Energy. Captains of industry drill for it, Einstein figured out the relativity of it, Richard Simmons obviously traded his soul for it—even Digital Slobs manage to summon enough of it to get out of bed four, maybe five, days a week.

But our gadgets always need a particular electrical version of it—and only it—regularly spoon-fed via specially designed spoons. This will remain the case for a while, unless Apple suddenly unveils a new line of pep-talk-bootable iPhones (actually, innovations that power devices via air waves or even the energy we generate as we walk are closer than you think—just not in-Wal-Mart-this-Christmas-Eve close).

For last-minute gift hunting, then, these facts on the ground present both a challenge and an opportunity.

Typically, the last name standing on your gift list is usually the hardest to buy for. But while the gadget-lovers in your life might have everything, keep in mind their everythings still need things like these to keep them going:

iCarPlay (monstercable.com, $80): This cool gadget plugs into your car outlet and lets you hear any iPod or iPhone wirelessly through your car speakers, while at the same time charging it or almost any other USB device. Problematic wireless transmitters for mp3 players have been around awhile, but this one automatically scans the FM frequency and offers presets to find the best, static-free station to pump out your music.

Medis Xtreme Power Pack (fuelcellstore.com, $50): The problem with most portable electronic power sources is how not-so-portable they are. Usually, you still need to tether them to a hotel wall outlet, a car socket or, in the great outdoors, some other bulky power generator. Solar-powered chargers are more travel-friendly, but can also be more finicky with less-than-optimal oomph.

This unique green device, however, gets its power from itself, as the first-of-its-kind consumer fuel-cell charger. Squeezing down and shaking it triggers an internal, mineral-based chemical reaction powerful enough to charge an iPhone.

Once it is used up, you can mail it back for recycling.

Moxia Energy USBCell NiMH batteries (USBCell.com, $18): Every time I travel, I forget I need AA batteries, and stand in line at the airport terminal newsstand waiting to be gouged. That gives me plenty of time to kick myself for forgetting to bring my rechargeable batteries, but then, who has room for that bulky base station in their carry-on?

These rechargeables, however, solve all these problems. Hiding under their removable tips are USB connectors, allowing you to plug them into any hipster Digital Age power source (i.e. laptop) for a charge before placing them into your device du jour. Soon the site will also offer AAA, 9-volt and iPhone batteries based on the same premise.

It can take a while to charge them, and the charge doesn't last as long as with other rechargeable batteries, but at least you can walk past the convenience store in Terminal A with some modicum of self-respect.