I'm using a Polarmate extended battery. I think it's about 12 hours or so, not sure. I haven't used it all day in some time now.
You can check them out here:
I have only two gripes with the Polarmates: first, mine has a really short cord for plugging into the tablet, so you have to keep it very close. It's about seven or eight inches! The one I saw in the ad had a spiral cord (like a telephone), but the unit I got didn't. Second, if you use it while you have your regular tablet battery in the tablet, and if the tablet is less than 80% charged, the alarm on the Polarmate will sound--- very loud and annoying. So you have to pop out the internal battery when using the Polarmate, which could be trouble if the Polarmate cord pops out entirely. It will shut off your system.
Other than that, it's not too heavy and lasts forever.
Given that M1400 batteries are likely to be obsolete in a few years (who will make replacements?), going with some external source may be required.