A couple of days after you first start running FruitJuice, it begins to calculate a “target battery time” for each day.
This is the recommended amount of time to run off of battery power for the day.
It looks at the average usage time (on battery or plugged in) for the previous 7 days and then takes 20% of that average to generate the daily target.
The target is a minimum amount of time to spend on battery. Beyond that, you are free to run on battery or plugged in as your situation dictates.
The “Battery Time for Today” is just that: a running total of time spent on battery today.