My RTL510, connected to my Edge 1030, lasts well beyond a century ride without a problem.
I would set the display light to 0%. Every vehicle appearing from behind is activating the display light and that is a real battery drainer. My Edge 520 battery is not the greatest anymore but Varia does not cause too much additional drain without backlight on.
Varia Radar has probably some impact on battery drain for sure, but I guess it is not main reason... my friend has Edge 820 and he is also experiencing massive battery drain, altough he doesn't have neither Varia radar, nor any other sensors, just Edge