I'm probably repeating a thread but this looks interesting;
Water Bottle Mounted Battery Pack
$559.00 (£345)
Introducing the TUBE PAK®
36V 12AH Lithium Battery Pack. Mounts in the triangle of the bicycle frame. Locks on and is removable. Keeps weight in the center and low. Keeps rear rack free for carry-on items. Panasonic
American site HERE
Water Bottle Mounted Battery Pack
$559.00 (£345)
Introducing the TUBE PAK®
36V 12AH Lithium Battery Pack. Mounts in the triangle of the bicycle frame. Locks on and is removable. Keeps weight in the center and low. Keeps rear rack free for carry-on items. Panasonic
American site HERE