Nissan North America Inc. has begun installing 30 solar-assisted charging stations at its Smyrna plant and its headquarters in Franklin. The advanced solar chargers are designed to charge the Nissan LEAF. The company soon will build the LEAF and the batteries that power it at the Smyrna plant southeast of Nashville. The company said in a news release Monday that the battery plant is 75 percent complete with an expected operational date of late next year. The solar chargers will be operational for use by Nissan employees and visitors by July 1.