Mac 86 951

Also could be a short in the cooling fans. Do as others have mentioned, check battery voltage with car off (~12V) with car running and cold (~14V), and with car warm and fans running it should still be ~14V. Many things can cause this issue such as a loose alternator belt, or even A/C clutch failing when warm, but you'll have to find it by toggling things on and off, or cold -> warm and watch the voltage.