Can you enter clamshell mode without the laptop plugged in?

Hi,

You need to have the laptop "plugged in." Per Apple (Use your Mac notebook computer in closed-display mode with an external display - Apple Support):

Requirements for closed-display mode

To use closed-display mode with your Mac notebook, you need:

• An AC power adapter
• An external keyboard and mouse or trackpad, either USB or wireless
• A USB-C to USB adapter if you're using a USB mouse or keyboard with MacBook (2015 and later) or MacBook Pro (2016 and later)
• An external display or projector

To operate in clamshell mode you need an external USB keyboard and external monitor connected. You don't need the AC charger.