Multi-cell battery

A flexible battery includes a first substrate layer with a first cell portion, a second cell portion, and a bridge portion connecting the first and second cell portions. An electrical bridge electrically couples a first electrochemical cell to a second electrochemical cell in series or parallel, and an electrical bridge is flexible and extends across the bridge portion of the first substrate layer. A second substrate layer is connected to the first substrate layer such that both of the first and second electrochemical cells are separately sealed. The flexible battery is configured to be folded over itself along the bridge portion such that the first and second electrochemical cells are arranged in a covering relationship. Optionally, an open gap area is disposed over the bridge portion of the first substrate to facilitate folding the flexible battery over itself along a line extending through the bridge portion.