USB 3.0 Data Sync Charger Cable Fast Charger Data Cable for Asus Eeepad TF101 TF101G TF201 SL101 TF300 TF300T TF301, 1m/3.3feet

while your asus transformer prime is plugged into the keyboard dock it will always use a battery from the keyboard dock and then transfer power to the tablet when it can do as you can see in this demonstration i have run out of keyboard and power so what i can do is unplug the tablet and plug the keyboard dock into the mains and even over tablets not plugged into the keyboard dock it will still charge the keyboard dock and i can carry on using the tablet this is useful if you want to leave a keyboard dock at home to charge while you take your tablet away on your travels