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Adding Generators to Your EV

This is a very frequent topic. Many people believe they can add generators to their electric vehicle and run around town for free, never exhausting their batteries.

Note: I am not talking about a gasoline-powered generator here. I am talking about mechanically connecting the generator to the vehicle in such a way that it generates electricity as the vehicle is in motion. The energy to crank the generator comes form the vehicle itself - from the electric motor and batteries or from the inertial mass of the vehicle as it is in motion.

Richard wrote to ask if he could add three 90-V generators that produced between 250 and 500 Amps each.

Here is my answer:

Richard,

You are not alone in thinking that you can add generators to replace electricity in the batteries while you drive. In fact, there is a very small car company making claims that they have such a vehicle and will make it available soon. These are scam artists.

Let's look at the issue scientifically. Assuming 100% efficiency in motors and generators, the conversion of power is 1 horsepower to 746 Watts of electricity. In your email, you mentioned that you have some permanent-magnet alternators (PMAs) that produce 250 to 500 Amps at 90 Volts. Let's use the 300 Amps you are hoping to get to calculate the actual wattage and required mechanical horsepower needed to produce it. Again, for the time being, we will assume 100% conversion efficiency.

Power = # Watts = # Amps X # Volts, So, in this case we have 300 Amps X 90 Volts = 27,000 Watts.

Now, to generate 27,000 Watts of electrical power, you will need at least 36.2 horsepower (27,000/746 = 36.2 hp). Two of these PMAs will require 72.4 hp and three will require 108.6 hp. This simply demonstrates that it takes real mechanical power to produce electrical power and vise versa.

Let's dig in now and see how much power the vehicle's electric motor is consuming from the batteries as you drive down the road. Assume the electric motor is using 90 Volts worth of batteries (15 batteries X 6 V each). Your average running current, without generators being attached, is around 150 Amps. So, the electrical power averages around 150 Amps X 90 Volts = 13,500 Watts. That is equivalent to 18.1 horsepower, assuming 100% efficiency. This is just the power that is needed to move the vehicle down the road.

With each generator you add, the power to drive them is going to come from the electric motor and the batteries. Now think carefully here, let's say that you add one generator to produce 150 Amps of current to replace what the motor is using. Sounds good so far, but wait - to produce the 150 Amps of charge current, the generator will require 90 Volts X 150 Amps = 13,500 Watts and 13,500/746 = 18.1 horsepower. So, the generator is requiring 18.1 horsepower from the electric motor and the batteries. Now, the electric motor must produce 18.1 horsepower to keep the vehicle moving plus 18.1 horsepower to drive the generator - that's a total of 36.2 hp that must come from the batteries.

Can we simply 'ask' the generator, or generators, to produce more current? Well, I think by now you are getting the idea - if the generator is asked to produce more current, it will take more horsepower, which comes from the motor and from the batteries.

What you are seeing here is that the generator can never make up for the power it takes to move the electric vehicle down the road.

What is more, neither the electric motor nor the generator are 100% efficient, which means you need much more than 1 horsepower to produce 746 Watts of electrical power and vise versa. In most cases, it's more like 1.2 horsepower to produce 746 Watts (as applies to the generators), where 1 horsepower produces only 634 Watts of electricity, assuming 85% efficiency. Looking at it from the Watts point of view (as applies to the electric motor), it would actually take about 878 Watts of electricity to produce 1 horsepower from the motor. This all means that you experience compounded losses that cannot be made up in the batteries-motor-generator circle of life.

Regenerative Braking

After having mathematically explained the above, in some cases it is still useful to have some kind of generating capability. Those are cases in which you wish to replace 'some' electrical power as the vehicle is slowing down to eventually stop or going down a long hill. This is called regenerative braking. Braking action is the result of the mechanical horsepower that the generator is demanding from the moving mass of the vehicle - it works to slow the vehicle down as energy is being removed from the moving mass.

Most hybrid vehicles today use regenerative braking to replace 'some' of the battery energy. This is a computer-controlled operation to ensure the smoothest and most efficient recapture of energy. Regenerative braking helps extend your range a little, not a lot. In your case, it may be useful to mechanically connect (toothed belt or chain) one of your PMAs to the back auxiliary shaft on the electric motor, assuming your electric motor has one. The electric motor I use does have one - the Advanced DC 9.1" motor #4001A.

You will need to add a high current diode and contactor between the generator and the battery bank. The diode should be a 150 V, 400 Amp Schottky diode available from DigiKey Corporation. The heavy-duty contactor is the same as used to connect the battery bank to the controller when you step on the gas peddle. Now you have two heavy-duty contactors, one for the controller and motor and the other for the generator. These two contactors must work opposite each other. When you press on the gas peddle, the motor contactor slams in and the generator contactor releases. This ensures that the generator does not generate electricity, and act as a powerful load, while you are trying to accelerate and cruise. When you take your foot off of the gas peddle to slow down, the motor contactor opens up and the generator contactor closes so the generator can pass current through the diode to the battery bank. The diode is needed to make sure that current will not flow from the batteries back through the generator - yikes. As long as the generator voltage is greater than the battery bank voltage, current will pass from the generator through the diode to the batteries. By the way, for this to work, you must leave the transmission in gear. In neutral, the electric motor and attached generator will simply stop as the vehicle continues to coast with no regenerative braking.

But wait! Why not use the motor as a generator to provide regenerative braking? Well, you can, but sometimes it is difficult. It is a complicated operation to get a series-wound electric motor to do that. It can be done, but it is very cumbersome. I decided it is not worth it. On the other hand, permanent magnet motors or parallel-wound motors make good generators. Unfortunately, there are not many available that have the starting torque to make good road-worthy full-size electric vehicles. In hybrid vehicles, the electric motor is specifically designed to also work efficiently as a generator to provide regenerative braking.

Best regards,

Mark