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Old 05-22-2008, 02:15 AM   #2 (permalink)
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Originally Posted by fizzleation View Post
This is a bit random, but and article on engadget sparked my curiosity...
Switched On: Green Plug tries to replace the worry warts (Part 2) - Engadget

The Green Plug they talk about turns off the flow of electricity to the device being charged once it has full recharged the battery. This is more efficient than charging with a wall adapter because those continue to feed power to the device even after fully recharged. This energy goes to waste.

So, here is my question: When charging via USB port, does the port lower the power being sent once the device is fully charged? I would assume some power is needed to keep the connection with the pc, but is that less than is sent out by a wall adapter?

here are some quick facts about USB ports and the Blackberry A/C charger...

USB Port: the average USB port is limited to 500mA, yet many USB 2.0 ports can offer 1000mA (1A) current if the device demands it...such as USB powered WD Passport drives. most smaller lighter laptops only offer 500mA ports, and almost all others have the ability to provide 1000mA per port. the voltage is 5V (+/- 0.25V).

Blackberry A/C adapter: transformer steps down the voltage from 120V down to 5V, or in terms of current it takes 0.2A (or 200mA) from the outlet and boosts it up to 0.7A (or 700mA). the adapter will send that exact current to the blackberry (700mA)

When you charge a device via the USB port, the charge is referred to as a "trickle" charge. a trickle charge is exactly what the word means...the charge trickles in and doesn't charge as fast as an A/C adapter. the reason behind this is the current draw limitations in that even though the blackberry when charged via the A/C adapter uses 700mA, it does not need all 700mA to charge. thus, if you use the USB port, the port will only provide the maximum the port can provide. the blackberry is not USB powered, and the computer can detect that and will not exceed the 500mA maximum for un-powered USB devices.

Thus, here is the breakdown of charging when comparing the USB Port method and the A/C Adapter method:

USB Port: 500mA --> 71.43% current of the A/C adapter (slower charging time)
A/C Adapter: 700mA --> maximum current (faster charging time)

USB hosts on computers will not "cut off" power to the USB port even if the USB host suspends itself, due to the fact that there are too many complications in getting the USB host out of the suspend state when power is cut off. i really don't want to go into specifics, but if you want to go with the "greener" method based on solely charging, then the USB method would be better since it charges the device slower and once the device is charged, it uses less "extra" power than the A/C adapter (500mA vs 700mA). However, you need to keep in consideration the power to keep your computer running overnight. if you normally don't leave your computer on, leaving it on just to charge your blackberry defeats the purpose of trying to charge using a "greener" method. if your computer is usually turned off at night, then you will want to use the A/C adapter method. another consideration is to take into account the extra power that your computer needs to draw in order to charge your blackberry. if you have a large desktop, then charging a blackberry won't affect the overall relative power draw of what your desktop is already pulling in. however, if you are using a laptop and charging the blackberry, the laptop may need to draw extra power which will result in more heat loss which will decrease the efficiency of the "greener" method you are trying to follow. also, i am not sure what the effect on the Lithium Ion battery using the USB method will do if used for an extended period of time. the battery charge/discharge quality testing of the battery design might have been based on using the A/C adapter at 700mA.

it's a tough call, but i'd say if you want to go green and your laptop/desktop is already on during the day and night, then use the USB method.

Last edited by JGBerry : 05-22-2008 at 02:17 AM.
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