I bought this one but there is a better one that's a little bigger and can even carry groceries in it because it folds out. This is the one I bought
http://www.amazon.com/Topeak-Trunk-B...9985495&sr=8-8

but if you are looking to get a quick release bag, you might want to get this one instead ...the panels fold out and down and I guess you can put groceries in it...it makes like two panniers on the sides
http://www.amazon.com/Topeak-Trunk-B...9996942&sr=8-4

It's been a while since I bought it. I'm pretty sure that is the right one, though. I bought the one with the biggest cubic capacity...I remember looking for the biggest one. 36v of Thundersky just barely fits into it with like 1/2 inch to spare. 48v of Thundersky won't fit in it but you might be able to put it in one of the side panniers or strap it to the top like I do now. There is a bungie strap on the top and one battery fits very nicely on top. It also comes with a shoulder strap. I took the shoulder strapped and wrapped it around the bike seat bar and it makes a nice "fail safe" device to prevent the batteries from sliding off. Not that I've had problems with it. I like being able to take it off if I decide to go to the grocery. You can put your controller in there too and just quick release both and put it in a shopping cart.
(I think Amazon.com has a deal buy both the bag and rack for $81)

The bike rack that I bought is just the explorer one...this one
http://www.amazon.com/Topeak-6310703...ef=pd_sim_sg_1

before buying, you might check below your seat to see if you have two holes because if you look at the picture, you need two holes in your frame to mount it too. On one of bikes, I didn't have those holes but I went to a bike shop and they gave me two 1/2' or 3/4" clamps ...or whatever they are called and it worked just fine. You can find those clamps at a hardware store too. Or you could just drill holes in the frame I guess.