[MattInMD]

Agreed, the ICE will NOT run to attempt to drain off excess battery power. If the prius did need to do so, It could always dump it into the electric motor.

That said, where would "excess" battery power come from? If the battery is full, the computer stops using regenerative braking and uses the brake pads. You can never get "excess" battery power, unless your computer malfunctions.

That said, it's important to know the energy display only shows when energy is being TRANSFERED. ie: it shows the state of the transmission. It does not show when the engine is running. If the engine is running, but the transmission to it is in neutral, it will show no arrows.

The computer often does this if it thinks you will wind up needing the gas engine really soon, or if it hasn't warmed up yet.

As long as the car is warmed up and the battery is charged, when you hit the brakes the engine will usually shut down right away. Read up on the web tips on "stealth mode" to understand this better. (note: this isn't a mode you select by gears, it just means your actions on the gas and brake have caused the computer to shut down the ICE).

However, it's also important to know that the prius WILL NOT stop the engine from spinning if you are doing over 42mph, and generally won't do it over 35mph. (42mph is the absolute top-speed the electric motor can propel the prius to when working alone). At this speed, the prius cannot react to your accelerator by using the electric. In order to ensure that the engine is ready if you suddenly need to hit the gas to avoid getting rear-ended, it keeps it going.

However, just because the ICE is spinning, doesn't mean it is burning gas. The prius can choose to let the pistons spin and shut off the fuel injectors. This adds drag, so it usually doesn't do it for long unless you're rolling downhill, but it does do it.