How is it powered ?

Basic facts about electric vehicles to answer questions often asked by people new to Battery Electric Vehicles
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timpootle
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How is it powered ?

Postby timpootle » Fri Nov 06, 2009 6:56 pm

Electric vehicles are powered from a very high capacity traction battery via an electric motor. To give an example of the relative capacity of the traction battery, the battery in a Berlingo Electrique is 16 KWh. This could power a 2 KW heater for about 8 hours and is 26 times the capacity of a normal 12V car starter battery (600Wh, 0.6kWh)
Tim Crumpton

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Night Train
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Postby Night Train » Fri Nov 06, 2009 8:03 pm

The choice of battery to use is also important.

Flooded Lead Acid (FLA) batteries come in 2v cells and are available in 2v, 6v, 8v and 12v packages and are heavy. They have been in use for a long time and are a known and robust technology in traction use. However, for short range and lower speeds they can be cost effective and give good performance. The type of FLA used in EVs are designed for deep discharge so they can give a lot of current for a long time. This is unlike the starter batteries in petrol or diesel cars which are designed to give a very short burst of high current for only a few seconds to start the car. In use the FLA battery pack is used at 50% depth of discharge meaning that only half the capacity of the pack is used to power the car. Exceeding that can shorten the life of the batteries.
Used well FLA cells should ahve a life of around 2000 charge cycles and would probably be replaced after a 3 year life.

Lithium Iron Phosphate (LiFePO4) is a more recent development of early lithium batteries. They come in nominal 3.2v cells and can be packaged to for any multiple of that as required.
LiFePO4 cells have the advantage of being lighter then FLA, also they can operate at 80% depth of discharge so more of the stored energy can be used without damage to the cells. This make them ideal for longer range and higher speed vehicles.
The downside is that LiFePO4 cells cost 3-4 times as much as FLA.
That said, the LiFePO4 cells have a projected 10000 charge cycles and a 10 year life though they have not been in use long enough to prove that yet. The latest projections for LiFePO4 life is even in excess of that.

Other battery types have also been used for electric vehicles but the FLA and the LiFePO4 will be the more commonly specified.

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Night Train
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Postby Night Train » Fri Nov 06, 2009 8:50 pm

Following on the electric car converst the electrical power in the batteries to mechanical power via an electric motor.

The motor comes in a number of varieties, not all are suitable for vehicle traction.

The first big difference in motors is whether it is alternating current (AC) or direct current (DC).

The most commonly used motor for a conversion is the DC motor. This runs on current that only flows in one direction in the circuit. This makes the design and development of the conversion both simple and relatively cheap as DC motors and motor controllers are available off the shelf suitable for use in a car.
The types of DC motors are generally series, shunt and permanant magnet.
In the series motor the current flows through the field windings to make the magnetic field in the case of the motor and then continues through to the armature of the motor, the part that spins, before flowing back to the battery. Therefore the field and the armature are in series.
In the shunt motor the current flows through the field and the armature windings in parallel and so the field shunts the armature.
In the permanant magnet motor the field windings are replaed by powerful magnets and so the current only needs to go through the armature.

Each type of motor has a different charateristic and use. The most common in an electric car is the series wound DC motor. These motors give a good start up torque and can run at high speeds. The speed is dependent on the voltage applied and so they have good speed control. They are also found in old fork lift trucks and are easily adapted to use in a car.

The shunt motor is generally not used as most controllers are designed to make best use of the series motor. The shunt motor works best producing a steady and consistant speed at any load but will keep on pulling higher and higher current in order to maintain that speed. This can lead to them overheating unless the controller is designed to work with it.

The permanant magnet motor is a more recent introduction to vehicle use and the most common are very efficient 'pancake' style motors. These motors are short, large diameter disc shaped motors where as the series and shunt motors are longer cylinder shapes.

Alternating Current motors are more often found in high performance electric cars like the Tesla. The AC motor is very good for use in a car as it can produce a lot of power for its size over a wide speed range.
The down side of AC motors is that the controller has to convert DC current into AC current. The AC current is also 3 phase so that would be three lots of AC current each cycling 120 deg apart. The current is also not at the 50hz that domestic supply runs at but at around 400hz. Then the controller has to vary the voltage and frequency to control the speed of the motor. All that makes it complicated and expensive and there are not many AC controllers available to the DIY converter.

The up side of AC motors is that they are more efficient and can also regenerate energy that can be easy fed back to the batteries to increase range. This regeneration is not easy to do with the series motor but is achieveable with the shunt and permamant magnet motors.


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