A well-designed battery pack needs to compete with petrol-based engines to appeal to customers. That’s a real challenge because electric batteries need to overcome complex issues that internal combustion engines (ICE) do not have.
Here are some common issues that can affect a battery pack’s performance:
- Cold temperatures prevent batteries from delivering their full power. Batteries operate best within a specific temperature range, which is between 68°F and 77°F (20°C and 25°C). Maintaining the right operating temperature is essential.
- Batteries lose part of their available power over time due to natural wear. EV manufacturers need to make sure that this power loss does not affect the driving experience.
- Battery cells need to be balanced to offer optimal performance, meaning that they must all have the same voltage. Battery cells are rebalanced during charges, but they lose their ability to maintain that balance as they age. In addition, rapid charges which are gaining in popularity are challenging the balancing performance.
To better understand the complexity behind battery technology, let’s go over the following subjects:
What is a Battery Pack?
A battery pack is a device that stores electrical energy to provide power to an electrical system, such as an electric vehicle (EV) or an energy storage system (ESS). The energy is stored in cells that are all connected to one another in the battery pack.
To provide sufficient power, battery packs require a minimum voltage level which a single cell cannot achieve. Multiple cells are therefore connected in series to boost voltage. Some designs use small-capacity cells. To achieve the desired battery energy, cells are connected in parallel to boost capacity. Cells connected in parallel provide power as if they were a single, larger cell.
Battery packs are made of multiple, smaller sections called battery modules (or sub packs). These modules include a smaller number of cells connected in series and parallel. They are usually at a lower voltage, which is safe for handling. Modules facilitate servicing when only a few cells are defective and if they can be replaced without replacing the entire battery. EV batteries are typically made of 4 to 40 modules connected in series to one another.
The Components of a Battery Pack
A battery pack is the most expensive part in an electric vehicle. It is a complex system made of a wide range of components. Here are some of the important components.
- Cells are the most important components of a battery pack. The mixture of materials comprising the cell is known as its chemistry. Different battery chemistries can achieve different performances and specifications. There are two common types of cells: energy cells and power cells. There are also many variants to provide the perfect trade-off depending on the application. In the EV industry, the lithium-ion cell (li-ion cell) is the most common chemistry. Alternative chemistries are sometimes used, such as Nickel-Metal Hydride (NiMH), which offers a slightly better lifecycle.
- Electrical connectors such as busbars, wires, or other distribution conductors are used to make series or parallel connections between cells and groups of cells. These connections are typically done using ultrasonic bonding or laser welding. Busbar connections between modules can also be done mechanically using fasteners.
- Thermal interface materials (TIMs) such as pastes, adhesives and gap fillers are inserted between battery components to join them mechanically while improving thermal properties between surfaces. With the rise of the structural battery pack, TIMs are becoming essential components.
- The Battery Management System (BMS) protects cells by monitoring key parameters such as voltages, currents, and temperatures. It is responsible for cell balancing (to maintain the optimal performance of the cells at the right voltage) and communicates with several systems such as engine management and temperature control. It also includes protection devices that can shut down the battery if needed.
- The Battery Thermal Management System (BTMS) controls the thermal energy in the electric vehicle’s powertrain and cabin, providing cooling or heating as needed to meet the battery’s thermal needs and protect the cells. The BTMS includes several components such as a heat exchanger, tubes, hoses, cold plates, pumps, valves, and temperature sensors.
- The Contactor System is a switch controlled by the battery management system. It can cut off the electrical connection between the main battery and the high voltage bus, which delivers current to the traction motor and other high-voltage components.
- The Housing is a rigid enclosure that protects the battery from environmental factors such as water, dust, and salt. It helps maintain a precise temperature and electrical insulation in the battery, and it prevents damages like rust and slow shorts.
- The Communications System ensures communication with other components in the electric vehicle. The most used protocol is CAN bus.
The 4 Main Types of Battery Pack Designs
12V Battery Packs for Accessories
With their low voltage, 12V batteries are used for low energy applications such as headlights, radio systems, and other accessories. In hybrid and petrol cars, they are used to start the engine. In electric vehicles, they are used as an energy source that can function without the main electric battery (traction battery). For example, it is used to activate the traction battery and provide power to some vital components if power has been cut off for safety reasons.
Traditionally, the most known type of 12V batteries were made using the lead-acid cell chemistry and were hence referred to as lead-acid batteries. The number of cells in these packs was limited to 6. The most recent 12V batteries are lithium-ion battery packs whose lithium cells offer better performance and lighter weight.
12V batteries are small and are typically placed under the hood. More recently, manufacturers have started placing them inside the trunk to improve safety, as it minimizes chances of short circuits during crashes. Since more collisions occur at the front, the battery is better protected from impacts when it is positioned at the back.
Hybrid Battery Packs
Hybrid batteries contain a smaller amount of energy than EV batteries and are much smaller. Still, today’s hybrid batteries typically have a range between 30 and 50 miles (50 and 80 km). They can be used for most short-distance trips without having to fall back on the internal combustion engine (ICE). That’s a major improvement compared to the very first models, which offered a mere 0.6 miles of autonomy (1 km).
Hybrid battery packs are built to complement the combustion engine when it is least efficient, such as when accelerating. The goal is to diminish petrol consumption as much as possible. The battery can also recharge itself by recovering wasted energy when braking (regen braking).
EV Battery Packs
Unlike other battery pack designs, EV batteries are full-sized batteries made to supply the entire range of the vehicle, including the traction motor and accessories. Current EV batteries offer between 20 and 130 kWh of energy and can use between 90% and 95% of that energy—a much higher percentage than other types of batteries. The Mercedes EQS is the electric car with the highest range, offering 485 miles of autonomy (780 km).
EV batteries represent a significant portion of the vehicle’s weight and volume. They can weigh up to 450 kg (1000 lbs), representing one-fourth of the car’s total weight. Different designs come with high voltage going from 400V to 900V. In the most recent designs, they are being integrated as part of the vehicle’s structure.
High Performance Battery Packs
High performance battery packs are batteries designed for Formula E races. They are divided in two categories: hybrid and pure EV. They are made with composite materials to obtain an ultra light structure. Some high-performance batteries are removable so they can be replaced during races.
Even though they are small, these batteries can deliver ultra high power. More precisely, they can deliver several hundred kW of power, which is enough to output power for an entire neighbourhood. Their cooling system is oversized due to the aggressive power demand.
High-performance battery packs are more energy efficient than other types of batteries. For example, they can recover a larger portion of lost energy during braking (regen braking).
The Evolution of Battery Pack Designs
Over the last decade, battery pack designs have evolved a lot to meet the demand for better battery range. The increased interest for EVs has also allowed to streamline production through more advanced manufacturing technologies like lasers to further improve quality and lower costs.