Fantastic little tool! One of those things where the more you use it, the more you find jobs for it to do.
It gets through the small batteries very quickly if you're doing continuous work with it, but being small, they also recharge really quickly, so it's not too much of an issue. Rather than getting the bigger battery which would restrict the tool's movement in tight spaces, I'd suggest getting a third small battery and rotate them, which should see no interruption in work.