is one of packaging and designing sources to fit inside cell phones, characterized by most in the know as an engineering problem, not a redefining or surmounting of the laws of physics. Solving the problems, though, is far from automatic. And when it comes to how much the power sources will cost the typical end user, no one's sure.

The main product drivers are here now. The various "air-breathing" technologies report energy densities up to 10 times that of conventional lithium-ion and competing battery types, which typically have two consumable electrodes. In this context, fuel cells-with two nonconsumable electrodes and an external energy cartridge added as required-and the somewhat related metal-air battery technologies (one consumable electrode) appear to have an edge. Sources with higher energy densities are going to power more-intelligent devices for much longer times.

The inherent properties of fuel cells and metal-air also work to their advantage. Their always-on service suits them for today's users, who are looking for instant and full-time Internet and e-mail access. Producing such high-energy-density sources in a large package apparently isn't that daunting. Preserving that density in a battery-size form factor, however, and doing it cost-effectively, are other questions entirely.

But the activity is mounting. The Trimol Group (Toronto) recently signed a cooperation agreement with Sagem SA, a European cell phone manufacturer, to jointly develop what they term an aluminum-air fuel cell. Mechanical Technology (Albany, N.Y.) has put together a team of noted scientists from several national labs to develop and mass-produce a methanol-based micro fuel cell. And AER Energy Resources (Smyrna, Ga.) is already sampling its zinc-air batteries in Nokia cell phones.