Another issue you need to consider is charging the batteries while driving. Both batteries need to be charged as some point while driving (both the 12 V battery under the hood for using lights, listening to the radio, and possibly charging and cell phone; and the NiMH battery used to propell the hybrid drive.) As we all know, you can't charge a battery being used to drive the vehicle. The NiMH battery is either being charged (regen braking or by the ICE) or it is being drained (propelling the vehicle.) The electrical system can't do both at the same time. However, the hybrid system can. It can drive propel the vehicle while charging the batteries. The FEH's ICE runs the AC for the 300V battery and the AC for climate control. If the ICE in unable to run, neither AC unit will turn on. I am also guessing the same it true for the Prius.

 

I ran my Prius II out of gas once. I got 66.5 miles, from the time the fuel gauge beeped. I got about 1.5 miles when the ICE died and had to run EV only.

 

The AC for the Prius II is electric, and runs off the HV battery, so that won't quit if you run out of gas, although if you do it would be a good idea to turn it off to minimize it's impact on the power drain. The Prius doesn't use the AC directly for battery cooling, it draws air from the passenger compartment into the batteries and then vents the hot air outside. The 12v battery is also charged from the HV battery and not directly from the ICE running like normal vehicles. The HV battery computer won't let you discharge below 30% SOC to protect the battery, normally it likes to keep it above 40% as a minimum, but in extreme situations it will drop to 30%.