External hard drive draining laptop's battery life?

I have issue with external Hitachi hard drive that is "self" powered with AC adapter. Issue is that ext. HD is drayning laptop's battery life despite being plugged to own power.

The problem is when I'm runing laptop on battery and have pluged usb 2.0 external Hitachi 1Tb drive then windows battery calculator says only 1:30 minutes left.. and keeps draining battery fast. But when I replug laptop to AC wall power the "bar" goes back to "standard" 3:30 minutes.. I don't know why this is happening because after all ex. HD has its own power adapter.

Any suggestions what to do or how to configure laptop/HDrive.

Thank's

 

Your laptop still uses battery power, because it still needs power to run the USB port (which is powered down when devices aren't connected). The USB 2.0 spec allows devices to draw 100mA - 500mA at 5V. So the moment you plug a USB 2.0 device in, you're drawing at least 0.5W and up to 2.5W.

Depending on the design of the external USB enclosure, it could either be drawing 2.5W from the USB connector plus supplemental power from the AC adapter, or it could be drawing 0.5W from the USB connector plus supplemental power from the AC adapter.

If you really want to see what's going on, download a program like BatteryBar, which will tell you the current power draw on your laptop's battery. You can do a before/after comparison, with the USB drive not/plugged in, and see how much power you draw.

 

As the above has said, your USB still uses power even if the HDD is using external sources.

 

Thank you both for replying. I'm especially thankful to *kent1146 for providing answer to exactly what I wanted to know!

"External" HDD manufacturer's should mention that in their user manual next time..