Think of what a second battery of the original's size would look like strapped in your bird. So all things being equal (for battery sets) more time = more batteries = more weight

Not all batteries are created equal- some rated for higher C and capacity get quite small when compared to others in the same voltage and capacity, and they can get expensive. I prefer to find a deal on some matched sets out there on the internet based on best price alone

I hope that helps...

T