Abstract: Mechanically and thermally improved rechargeable batteries, modules and fluid-cooled battery pack systems. The battery is prismatic in shape with an optimized thickness to width to height aspect ratio which provides the battery with balanced optimal properties when compared with prismatic batteries lacking this optimized aspect ratio. The fluid-cooled battery pack includes; 1) a battery-pack case having coolant inlet and outlet; 2) battery modules within the case such that the battery module is spaced from the case walls and from other battery modules to form coolant flow channels along at least one surface of the bundled batteries; and 3) at least one coolant transport means. The width of the coolant flow channels allows for maximum heat transfer.