That is a common issue at our shop.
Simple question:
Are you updating connected to a power source?
If you are not, this is normal. MAC OS X is not going to update ANYTHING critical as (any) firmware if it is not connected to an external power source. Battery is not considered by MAC OS X as a reliable power supply, therefore it will prevent ANY failure during such a critical process (that must not be interrupted in any way) by not allowing it at all.
Therefore in that case you will experience the following:
- Update will download.
- Update will uncompress and install will start.
- After installing software it will reboot.
- A gray screen will appear, then the apple will show in darker gray.
- Then a status bar will show up, eventually by about 1 second (that is the time the EFI delays to know if it's magsafe is connected or not).
- Then the process will be interrupted by itself as a safety precaution in the moment it notices it is not connected.
So next time do it normally, but TAKE CARE you DO CONNECT your magsafe to your computer before you start the process, and it hopefully will go smooth as... (think in something really smooth

he he)
Hope this helps someone.
Cheers from Mallorca (Spain)