I bought my ebike two months ago. It has a bag that attaches to the seatpost and saddle, in which the battery is stored. The bottom of the bag is starting to tear so it's only a matter of time before the battery will fall, and it will most likely happen during a ride.
I am looking for a new bag that attaches to the top tube of the bike (this is a folding bike, so there's no triangle in the frame, like BionX batteries do. There are two reasons for that: one is that the rear wheel has the motor, so I think it's best to place the 2.5KG battery on the front, and two is that I'd like to easily be able to remove the battery bag from the bike when I lock it (so far, I've been carrying the whole seatpost + saddle, which is big. A battery bag alone would fit inside my messenger bag).