Plugging into the grid

The PRIUS+ project from CalCars (link to PDF) adds extra batteries extending electric-only operation up to 34 MPH and providing extra (as compared to normal Prius battery pack) electric assist during normal hybrid drive. To get the improved "gas millage" (they also figure the cost of electricity used to recharge the batteries into this calculation) 300 lb. of batteries must be added. With lead-acid batteries used in early PRIUS+ model, this added weight gives you 10 electric-only miles. Anything past that is hybrid miles. Using Li-Ion batteries extends the all-electric millage to 35 miles. BTW, these miles also have to be at speed at or below 34 MPH.

Let's say that taking out the gas engine and supporting systems will reduce the car weight by 30%. With the same Li-Ion battery pack, the all-electric range increases to 45.5 miles (all driven at or below 34 MPH!) between recharge stops, which cripples the car's usefulness in my opinion.

To get beyond the 34 MPH limit, I image you'd need a bigger electric motor, which increases the weight. The current Prius electric motor delivers a tiny 26 HP.

Anyway, there's trade-offs and engineering compromises all over the place here depending on the expected use of the vehicle.

Reasons for the internal combustion engine are 1. helps accelerate the car to higher speeds (where the electric engine becomes inefficient) and 2. allows for indefinate range (gas stations easy to find; place to plug your car in hard to find.