motor with peripherals. A typical 273 volt battery pack as used in the Toyota Prius Hybrid (up to 500 volts with the use of the inverter) weighs around 53 kilograms (117 pounds) in the latest version. They are typically of a type called nickel-metal hydride. There is a newer and more efficient type of battery called lithium-ion, but the cost, so far, has been prohibitive for automotive applications.

The Prius battery stack consists of 38 prismatic NiMH modules connected in series. It delivers a nominal 273.6 Volts and has a 6.5 Ah capacity. The modules are stacked side by side and then compressed together in a rigid, non-expandable structure that prevents expansion from internal pressures. The complete battery pack consists of the battery stack, enclosure for structural support and airflow, battery electronic control unit/monitor, relays, and safety switch. The weight of the complete battery pack is 53.3 kg. The pack is horizontally positioned in the trunk of the vehicle, partially under the back seat. Power electronics (inverter, DC-AC converter) are under the hood and a blower for moving air and associated air ducts are in the trunk. Discharge power capability of the Prius pack is around 20 kW at 50% SOC with regenerative capability of 14.5 kW at 2C. The power capability increases with higher temperatures and decreases at lower temperatures. Active thermal management can improve power capability at lower temperatures

The electric motor and controller is a weighty item. As an example, a 100 horsepower UQM electric motor and controller weigh around 163 pounds. Then, of course, you need an internal combustion motor to “top up” the batteries as they are discharged. If you were to use a very efficient 60 horsepower motor (this motor would only need to cover the cruise power requirements, not peak power) to take care of this function, figure in at least 80-100 pounds. So add it all up and you have around 350 pounds of powerplant to provide 100 horsepower worth of thrust. Not very compelling compared to 240 pounds or so for a plain old Continental O-200. If you take my car, a Honda Insight Hybrid, as an example, the weight of the battery pack and powerplant has been offset by extensive use of aluminum chassis elements

and plastic or composite body panels. Our airframes are already pretty light, so it is hard to compensate for the weight of the hybrid system in the same way the auto makers do. However, there is an offsetting weight reduction in that, for the same airplane range, you are likely to need less fuel to fly the mission. So perhaps you carry around 50 pounds less fuel in your two-seater.

THE BOTTOM LINE?
The hybrid (as we know it today) is not likely to make the transition from automobile to light aircraft anytime soon.  

FUEL CELLS
A real possibility for general aviation? You have probably read about several efforts to adapt fuel cell technology for aviation. So what is a fuel cell? Well, with a typical PEM (proton exchange membrane) fuel cell, hydrogen and air go in and electricity and water come out.

A typical PEM fuel cell from Intelligent Energy, UK

PEM (proton exchange membrane) fuel cell, hydrogen and air go in and electricity and water come out. 

Let’s grossly simplify things and say it's like a battery that is constantly recharged by putting in hydrogen. It is almost twice as efficient at generating power as an internal combustion engine, hence the level of interest and investment by the auto companies.  

Boeing has been researching the use of fuel cells and has targeted the APU (auxiliary power unit) as the place where a fuel cell might make sense in commercial airliners. In order to investigate the challenges of using this technology in a flying aircraft, Boeing is building an airplane known as the Fuel Cell Demonstrator. Based on the airframe of the Diamond Dimona motorglider, the Demonstrator serves as a flying laboratory to investigate the integration issues necessary to make a fuel cell function in flight. The aircraft is expected to fly by the year 2007. 

The Boeing Demonstrator is configured with a type of hybrid powerplant (not exactly the same as previously described), with a fuel cell providing enough electrical