My wife uses a Garmin Forerunner CX405 with HRM and cadence unit for cycling. I was thinking of going with a 305 or a 500, but was concerned about the battery life. Her Forerunner last about 3 hours when the GPS is turned on. My daily rides are just below or right at two hours, but my charity rides are, most often, over three hours. How's the battery life on the 500?