Chapter 6 — EV powertrains: motor, inverter, battery and charging

An electric vehicle (EV) powertrain replaces the reciprocating mass of an internal combustion engine with three primary components: a high-voltage battery pack, a power inverter, and an electric motor. While a petrol engine relies on thermal expansion to push pistons, an EV uses electromagnetism to rotate a shaft. This process is significantly more efficient, typically converting over 85% of stored energy into kinetic motion, compared to approximately 30-40% for modern petrol or diesel engines. Understanding the modular nature of these systems is essential for assessing the reliability and performance of modern battery electric vehicles (BEVs).

01Traction Motors: Permanent Magnet vs Induction

The motor, often referred to as a traction motor, is the unit responsible for driving the wheels. In the UK market, two primary designs dominate: Permanent Magnet Synchronous Motors (PMSM) and Induction Motors (IM). A PMSM uses fixed magnets—often containing rare-earth elements like neodymium—mounted on the rotor. These magnets react to the rotating magnetic field generated by the copper windings in the stator, providing high efficiency at low speeds and a compact physical footprint. Their high power density makes them the standard choice for the primary drive units in cars like the Tesla Model 3 and Hyundai Ioniq 5.

Induction motors, famously pioneered in modern EVs by early Tesla models, do not use fixed magnets. Instead, they rely on 'inducing' a magnetic field in the rotor using an alternating current. While slightly less efficient than PMSM units at low speeds, they offer lower drag when coasting and do not require expensive rare-earth materials. Some high-performance all-wheel-drive EVs use a combination: a PMSM on one axle for efficiency and an induction motor on the other, which can be 'switched off' magnetically when not needed to reduce energy consumption.

02The Inverter: The Powertrain's Brain

The battery stores energy as Direct Current (DC), but the majority of EV motors require Alternating Current (AC) to operate. The inverter is the critical bridge between these two states. It takes the DC from the battery and converts it into a three-phase AC signal. By varying the frequency of this AC signal, the inverter controls the speed of the motor; by varying the amplitude, it controls the torque.

The inverter also manages regenerative braking. When the driver lifts off the accelerator or applies the brake pedal, the motor's role reverses, acting as a generator. The inverter takes the AC power generated by the wheels' momentum, converts it back to DC, and feeds it into the battery. This process recaptures energy that would otherwise be lost as heat through friction brakes, significantly extending the vehicle's urban range. High-voltage cables, typically identified by bright orange insulation for safety, connect these components.

03High-Voltage Batteries and State of Health (SoH)

The battery pack is the most expensive and heaviest component of an EV. Most current models use Lithium-ion chemistry, consisting of thousands of individual cells arranged into modules. These cells are monitored by a Battery Management System (BMS), which ensures each cell charges and discharges evenly. The BMS also manages the thermal system, using liquid coolant to keep the cells within an optimal temperature window, typically between 15°C and 35°C.

A critical metric for used car buyers is the State of Health (SoH). Unlike a fuel tank, which remains a constant size, a battery's capacity degrades over time through chemical ageing and cycles. SoH is expressed as a percentage of the battery's original usable capacity. For example, a car with 90% SoH can only hold 90% of the energy it could when new. Under UK consumer law and manufacturer warranties, most batteries are guaranteed against dropping below 70% SoH for 8 years or 100,000 miles, whichever comes first. Factors that accelerate degradation include:

• Frequent use of DC rapid chargers, which creates high thermal stress.
• Maintaining the battery at 100% state of charge for extended periods.
• Deep discharging the battery to 0% frequently.
• Operating the vehicle in extreme ambient temperatures without pre-conditioning.

04AC Charging: Single and Three-Phase

Charging an EV is divided into two distinct categories: AC and DC. When charging at home or at most kerbside points, the car uses its On-Board Charger (OBC). The OBC is a component within the car that converts AC from the grid into DC for the battery. This is inherently limited by the OBC’s capacity, not just the power of the wallbox.

• Single-phase AC: Common in UK domestic settings, typically capped at 7kW. A standard 7kW home wallbox will add roughly 25-30 miles of range per hour.
• Three-phase AC: Common in commercial premises and some public car parks. This allows for 11kW or 22kW charging. However, many EVs (such as the Jaguar I-Pace) have an OBC limited to 7kW or 11kW, meaning they cannot take full advantage of a 22kW post.

The standard connector for AC charging in the UK and Europe is the Type 2 (Mennekes) plug. Drivers should note that using a 'Granny Cable' (a domestic three-pin plug) is generally restricted to 2.3kW and is intended for occasional use rather than a daily charging solution.

05DC Rapid Charging and the Charging Curve

For long-distance travel, DC rapid chargers bypass the car’s On-Board Charger and feed high-voltage DC directly into the battery. These units, found at motorway service stations and dedicated hubs, typically range from 50kW to 350kW. The connector used for this in the UK is the CCS (Combined Charging System), which adds two large DC pins below the standard Type 2 inlet. Narrowly, older models like the Nissan Leaf use the CHAdeMO standard, though this is becoming less common on new infrastructure.

It is a common misconception that a 150kW charger will deliver 150kW throughout the entire session. Charging speed follows a 'charge curve'. As the battery fills, internal resistance increases and the BMS slows the intake of power to protect the cells. Typically, an EV will charge at its maximum rate between 10% and 50%, with a significant 'step down' after 80%. This is why manufacturers often quote 10-80% charge times rather than 0-100%. For optimal efficiency and battery longevity, mid-journey stops are best kept to the 10-80% window.

The transition to electric powertrains shifts the focus from mechanical wear of pistons and gears to the electrochemical health of the battery and the thermal management of the inverter. While simpler in terms of moving parts, the complexity lies in the software and power electronics. Maintaining an EV requires less routine fluid-based servicing but demands a disciplined approach to charging habits to preserve the long-term value of the battery pack.