Introduction to Electric Vehicle Battery Systems

The fundamental piece of any electric vehicle (EV) is its battery. The battery must be designed to satisfy the requirements of the motor(s) and charging system that a vehicle utilizes. 

This includes physical constraints such as efficient packaging within the vehicle’s body to maximize capacity. As the main contributor of weight in an EV, designers must also consider the battery's placement within a vehicle as it can affect power efficiency and vehicle handling characteristics (which is typically why you'll often see batteries placed under the floor pan of the vehicle).

Here's an overview of some of the specifications, safety considerations, and management systems involved in EV battery design.

 

EV Battery Specs: Voltages and Capacities

An electric vehicle battery is often composed of many hundreds of small, individual cells arranged in a series/parallel configuration to achieve the desired voltage and capacity in the final pack. A common pack is composed of blocks of 18-30 parallel cells in series to achieve a desired voltage. For example, a 400 V nominal pack will often have around 96 series blocks (as in the Tesla Model 3).

Common nominal pack voltages in current vehicles range from 100 V-200 V for hybrid/plug-in hybrid vehicles and 400 V to 800 V and higher for electric-only vehicles. The reason for this is higher voltages allow more power to be transferred with less loss over the same diameter (and mass) of copper cable.

 

An example EV battery system with individual cells in series.

 

The drawbacks of higher voltages include the necessity for higher-voltage-rated components in the entire system. They also prevent the use of DC fast-charging stations of a lower voltage without incorporating some type of DC-DC boost converter in the onboard charger.

Common battery capacity ranges, on the other hand, are as follows:

• Hybrid vehicles: 0.5 to 2 kWh
• Plug-in hybrid vehicles: 4 to 20 kWh
• Electric vehicles: 30 to 100 kWh or more.

 

Safety in EV Batteries: Contactors (and Pyro Fuses)

The battery represents multiple safety challenges when it comes to design, as well as regarding the high voltages permanently present within them.

Fuses are present inside the battery pack before the output connector, often on both the positive and negative side. Special high-current, sealed relays known as contactors connect the internal fuses to the battery itself.

 

A series of Panasonic EV relays/DC contactors (left) and a breakdown of a contactor's structure. Images used courtesy of Panasonic

 

Contactors incorporate features such as sacrificial contacts to prevent increasing resistance due to contact pitting. They also often incorporate an auxiliary contact to detect internal welding which may occur if the contactor is intentionally or unintentionally opened while a large current is passing through it.

The contactor coil power supply is usually passed through an HVIL, or high-voltage interlock loop, which loops through all the high-voltage components in the system alongside the high-voltage cables (usually incorporated into each connector). Thus, the contactor cannot receive power to close unless all the high-voltage connections are securely plugged into the battery.

A pre-charge contactor closes before the main contactors to allow a small current to flow into the system through a large resistor. This limits the inrush current into all the large capacitors in the system and allows the battery management system to detect short circuits before the high-current path is completed.

Isolation is continually monitored, usually on both sides of the main contactors. A fault will occur if the isolation from either side of the high-voltage system to the chassis drops to less than 500 ohms per volt.

Tesla also incorporated a new safety device into their Model 3 and newer packs, known as a pyro fuse. This device can be blown open by a small pyrotechnic charge if the contactors become welded, which allows them to use less robust contactors. A discharge resistor and contactor are sometimes included across the output of the battery to allow the system to be actively discharged to a safe voltage after shutdown.

 

EV Battery Monitoring PCBs

The battery’s blocks of cells need to be monitored and kept in balance and specialized circuit boards are included within the pack to perform this task. These boards must include an isolated communication interface, as each board’s ground reference will be hundreds of volts different from each other and from the main battery management system.

These boards monitor the voltage and temperature of each block as well as the temperature of the interconnects between blocks. They also contain small groups of resistors to carry out the task of balancing.

The blocks of cells inside the pack must be kept within a few millivolts of each other to allow maximum power to be transferred in and out of the pack. Due to natural differences in the manufacturing of the cells, some blocks will charge or discharge slightly faster than others. To combat this, during charging, balancing is performed, which drains a small amount of power from the highest voltage blocks to bring them close to the others.

These block monitoring boards also provide an additional safety feature of the pack. They allow the temperature of the cells and interconnection points within the pack to be monitored very precisely. In cases of damaged cells, this means that a fault can be raised before serious damage or even possibly a fire can occur.

 

Battery Management Systems 

Finally, the battery management system, or BMS as it is commonly known, manages monitoring and controlling all aspects of the battery pack.

Current shunts report various information to the BMS, including the total charge transferred in and out of the pack. Voltage measurements before and after the contactors allow monitoring of the pack system voltages. Contactor control and economizer circuits manage the contactor closing and minimize static current through the coils after the contacts have pulled in.

The BMS is also in constant communication with the block management boards to monitor cell voltage and temperature and control balancing.

 

Reference design block diagram for a 400V battery pack. Image used courtesy of Texas Instruments

 

Overall system and connector temperatures are monitored to detect any high resistance connections caused by loose connectors or bolts.

System and pack isolation are also continuously monitored, and other potentially redundant safety features can be incorporated. The BMS also exposes a communication interface to the rest of the vehicle—often over either automotive ethernet or CAN bus—where it communicates with the inverter, charger, and other systems. It calculates and provides charge and discharge current limits, pack state-of-health and state-of-charge, and notifies other systems when the contactors must open so, ideally, they can open without a load present.