convert ac to battery

Hi….i have a project to convert my old surround sound system to work in my car. It runs on 240v AC. There is a transformer that converts this to 12v AC……on the coil there is a label that shows; “SEC:11.5v — 0v — 11.5v - 1.7A” and directly below that (and in line) “YEL — BLK — YEL”. Obviously...