I could not find any article in Xbox.com explaining the console charge mode, so here it goes:

If the console is turned off while a wireless controller using a rechargeable battery pack is connected to the console via a Play & Charge cable, then the console will enter a low-power mode where it keeps passing enough energy to charge the battery pack. During this mode the light of the play & charge cable will be red and you may still hear some fan noise. Once the battery pack is fully charged, the light of the Play & Charge cable will turn off and the console will enter stand-by mode, where the fans are no longer running.