(Source: Chicago Tribune)In the last three years or so, companies have been developing solar-powered portable chargers that soak up sunlight and store the energy, ready to give your gadget a jolt when the "low battery" light starts blinking. The products that have been rolling out over the past three years vary in size _ some fit into a purse or messenger bag, others can clip onto a backpack _ and range from $30 to more than $100. They come with a handful of adapters that fit hundreds of phones and other devices.

CONSIDERING A SOLAR-POWERED CHARGER?

Keep this in mind:

_ They work best with small gadgets, such as cell phones and iPod-like media players. They don't deliver enough juice to power a laptop.

_ They are meant to charge your device while you're not using your gadget or to extend your battery life just long enough to wrap up a phone call or hear a few more songs on your music player.

_ The solar panels store energy faster when in direct sunlight, preferably with the panels tilted toward the sun.

So, as the sun moves through the sky, you should reorient your solar panels for maximum efficiency. Bear in mind, though, that it's not a requirement. They will collect power even on overcast days, just not as fast as in direct sunlight.

_ You have to expose these chargers to the sun for 12 to 48 hours to give your gadget a full charge, but you can get up to 20 minutes' use _ enough to make an emergency phone call _ after charging for an hour.

_ They don't work solely on sunlight. You can plug them into electrical outlets or a computer's USB port to store up juice faster than they can via the sun's rays.

_ Before buying a portable charger, check the manufacturer's Web site to make sure it will work with your gadget. Though the charger comes with adapters that fit many devices, yours may not be among them, or your device may need more energy than the portable charger can deliver.

_ The time it takes a portable charger to fully charge your gadget depends on the charger and on your gadget's power requirements. A charge time of six to eight hours is not uncommon.

_ As we increasingly demand more from our portable devices, newer ones demand more and more power, so the charger you buy today may not kick out enough energy to sustain the gadgets you buy next year.

_ All solar-powered chargers are not created equal. The most useful models deliver enough juice to fire up your gadget and keep it going, and they have their own replaceable battery, so when the charger's battery is used up after 500 to 1,000 charges, you can simply replace the battery instead of throwing away the charger. The best chargers also should be able to store a lot of energy in reserve.