Scotty,

My "Sheila" (2004 Ford Explorer Limited) has the 4.6L V8 with an automatic transmission. About 164k miles. As I was getting off the interstate yesterday to bring my son to a park, the car beeped and a warning came up that said something like "Check Power Equipment," followed with a battery warning light. As we got closer to the park, I watched the battery go from close to H down to just above L before we parked. I had it towed to a shop and they'll get back to me tomorrow on what might be going on. 

Is it safe to assume it's the alternator going out? Or could something else be the problem here? Any recommendations on questions to ask the shop will be greatly appreciated.