Electric vehicles (EVs) are at the forefront of automotive innovation, and building a reliable and safe battery pack is essential to their performance.
Whether you’re working on a custom EV or converting a traditional gas-powered car, incorporating the latest safety measures into your battery design is critical. Below, we’ll outline the best practices for constructing a safe EV battery pack, with an emphasis on ventilation, thermal management, electrical isolation, disconnect systems, and real-time monitoring.
Maintaining the right temperature range is crucial, as it enhances battery performance, extends lifespan, and ensures safety. Each battery cell operates best within a specific temperature window; straying outside this range can lead to reduced efficiency, shorter battery life, or even safety risks like overheating.
There are two primary methods of thermal management: passive cooling and active cooling.
Passive cooling uses natural methods such as heat sinks or airflow to dissipate heat. This approach is simpler, requires less energy, and works well for smaller systems or less demanding conditions. However, it’s less effective during heavy use or in extreme climates.
Active cooling, on the other hand, is more advanced and involves using liquid coolants or other methods to actively regulate temperatures. Active systems are particularly important for high-performance or larger electric vehicles, where batteries generate more heat under heavy use. These systems are highly efficient, providing consistent cooling even under extreme conditions, which in turn helps to prevent overheating and thermal runaway.
Typically, a series of chill plates is installed inside the battery pack and connected to pumps, radiators, valves, and sometimes heaters or phase-change cooling devices (such as air conditioning). The EV control system manages these components to optimize the battery’s temperature, ensuring it stays within the ideal operating range.
Ventilation is a crucial aspect of EV battery design. Battery packs consist of numerous cells, and the air inside can expand or contract as the vehicle moves through different temperatures and altitudes, so it is important that there is a way for pressure to equalize. Additionally, in the rare case of a failure, such as thermal runaway—a condition where the temperature rapidly escalates—batteries can release gasses. To mitigate this risk, it’s essential to include ventilation systems that control gas release and prevent pressure buildup. This ensures that internal components remain intact even under abnormal conditions.
In most cases this ventilation function is accomplished by a dual-stage vent.
Dual stage vents offer important features like sealing out water and contaminants, balancing pressure, and releasing damp air to prevent condensation. In case of thermal runaway, they quickly release gas. The dual-stage design handles both pressure balance and emergency venting, keeping the battery safe and secure. This is crucial for battery safety, reducing damage risk, and extending battery life.
Safety is critical when working with high-voltage systems, especially during maintenance or emergencies. To protect both technicians and the system, High Voltage Battery Disconnects (HVBDs) and Manual Service Disconnects (MSDs) are used to safely isolate the battery from the rest of the vehicle’s electrical system, ensuring no voltage is present during servicing.
HVBDs are usually installed in easily accessible locations, allowing technicians or first responders to quickly disconnect the battery in case of an emergency. This prevents electrical hazards, like short circuits or electric shocks, during routine checks or repairs.
MSDs provide a manual way to cut power to the high-voltage system by removing the component. These disconnects are often designed with a locking mechanism to physically break the circuit, providing a clear visual indication that the battery is safely isolated. MSDs are particularly useful for routine service, as they allow technicians to ensure the system is fully de-energized before performing repairs, reducing the risk of accidental electrical contact. Furthermore, they also often have HV fuses and HVIL built into their design.
Both HVBDs and MSDs are essential safety features that protect not only the people working on the vehicle but also the vehicle’s electrical components, preventing damage from unexpected voltage during maintenance or emergency situations.
The High Voltage Interlock Loop (HVIL) is a key safety feature in EVs, acting like an electrical alarm by monitoring all high-voltage connections through a continuous loop. If a connector is unplugged or damaged, the HVIL circuit is broken, which will shut down the high-voltage system. This can be especially important in custom EV builds, where factory-level safety checks may be limited.
By integrating HVIL, high-voltage components only function when all connections are secure, reducing the risk of electric shock and adding critical protection for both users and technicians. It serves as a failsafe, automatically disabling the system if any connections are compromised.
Fusing is another critical safety element in EV battery design. Fuses protect the battery pack from overcurrent situations that could lead to overheating, fires, or damage to individual cells. Ideally, fuses should be placed as close as possible to the power source to provide maximum protection. When an overcurrent is detected, the fuse will blow, effectively cutting off the circuit and preventing further damage.
Selecting the right fuse for your battery system is crucial. It should be rated to handle the expected current levels while also acting quickly in case of overcurrent, ensuring that the system is protected from any sudden power surges or faults
Proper isolation monitoring ensures that the high-voltage system remains safely insulated from the vehicle’s low-voltage components. This protects against potential electrical shocks and short circuits, which could cause severe harm or damage to the vehicle. Isolation monitors work by continuously checking the insulation between high- and low-voltage systems, ensuring that any faults are detected early before they escalate into more significant issues.
The standard insulation resistance value for an EV is 500 Ohm/V—a measure that ensures safe separation between electrical systems. By catching insulation problems early, isolation monitoring can prevent costly and dangerous failures that might otherwise go unnoticed until a critical moment.
An example of effective isolation monitoring is the Thunderstruck Ground Fault Monitor, which detects insulation faults in high-voltage EV systems. It continuously monitors for any unintended electrical connections between the high-voltage system and the vehicle’s chassis or ground. If a fault is detected, it alerts the user, allowing for quick action to prevent potential hazards like electric shocks or system damage. This type of monitor adds an extra layer of safety, ensuring any insulation issues are caught before they become serious.
To protect against environmental factors like dust, moisture, and debris, EV battery enclosures should adhere to IP (Ingress Protection) ratings. These ratings define the degree of protection the enclosure offers against environmental contaminants. For EV battery packs, IP67 or higher is the standard, as it guarantees that the pack is both dust-tight and water-resistant for short periods.
IP67-rated enclosures protect the battery pack from exposure to the elements, which is essential for long-term safety and performance. By sealing the pack from environmental contaminants, you prevent corrosion, reduce the risk of short circuits, and ensure that the battery continues to function optimally in various conditions.
The Battery Management System (BMS) is the nerve center of your EV battery pack, acting as the primary safety and efficiency control for the system. In addition to monitoring critical data points such as voltage, temperature, and charge levels across individual cells and modules, the BMS plays a crucial role in the broader ecosystem of the electric vehicle. By utilizing a Controller Area Network (CANbus), the BMS communicates with other devices and subsystems within the vehicle, making it a key part of the overall powertrain and thermal management strategy.
Through CANbus, the BMS shares real-time data with the vehicle’s primary control unit, enabling integrated actions to ensure the battery pack operates within optimal conditions. For example, if the BMS detects high temperatures within the battery pack, it can communicate with the vehicle’s controllers to limit power output or adjust charging rates. This proactive approach prevents overheating, which could lead to thermal runaway, a condition that can severely damage the battery or cause safety hazards.
Additionally, the BMS - either directly or through a VCU can control ancillary systems such as coolant pumps, fans, or heaters, helping to maintain the battery’s temperature within a safe operating range.
In regenerative braking scenarios, the BMS can communicate with the motor controller to regulate the energy being fed back into the battery, ensuring that no cells are overcharged. By preventing overcharging and undercharging at both the cell and pack levels, the BMS extends the battery’s life, optimizes performance, and safeguards the overall health of the powertrain.
In short, the BMS, through its communication with other devices via CANbus, plays a central role in ensuring that the EV’s battery and overall electrical system remain safe, efficient, and reliable under all driving conditions.
For even more advanced monitoring, products like Metis Engineering’s Cell Guard take battery safety a step further. This cutting-edge sensor provides detailed, real-time information about the health of lithium-ion battery packs. Unlike traditional systems that only monitor temperature and voltage, Cell Guard tracks a range of environmental parameters, including volatile organic compounds (VOCs), pressure, humidity, and dew point within the battery pack.
Cell Guard can also detect cell venting—a key warning sign of battery failure—and take preemptive action, such as isolating the battery to prevent thermal runaway, a dangerous condition where heat in the battery escalates uncontrollably. Additionally, Cell Guard’s shock monitoring capabilities (up to 24G) help assess the impact damage, making it an invaluable tool for both consumer EVs and high-performance applications.
If you plan to store your EV for an extended period, it’s important to take steps to maintain battery health. Long-term storage requires the battery to be kept at a state of charge (SOC) between 30% and 80%. This prevents over-discharge or overcharging, both of which can reduce the battery’s lifespan. Furthermore, it’s advisable to store the vehicle in a climate-controlled environment, as extreme temperatures can degrade battery components.
When storing the vehicle, consider disconnecting the 12V battery to avoid unnecessary drain. If the battery pack has an HVBD, it should be used to fully isolate the high-voltage battery from the rest of the system. These steps will help preserve the battery’s health and ensure that it’s ready for use when needed.
Building a safe and reliable EV battery pack requires attention to multiple factors, from ventilation and thermal management to real-time monitoring and circuit protection. By following these best practices—incorporating essential safety features like HVBDs, isolation monitoring, fusing, and IP-rated enclosures—you can ensure that your EV battery pack operates safely and efficiently for years to come.