Insulation Failure Detection in EV Batteries

One of the issues with electric vehicle (EV) batteries is insulation failure, and the ability to detect and correct it is critical. A proven approach lies in ground-fault detection, requiring solid-state MOSFET relays. However, as higher voltages become increasingly common in EV battery systems, finding the right type of MOSFET to handle these voltages reliably is vital.

 

As electric vehicles become more powerful and require more voltage, MOSFET relays with higher operating voltages are necessary. Image used courtesy of Pixabay

 

Insulation Failure in EV Batteries

When insulation materials lose their insulation ability, it can cause resistance to decrease, leading to hazards for the battery management system (BMS) that can lead to shorter battery life or a risk of fire. 

Therefore, in the design and use of a BMS, it is essential to be aware of these factors that can cause insulation breakdown and take appropriate measures to prevent it. Such factors include:

• Overload: Applying too much voltage to insulation materials can cause a breakdown.
• Contamination: Dust, moisture, or other pollutants can weaken insulation materials, causing breakdown.
• Aging: Over time, insulation materials may break down due to thermal, chemical, or mechanical degradation.
• Physical damage: Scratches, cuts, or other physical damage to insulation materials can create a current path, causing breakdown.
• Voltage stress: Over time, the application of AC electric fields can cause insulation materials to break down.
• Temperature: High temperatures may make insulation materials brittle and prone to breakdown.

 

Early EVs had issues with slow charging times and short ranges, which led to engineers increasing the total voltage and current rating to improve these characteristics. However, because of higher currents and voltages, there was the potential for shorter battery life and overheating to the point of fire. Engineers began developing insulation monitoring functions for EV BMSes to address this issue. 

 

Preventing Insulation Failure

The most common method for preventing insulation failure is measuring the resistance of the dielectric by detecting the ground-fault current. 

When the insulation of a battery cell fails, the energized conductor will come into contact with metal that is not intended to carry current. That metal is usually bonded to part of the equipment-grounding conductor and becomes a path of least resistance to electrical currents, constituting a ground fault. The presence of a ground fault can be used to activate an alarm signal using a MOSFET relay between the current sensors and the ground. This insulation monitor/detection function in BMS ensures that the battery insulation is healthy and no leakage occurs. The insulation detection system aims to identify and isolate faults, ensuring the safety and reliability of the battery system and protecting the batteries from premature failure. 

In the ground fault detection approach, the MOSFET is switching high voltage from the BMS through a non-contact relay and a set of series/parallel resistors, as shown in Figure 1. The MCU (microcontroller unit) then measures the voltage drop to calculate the insulation resistance of the BMS. The insulation resistance value must comply with safety regulations: AC 500 ohms/V and DC 100 ohms/V; if it is too low, an alarm signal is activated to provide immediate protection against potential hazards. 

 

Figure 1. A typical circuit for ground fault detection. Image used courtesy of Bright Toward

 

Furthermore, it is necessary to promptly check and repair equipment or systems to restore them to normal operation. Maintenance checks can also be performed on the insulation detection system to ensure its proper functioning and provide accurate data. 

However, when selecting the MOS relay, it must withstand a higher voltage than the battery pack's nominal voltage. For example, a battery pack with an 800 V nominal voltage typically requires a relay with a load voltage greater than 1600 V.

 

SiC MOSFETs Versus Si MOSFETs

SiC (silicon carbide) MOSFETs provide some definitive benefits compared to the Silicon-based equivalents. SiC-based Opto-MOSFET relays, in particular, offer greater load voltages, excellent switching speeds, and more energy-efficient performance. And while they are used in a range of applications such as industrial robotics, security, and telecommunications, they have been extremely useful for insulation failure detection in electric vehicles–especially those involving higher voltages.

 

Why Opto-SiC MOSFET Relays are a Better Solution

The relay used in BMS insulation detection has changed over the years, as illustrated in Figure 2, beginning with reed relays when the nominal voltage was 380 V. As the nominal voltage for battery systems increased, the operating voltage for the relays also increased. That increase required MOSFETs to switch to much higher voltage levels. 

Note that when the nominal voltage was 400 V, Si MOS Relays were sufficient. However, a different semiconductor material was needed to handle greater voltages efficiently. 

 

Figure 2. How BMS insulation detection system relays have evolved through the years. Image used courtesy of Bright Toward

 

A Si-based Opto-MOSFET relay's physical limit is around 1500 V, which is not high enough for newer, higher-voltage battery systems that demand 1800 V operating voltages. Hence, the move to SiC MOSFETs. 

 

Opto-SiC MOSFET Relays

Bright Toward’s Opto-SiC MOSFET relays for automotive applications are ideal for EV insulation failure detection, including some rated for 3300 V. The two main series are the 58 Series and the 53 Series. 

58 Series is rated for a peak load voltage of 1800 V, with the 53 Series rated at 3300 V. Also, note that 6600 V Opto-SiC MOSFET Relays will be released soon.

The AA58 series is AEC-Q101 certified and rated for a peak load voltage of 1800 V. They are used not only for EV BMS but also for energy storage systems and automatic test equipment -- and represent the most innovative and highest voltage MOS relay in the market. The AS58F series has similar ratings and applications as the AA58 series but also includes creepage clearances of  ≥ 8 mm for input-output and ≥ 8 mm between drain pins of MOSFETs for safety certification requirements. Major automotive companies have already validated the 1800 V Opto-SiC MOSFET Relays (AA58, AS58) with ongoing mass production. 

As the demand for higher load voltage solid state relays increases, Bright Toward has developed SiC-based Opto-MOSFET Relays to improve and increase load voltage for applications, including EV battery insulation fault detection and BMS battery balancing and other applications in industries as diverse as telecommunications and aviation.