When is the battery really full?

Just drive it, the car is smart enough to take care of these kind of things, the more I treat mine like a regular car the more I enjoy it.

 

it is never full or empty, and you don't have to be concerned, it takes care of itself. if you want to concentrate on something, study the threads on mpg's and practice improving them every time you go out. it's addicting!

 

+1 Just enjoy the car.

 

As everyone has already said, just drive it!

But to answer your question, when the MFD shows "full" (all green) you are around 80% actual SOC on the HV battery. When the MFD shows 1 purple/pink bar (it is never completely empty in READY) it is around 40% SOC on the HV battery.

You would need an external charger to bypass the Prius safety limits and charge past 80% which is highly unlikely, unsafe, and not worth anything.

It is however possible to get below 40%. If you run out of gas, the Prius will allow you drive on the traction battery alone in EV mode to limp to the side of the road. However if you count on this and try to drive a mile or two to the nearest station, you may make it, but your SOC will more then likely dip way below 40%. This is bad for the traction battery. Worst case scenario is you deplete the pack to about 12% to 15% which is too little to spin MG1 and thereby start the ICE. Now you have a chicken and egg problem. You need to recharge the battery which only happens with the ICE spinning, but your battery is too discharged to spin MG1 to start the ICE. The only solution is to recharge the pack externally which Toyota can do with a specialized wall charger that is flown in from Japan on an as-needed basis for $$$$.

 

^ Moral of the story: stretching tanks is a fool's game. Really, it's not improving your mileage, it's just a numbers game.