The best way to ensure that the battery does not get depleted is to remove it entirely from the powershell and store it separately after it has been fully charged. There is no way to turn off the flow of charge from the battery to the Z1 if they are both in the powershell together. Additionally, the powershell itself will draw a small amount of charge from the battery and drain it in 2-3 days.
Note: Make sure that your battery is actually charged fully before disconnecting it from the power supply. The Z1 screen will stop flashing when the battery is fully charged. It will start to show "full bars" when the charge is at 75%. Please wait till it stops flashing.