Thinking electric

1 August 2022

ICE-powered vehicles still vastly outnumber EVs, but that will change

Electric vehicles can be a challenge to those accustomed to thinking in terms of internal combustion engines. While this is most common for those outside the automotive trades, some non-electrical auto trades are also unfamiliar with the rapidly emerging subject of EVs, and the terms defining them. This basic outline will help. Batteries are fundamental to the subject, so we’ll start there.

A cell is the individual unit of electrical storage/supply. It has a nominal voltage. A battery is an assembly of cells combined to create the desired system voltage. Lithium-ion cells are currently the most common in EV applications. Lithium has high energy density (energy stored per unit mass) and also a high nominal voltage of 3.7 volts. This means fewer cells need to be connected in series to achieve a desired voltage. In turn, this means, for a given volume, more cells can be connected in parallel for higher current and greater power.

There are three main cell designs used in EV applications. Cylindrical cells are the type that most commonly come to mind. Tesla uses several thousand cylindrical 18650 cells in its battery packs but is also transitioning to the newer and bigger 2170 cylindrical cell. Other manufacturers like BMW use prismatic cells, although it’s suggested BMW will move to cylindrical cells by mid-decade. BMW also uses pouch cells, along with Volkswagen.

Why the differences?

Cylindrical batteries are cheaper to manufacture, and they have high thermal efficiency. Prismatic cells have high cost because of their cladding, lower thermal efficiency but better packaging efficiency than cylindrical cells. Pouch cells have high packaging efficiency but lower thermal efficiency. Their cost is medium because they don’t have as much casing as prismatic cells. Both prismatic and pouch cells are subject to swelling because of their large flat areas. Cylindrical cells are also subject to swelling, but their form better resists the strain. Cylindrical cells are stronger in general.

Lithium-ion cells can undergo more charge/discharge cycles than other battery chemistries generally suitable for EVs. In the name of accuracy, it must be said the internal components of a lithium-ion cell don’t undergo a chemical change during charging and discharging. Managing the charge/discharge profile of a battery pack can extend the basic cycle life. Reducing Depth of Discharge (DOD) to about 80 per cent of full capacity will greatly extend service life. This is one reason battery management systems (BMS) are so important. Another is cylindrical cells can vary slightly in capacity and this variation, small though it is, must be managed.

Passive and active balancing are the two fundamental methods of equalising the charges between cells. In passively balanced systems, charge is reduced in cells with higher voltages than others. The charge removed is wasted by dissipating it through resistive circuits. Active balancing removes charge from higher voltage cells and transfers it to cells with lower voltages. This is a more expensive method but it’s also more efficient. The BMS controls cell balancing.

Cell capacity is the amount of charge a cell can hold. It’s expressed in Ampere-Hours (Ah). As an example, a 10Ah battery can in theory deliver one amp for 10 hours. C-rate represents the discharge rate of a cell per hour. The C-rate will be 1C when the discharge rate equals the amp-hours specified. So, if a 10Ah battery can be charged/discharged at 10 amps per hour the rate will be 1C. If it can only be charged/discharged at five amps per hour, the C-rate will be 0.5. If it’s charged/discharged at 20 amps per hour, the C-rate will be 2.

During discharge, the anode is the negative electrode while the cathode is positive. This is what’s almost always meant when mentioning anodes and cathodes. However, confusion can arise because, during charging, the terms are reversed. The anode is positive and the cathode negative.

Voltage, current and temperature indicate the state of a cell. These are monitored by the BMS and indicate the State of Charge (SOC), which simply refers to how much charge a battery contains at a particular moment. There are a number of methods of measuring SOC. The State of Function (SOF) indicates how much power is available from a battery at any moment. 

These factors, measured over time, indicate the State of Health (SOH) of a battery throughout its service life, which in turn indicates how much charge a battery can hold. This obviously indicates what the range of a vehicle will be.

The BMS also controls the temperature of the battery pack. The temperature of a cell or battery must be held between certain limits. This is to ensure greatest efficiency, optimum charging times, best power availability, safety and maximum service life. Coolant is routed between the cells for this reason. Thermal runaway is a particular concern because of the danger of fire. On the other hand, battery packs operating in cold conditions are heated to maintain an acceptable temperature. BMSs can be centralised or distributed. In a centralised system, all the hardware is in one enclosure. In a distributed system, hardware is distributed throughout the battery pack according to function.

EV battery packs have to be charged. Level-1 charging draws power from a standard AC wall socket. In Australia, that’s 240V and 10A for a calculated supply of 2.4kW (240x10). A Nissan Leaf has a battery capacity of 62kWh. Dividing that figure by 2.4kW gives about 25 hours for a full charge – if that level of charge can be maintained. However, it’s important to remember a BMS won’t allow a pack to be fully discharged or fully charged, so charging a pack within the allowable range will take less time. Also, if the usable capacity of the pack isn’t exhausted, then the charging time will be further reduced and could be completed overnight. In a larger EV, like a Tesla with perhaps a 100kW pack, things will take longer. Again, though, if the full capacity of the battery hasn’t been exhausted then an overnight charge from a wall socket might suffice.

Level-2 charging of several kilowatts can also be obtained from a single-phase supply through a special wall-mounted EVSE (Electric Vehicle Supply Equipment) unit, which allows a direct connection from the switchboard via an isolator. This arrangement passes more of the total power available to a household than a standard 10A wall socket/RCD (Residual Current Device) combination and results in faster charging. The EVSE unit also controls the amount of current the car can draw depending on supply.

Going one step further and utilising a three-phase supply of 400-415V will provide much quicker charging again, although such a supply is still classified as Level-2. This is the most common type of connection used at public/propriety charging stations. AC supplies have to be converted to DC for the battery pack. Such conversion takes place in the vehicle through the OBC (On-Board Charger). On-board chargers are needed because the specifications of batteries vary considerably across manufacturers. Trying to make charging points that cater to them all just wouldn’t be possible.

DC fast charging is a step beyond Level-2 and is the most rapid charging system available. It’s commonly referred to as Level-3 charging, although this is not correct. As an example, a Tesla Supercharger station uses 480V DC and can supply up to 150kW per car. Although rapid charging is convenient, in many cases it can cause degradation of the battery pack if used all the time. In fact, software within the car usually only allows a couple of fast charges and then specifies a Level-2 charge to promote rejuvenation of the electrodes. Charging, in general, has to be carefully managed. When low on charge, a battery can take relatively high charging currents but as it approaches maximum capacity the charging current must be reduced. Further, not all EVs are compatible with higher charging rates.

Charging and discharging of an EV battery pack is controlled by the BMS in conjunction with a DC/DC controller, which varies the DC voltage to and from the pack. Additionally, EVs also have standard 12V batteries to supply conventional components like lights, indicators, entertainment units, etc. The DC/DC converter supplies 12V to such components. This eliminates the costs developing new components to use the higher voltages would entail.

AC electric motors have two main components. These are the stator and the rotor, just like an alternator. Despite the wide range of electric motors available, the Permanent Magnet Synchronous Motor (PMSM) is the most widely used in current battery electric vehicles. It’s quite similar to an AC induction motor in that it operates on three-phase AC power, and as with an induction motor, the stator has rotating fields. At least this is the common description; the reality is that the fields are turned on and off extremely rapidly around the circumference of the stator windings. They only seem to rotate.

The rotor has embedded within it very strong rare earth magnets that create the rotor’s magnetic fields. The magnets in the rotor are attracted to the rotating fields in the stator and follow them, causing the rotor to rotate. A feature of the PMSM is that the rotor magnets become locked to the rotating stator fields, and they turn at the same speed, or, synchronously. PMSMs provide high torque at low speeds and can maintain the same rotational speed under varying load (within design parameters).

Induction motors are also used in EVs. Indeed, the Tesla Model 3 and Model Y use a permanent magnet motor to drive the rear wheels and an induction motor to drive the front wheels, although newer versions are reported to have a PMSM at the front. Specifically, the Tesla PMSM is actually an IPMSynRM (Internal Permanent Magnet Synchronous Reluctance Motor). Internal because the magnets are inside the rotor rather than on the outside and reluctance refers to the rotor’s tendency to achieve a low reluctance state. That is, it creates magnetic fields that interact more favourably with the stator fields for slightly increased efficiency.

As with induction motors the rotating speed of the stator field of a PMSM can be regulated by vector control. Vector Control is a term relating to the control of AC induction motors. Basically, it means altering the frequency of the AC supply to the stator coils in a particular way. We intend to run an article detailing motor control shortly. For now, suffice to say lower frequencies mean the fields will rotate more slowly. Higher frequencies will cause the fields to rotate at greater speed. The stator fields attract the rotor fields, so slowing the former also slows the latter and vice versa. This is the most commonly used method for governing the speed of an EV induction motor.

Apart from basic form, motors are operationally defined by power and torque. These both have continuous and peak values. Operating a motor at peak values will result in heat build-up and damage. Higher continuous power and torque can be designed into a motor but doing so would call for a bigger, heavier and more expensive unit. Applications demand compromise and EV motor design for any particular application probably represents about the best compromise.

Electric motors for EVs spin much faster than the wheels they drive, so some sort of gear reduction is needed. Again, we turn to the Tesla because it’s the one about which we know the most. The reduction occurs in two steps. First, there’s an intermediate gear shaft that’s much like a normal layshaft. It meshes with and is driven by the drive gear on the rotor shaft. Then, the intermediate shaft meshes with the ring gear on the differential. The whole assembly gives a reduction of about 9:1. EVs only need the one ratio. A differential is still necessary because there’s only one motor driving each pair of wheels, although the latest Model S Plaid actually has three motors, two at the rear and one at the front.

One of the other main components of an EV is the motor controller. This vital component takes DC power and converts it to AC power of the required frequency, amplitude and phase. There’s a great deal to say about motor control but as we mentioned earlier, Australasian Automotive will be covering it in another article shortly. 

Words: Paul Tuzson.

As featured in Australasian Automotive August 2022

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