Miles....

I just charged my batteries for the 2nd time since March and I have almost 2000 miles since then.

I just replaced my broken Garmin 510 with a 530. The 530 does have a display for batter level but it only shows the RD battery. I noticed that it dropped 1 out of 4 bars so I checked the RD and the green LED was now red. So I swapped FD to RD and did went on my ride. When i got home I decided to charge both.

Obviously the RD battery will drain quicker than the FD. The 530 will tell you how many times you shift each and since I like on Long Island, relatively flat I can have 2000 shifts on the RD during a 50 mile ride and 0 FD shifts. But even if you don't shift, there is a drain when in motion. They tell you when transporting the bike you should remove the batteries to save the charge.