I have a Quickie S646-SE (same as just the 646 but with faster motors) that I use for daily use and wheelchair tennis. I often get asked about my batteries and what works best. I have had really good luck with the DCM0075 batteries from Interstate Batteries.
They last at least a day even with lots of travel. I usually charge every night, but can let it go two nights if I forget or am being lazy.
You want to make sure you get a mobility-specific battery, essentially a sealed gel cell, deep-cycle battery for this chair. What I like about Interstate is that they have a ton of different batteries so no matter what wheelchair you have they can usually set you up. I know several medical vendors that get the batteries from Interstate then tack on their medical markup, so you might as well just go straight to the source.