I don't have experience of that particular cam but as a general rule the lower the light the slower the focus, it will be nothing to do with your battery.

Having spoken to people who do such photography they tend use fast short lenses where possible eg 50mm F1.4 seems to be popular on a Canon.

Perhaps if Donald gets a chance to return to the forum he might be able to enlighten more on what set up he used for the gig shots he recently posted.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html