Design Engineering EV Battery Management Systems (BMS)

Optimizing This Crucial Component for Electric Vehicles

Battery management systems are found in many markets and applications, including Industrial and Renewable Energy. But for hybrid electric vehicles (HEVs), plug-in hybrids (PHEVs), and full electric vehicles (BEVs) they’re essential for safe and reliable operation.

These innovative systems provide real-time data collection to:

Optimize battery performance
Alert the driver/operator of issues and malfunctions
Communicate the battery’s state of charge (SOC) or amount of energy currently available
Communicate the battery’s state of health (SOH) or remaining capacity/life

Analyzing voltage, temperature, and current, the BMS provides a constant feedback loop to help drivers maintain safety while maximizing the driving range on a battery’s charge. The BMS uses this information to help prolong the battery’s life by stopping the charge function when it’s full and shutting down operations when it is overheating.

Battery management systems must be designed to incorporate the appropriate levels of:

Overcharge protection
Over-discharge protection
Short circuit protection
Thermal runaway protection

Charging and discharging must be carefully managed throughout the life of the battery. Initially, the BMS will not charge the battery to full capacity, as later in its lifecycle, it will need more charge to hit the desired range. Additionally, the rate of charge in each battery cell will vary (even in a new battery), creating the need for constant balance/equalization.

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How do battery management systems work?

The typical BMS setup includes multiple lithium-ion batteries connected to a control unit and sensors by connection wires.

While there is currently no global standard, the power topographies are typically set up in one of two ways:

Centralized architecture

With a centralized architecture, one controller unit manages all of the battery cells. This single assembly board is simpler to design but can be bulky and costly. Also, if one battery cell fails, the whole unit must shut down as a safety measure.

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Primary-secondary architecture

With the primary-secondary or decentralized architecture, the cell monitoring and intelligence circuitry are monitored by multiple control units. One advantage of this architecture is if one cell fails, the others are still able to function.

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One primary function of the BMS is balancing the battery cells, which each have their own rate of charge at any given time. The battery monitoring integrated circuit (BMIC) monitors the individual cells in real time, then informs the cell management controller and battery management controller. This can be achieved in two ways:

Passive cell balancing - energy is dissipated from the battery’s most-charged cell in the form of heat, which may impact overall performance and efficiency.

Active cell balancing - instead of dissipating the extra energy when a cell reaches higher voltage, it transfers that excess energy to another battery cell with lower levels of charge. A low equivalent series resistance (ESR) MOSFET is utilized as a power switch to help minimize energy loss.

A protection device is used to help safeguard this vital EV component from harmful electrostatic discharge (ESD).

EV BMS Block Diagram

Design Considerations

Compared to other parts of the vehicle, the EV’s battery system is the most expensive. Therefore, the BMS can make or break the performance and longevity of the EV itself. Proper component selection is key to a BMS drivers can rely on. Critical requirements include:

Charge/discharge - cell balancing by charging and discharging of individual cells with different levels of charge can impact the overall performance and life of the battery.
Energy efficiency - lower energy usage enhances fuel economy and requires less power from the vehicle’s battery, which improves the range for EVs.
High-temperature capability - components must withstand the worst-case scenario in thermal surges caused by torque assistant pulses.
High current handling - the system should perform reliably under the worst possible conditions and current surges. BMS components must provide overvoltage protection (OVP).
ESD protection and transient suppression - in addition to protecting from high heat and current, the BMS design should provide the appropriate levels of protection from electrostatic discharge and ensure any transients are effectively managed.
Low on-resistance - minimizing heat generated during operation extends the life of the component, and low RDS(on) plays a crucial role while enhancing system responsiveness.

Fast switching speed - switching should be fast enough to ensure smooth operation amid ever-changing conditions.

Low reverse recovery time - low Trr is needed to enable faster switching at high frequencies and improve efficiency.
Applicable industry standards - for maximum safety and reliability, adherence to AEC-Q101, ISO 26262, and other standards is required for certain components.

Design Engineering EV Battery Management Systems (BMS)

Design Engineering EV Battery Management Systems (BMS)