A retaining structure for a power supply may include top and bottom rails connected by columns, which carry loads that would otherwise be applied to the power supply. End plates may be attached to the rails to inhibit movement of the cells in directions parallel to the top and bottom rails. The rails may have a cross section configured to carry at least some of the loads applied to the battery pack. For example, at least a portion of the cross section may be configured in a shape that acts as a spring and deflects upon loading.