Charge while poweroff

Hi, I've noticed that the device powers on each time I put it on charge, I'd like to keep it powered down so it charges faster.

I thought I'd try "fastboot oem off-mode-charge 1". I can get recover menu (home button+power button combi), but I can't connect with fastboot yet.

I'm with the 2.3.1 kernel and I don't want to root the device. Does anybody have suggestions?