Good Morning,
I'm on my second EBW1150 (sold the first like a fool) and always get at least 20 hours run time with backlight on first or second step. The most recent firmware has made the meter more accurate but the manual states that to get the most accuracy, the ebook needs to on while charging. This may not help you since I fear that your battery may be on it's way out.
You may be able to buy a replacement battery from ebookwise or from the manufacturer, ebooktechnologies.com. The battery is a 6 cell Li and should be treated with respect as far as handling it outside the ebook. Here in the States there are outfits that rebuild battery packs at a resonable cost, maybe you can find a source locally.
I've never tried the batch conveter from ebooktechnologies myself so can't help there but you may want to give the programs at
http://www.processtext.com/index.html a look see. I use Amberlit to convert .lit files to .RB or FB2. Put the .RB files in the ebooks to be converted folder and Librarian should ask first the do them all in a batch. I use the .FB2 files in Bookdesigner 4.0 when I want to do a bit of cleanup. Amberlit and it's cousins do not correct formatting and will produce a unreadable ebook if the original file has errors.
Also, check out the wiki,
https://wiki.mobileread.com/wiki/E-book_conversion for a pretty comprehensive list of whats out there.
Michael