Usually you use this option for extra power and launch control (for insane or ludicrous mode).
I have noticed when pulling up to a red light, if you want to smoke the sports car next to you (instead of just beating it badly), you can't get the "max" power unless you warm up the battery.
So if you are looking for some extra power immediately, you have to plan ahead as you never know when you will pull up to that fancy sports car and need the extra juice.
My suggestion: Select "max battery" after starting the car, which takes ~25-30 min to warm up.
During the warming up process, you can see your Wh/mi run much higher (i.e. closer to 500-700 Wh/mi when driving normally). Assuming you don't need the range that day.
Once the battery is warmed up you will see it say "ready!"
As you might not need the power just then, you can select to turn off "max battery" and drive for a couple miles on a lower Wh/mi, then if you pull up to that red light with a sports car next to you, then you can select "max battery" and it will immediately show "ready!", however my suggestion is you continue to toggle back and forth with turning "max battery" ON and OFF every 2-3 miles to balance your range (i.e. to give you a lower Wh/mi), although you will likely be generating 450 Wh/mi doing this while driving normally.
I later tested to turn OFF the "max battery" after it was warmed up, and drove 10 miles on the freeway and when tried to turn ON "max battery" again, it said would take 10 minutes to warm up again.
I am assuming that after warming up the batter there has to be a certain mileage with "max battery" OFF that you don't need to warm up the battery when you select ON again.
My guess is for every mile after 5 miles driven when battery is warmed up already, you will need that many minutes to warm up the battery again to get "max battery" power.
Anyone else have any observations or a formula for calculating how many minutes are needed to warm up battery?