If it is sunny, just turn off the backlight. The diodes still function and you can read the 820 display without the backlight perfectly well in the sunshine.
The only time I've found an 820 to have problems with battery life is when using a Varia radar tail light. As you might expect, the constant communication and audible alerts and display eat battery worse than anything else I've seen. With the Varia going and the display on bright, I've seen as little as 5.5 hours battery life from an 820. But without the Varia, I've had more than 12 hours battery life from the same unit.