I'm considering buying a 2004 Prius with 104,000 miles. I took it for a test drive in the mountains and as I began to climb in elevation the battery level gradually dropped to 1 bar and speed slowed from 60 mph to 40 and running on gas engine only then red triangle indicator in dash came on. I stopped and headed back down the hill. Battery charged back up to about 3/4 and red light went off. Can anyone tell me what this means?