Yamaha Virago 125

Hello, I have the following problem. This morning everything started. Without a problem a track was driven and then on the way back, the Baterie Death was pushed, then. Until then again good 20km drove, but the Baterie did not charge at all (also highway ride and from time to time the light turned out during the ride) My tension goes into the cellar as soon as you give gas. Was just in the garage and measured, at idle almost 11,5V, as soon as I give gas under 5V. Have a regulator u and an empty Baterie removed from my butcher and installed there and even worse. Of course I had to push to start. I drove the few km only with max 20 km/h, otherwise the moped went out. mfG mario