Chipmaker Freescale has designed a USB charger that knows when to stop working.

At the Consumer Electronics Show in Las Vegas this week, Freescale will announce availability of Watt Saver, a system that eliminates the "vampire" power draw of wall chargers for mobile phones and other small electronics.

The first generation of Freescale&39's zero-draw technology is used in AT&'T phone chargers.

Even when wall chargers are no longer plugged into phones, the chargers still draw a small trickle of electricity, ranging from less than a watt for efficient chargers to over 5 watts. The Freescale system can detect when a phone's battery is full or when it's been unplugged and cut the power draw to zero.

Last year AT&T introduced its Zero Charger, available for $29.99 in stores, using Freescale's technology. Having tested the system with AT&T, Freescale is now talking to other phone and gadget companies about using Watt Saver, said Glen Burchers, director of consumer segment marketing at Freescale.

Watt Saver adds three components to a traditional wall charger, which are typically made by third-party companies rather than electronics manufacturers themselves.

There is a micro-controller which detects when a phone has been unplugged, a capacitor to power the microcontroller, and a relay to shut off power, Burchers explained. The additional components add about $1 more to the charger's cost at retail.

It's designed for wall chargers that have a USB output, but the technology can be used to eliminate vampire power in different power packs, according to Freescale.

On an individual level, the dollar savings from cutting the vampire draw to zero is not very significant--perhaps less than one dollar over the course of a year, Burchers said. But in aggregate, the energy reductions make a big difference, given that there are 4 billion mobile phones already being used and another 1.4 billion sold a year.

Freescale estimates that vampire power from cell phones is about 1,200 megawatts, which is the output of a large power plant. It plans on giving 1 percent of the proceeds from sales of the Watt Saver to a nonprofit dedicated to preserving natural resources.