No second screen on battery power ?

My does run a second display while on battery - 17" MBP from 2009 with 9400M.

If you still have warranty you should have Apple take a look at it.

 

it won't turn it off after unplugging, but it won't start if your MBP isn't plugged in to start with.

 

Okay, I just tested it again and I run on battery, unplugged the monitor and plugged it back in and it took a bit longer (two to four and a half seconds) to actually recognise the monitor compared to when running the MBP on AC, but right now I run a second monitor while on battery.

Btw, I use a MDP to VGA adapter, if that is worth anything.

 

From: Apple Portables: How to use your computer in closed clamshell (display closed) mode with an external display

 

Erm ... I don't see anything in the OP to suggest that he is talking about closed clamshell.

cheers,

Henry

____
Read the message carefully before you post

 

It'll work on battery but will drain substantially more energy.
I don't think it's normal that it switches off, but I haven't tried with 10.6.5 yet.

 

And we were supposed to know that ... how?

The ol' rule still applies: the more information you provide, the better advice we can give.

cheers,

Henry

 

The OP got their answer. That's all that matters.

 

I use my 15" i5 on battery most nights running video on a 42" HDTV using mini displayport to HDMI and it always works fine.
It does seem to use a lot more power though (understandably I suppose).