What is a battery electric vehicle (BEV)

Battery Electric Vehicles (BEVs), compared to classic internal combustion engine (ICE) vehicles, are fairly simple and easy to operate. The simplest powertrain architecture consists in a high voltage battery, an electric motor with power electronics controller and a single speed gearbox. BEVs are also called pure electric vehicles, in order to distinguish them from Hybrid Electric Vehicles (HEVs), which have a hybrid powertrain (internal combustion engine plus electric motor).

In a BEV the propulsion is based exclusively on the electric energy stored in the high voltage battery.

Battery electric vehicle are increasing their market share because they are the most viable way towards a clean and efficient transport system. Compared with ICE vehicles, the most important advantages of a BEV is the overall high efficiency, reliability and relatively low cost of the electric motor. The main drawback is the low energy density of the high voltage battery.

The range of an electric vehicle is the maximum distance that can be covered with a “full” battery. Take into account that the range is given for a particular homologation test cycle (NEDC, FTP, etc.)

Depending on the range and maximum speed and acceleration performance, battery electric vehicles can be classified as:

• neighborhood electric vehicles: small vehicles, very low range (less than 25 km)
• city electric vehicles: small vehicles, low range (less than 50 km)
• full performance battery electric vehicles: these are the equivalent of the classic passenger vehicles, with the range between 100 and 600 km

In this article we are going to focus on the full performance (passenger) battery electric vehicles (BEVs).

Most of the BEV architecture have the powertrain on the front axle and the high voltage battery in the floor, between the front and rear axle. This configuration gives plenty of volume for the passenger area and boot/trunk.

The high voltage battery, being the heaviest electric component of the vehicle, is positioned very low, in the body floor. This give another advantage, a very low center of gravity, which improves the overall stability of the vehicle.

High performance BEVs, like Tesla Model S, have two electric motors for traction, one on the front axle, the second on the rear axle. Both motors have their own power electronics controllers. This configuration gives all-wheel drive (AWD) capabilities as well as very good performance in terms of acceleration and driving dynamics (torque vectoring).

Very high performance BEVs, like Rimac Concept_One, takes performance and driving dynamics to an extreme level. The powertrain consists of 4 motors in total, one for each wheel. Each motor has its own gearbox, in the front there are single-speed gearboxes while in the rear there are two-speed gearboxes with carbon fiber clutches. The high voltage battery is displaced in a “T” shape, between the front and rear axles. Rimac Concept_One is the first battery electric hypercar.

The energy storage component in a pure electric vehicle is the high voltage (HV) battery. The nominal voltage is, in most of the cases, between 360 and 450 V. A BEV has also a low voltage battery, the usual 12 V battery, which is used as a power supply for the auxiliary equipment (lightning, multimedia, etc.).

The battery is the key component of the EVs because:

• the range of the vehicle depends almost entierly on the HV battery
• it is the heaviest electrical component
• it is the most expensive electrical component

There are different types of high voltage batteries, the chemistry being the main distinct factor. The most common HV batteries for BEV are the lithium-ion batteries. These have also different “flavours”:

• metal-oxides (e.g. Lithium Manganese Oxide, LiMn₂O₂)
• phosphates (e.g. Lithium Iron Phosphate, LiFePO₄)

In automotive application phosphate lithium-ion batteries are more suitable because they are safer in terms of chemical and thermal hazard.

Legend:

1. power electronic controller
2. stator
3. rotor
4. single speed gearbox and differential

The torque is provided by an electric machine. In passenger vehicle applications there are mainly two types of electric motors already in use, with interest for the third:

1. permanent magnet machines
2. inductance machines
3. switch reluctance machines

It’s more appropriate to call them electric machines instead of motors because they can also generate electrical energy during vehicle braking. This mechanism is called energy recuperation/regeneration.

When the vehicle accelerates, the electric machine takes electrical energy from the HV battery and produces torque. This is the motor phase. When the vehicle is braking, the kinetic energy of the vehicle is used by the electric machine to produce electrical energy. This is the generator phase.

The main difference between the electric machines consists in the way they produce torque (permanent magnetic field from magnets, induced magnetic field in the rotor windings or magnetically conductive path in the rotor aligned with the stator field).

Legend:

1. rectifier
2. DC-DC converter
3. input filter
4. inverter

The power electronics control module has several subsystems, each responsible with a control function. When the vehicle is charged from a home electrical grid (e.g. 220 V), the rectifier converts the alternating current (AC) into direct current (DC), which is fed into the high voltage battery. The DC-DC converter is responsible with the lowering of the high voltage (e.g. 400 V) to the low voltage network (12 V).

The inverter controls the electric machine speed and torque by converting the direct current from the battery into alternating 3-phase current for the electric machine. When the vehicle is in energy recuperation phase (braking) the inverter is doing the opposite conversion, from 3-phase AC to DC.

This purpose of this article is to give the reader a brief introduction of the main components of an battery electric vehicle. In future articles we will discuss each component in much more detail.

For any questions, observations and queries regarding this article, use the comment form below.

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