I've really only used CenterArt after the stock theme, and the battery meter seems to be working just fine but I haven't fully drained the battery yet so I'll have to report back if I notice anything. It could be that the values or coding in the theme files are causing issues since most of these themes were designed for the iPod. Other than that, I'm not sure what it could be but possibly something in the RB port itself. I did take the latest CWM version of RB and unzipped it, put in some custom themes (including CenterArt), the font packs and a few backgrounds and re-zipped so it's ready to be installed using the current install method (https://bitbucket.org/isergachev/rockbox/wiki/Home) if anyone is interested:
https://docs.google.com/file/d/0B_hHZnvhPk2TQ1FGNk51bDE0UHc/edit
https://docs.google.com/file/d/0B_hHZnvhPk2TQ1FGNk51bDE0UHc/edit