Stretchable batteries are disclosed. Various designs and embodiments of the present disclosure are included. In one "stack and slide" design, solid flat Li ion batteries are stacked and encapsulated with a protective and stretchable plastic material such that they slide as the battery assembly is stretched inplane and retain electrochemical functionality. In another "out-of-plane spring" design, a multi-layer solid battery is designed in the form of a flat spring such that it can stretch out-of-plane. In another "particle battery" embodiment, solid conventional electrodes are replaced with particle grease or composite so that the active battery materials can stretch.