D5100 AE Lock (Hold) question

Just wondering if this is how it works for others with the 5100.

I have the AF/AE button set up for AE-Lock (Hold)
The auto off for this is set to 30 mins.

I take a picture, it appears on the LCD. Wait a few seconds and the screen turns off.
Then I try to take a picture again but the AE-Lock is turned off.

I try locking the exposure again. Then I take a pic, but before the screen turns off I half press the shutter. This then locks the exposure once again and I can take another pic with the locked exposure settings.

Is this how it works for others? Do you have to half press the shutter to keep it locked?