Connecting a MacBook to an external display and closing the lid (going clamshell) is particularly useful for showing a larger screen but minimising desk space. If you do it the way Apple tells you, however, you will have to connect your MacBook to your AC power connection at the same time. Using my MacBook with an external display for days, if not weeks, at a time with frequent small trips away from the desk, I didn’t want the potential reduction in battery life that comes from recharging every couple of hours. I’ve found a way to display a MacBook on to an external display, without charging constantly.
Showing the display
1. Connect your MacBook to the external display
2. Plug in your AC charger
3. Wait for the external display to show up.
4. Shut the MacBook lid and wait for it to sleep
5. Trigger the display to show up using your mouse/keyboard.
6. Activate InsomniaX
7. Remove the power cable.
This has the advantage of keeping the screen at full external display resolution and sleeping the MacBook’s screen. If you enable Insomnia from the MacBook whilst it’s open and then close the lid, the resolution won’t adjust to your external display and the screen will stay on inside the closed MacBook. If you do that, you’re going to lose most of the benefit of using an external display and battery life will be decreased by keeping the MacBook’s screen running.
The only downfall is the fact you have to let the MacBook start charging whilst running through this sequence. But, if you get really quick at following the sequence, the only real time spent charging is during the sleep and wake. I’m not a battery geek, but from what I know this won’t affect the battery life. It’s a little like syncing your iPhone – it doesn’t have a detrimental affect on battery life.

