Why different Battery Pack has different Insolation Resistance Value ?

 

Illustration of IR Measurment 

The Fundamental variation in insulation resistance (IR) readings of Lithium Batteries are due to below reasons namely,

1. Battery Pack Design and Construction
2. Voltage Level
3. Environmental Conditions
4. Contamination or Aging
5. Measurement Path and Method
6. Capacity/Size of the Pack

Different OEM's battery Pack Types have different IR readings through their software like Tesla Model S is having 25MΩ or 25000KΩ, The Indian Manufacturing giant TATA and its Vehicle's battery possibly have 5MΩ or higher.

The above measurments are dipendent on the applied Voltage levels to measure IR readings, Higher Voltage levels to measure leads to lower IR Values.

As per international standards like below;

1. SAE J1766 (Society of Automotive Engineers),
2. ISO 6469-3 (Electrically Propelled Road Vehicles – Safety Specifications),
3. IEC 61851-23 (Electric Vehicle Conductive Charging System),
4. UN ECE R100 (Uniform Provisions for Battery Electric Vehicles)

All the above standards specify minimum IR of 100 Ω/V for DC systems and 500 Ω/V for AC systems, measured between HV components and the chassis, also at the time of crash or vibration, thermal shock, environmental tests like water ingress IR must remain above these thresholds.

For an 800V system (e.g., BYD Seal EV Vehicle):

• DC Side IR Measurment: 100 Ω/V × 800V = 80 kΩ.
• AC Side IR Measurment: 500 Ω/V × 800V = 400 kΩ.

And for EV Chargers , it must maintain IR >1 MΩ under normal conditions to prevent leakage to the grid or chassis, It has to ensures safe interaction between the vehicle and charging infrastructure. Diagnostic monitoring Devices monitor IR has to detect faults during charging.

EV manufacturers like Tesla and BYD calibrate their Battery Management Systems (BMS) and diagnostic tools to monitor IR in realtime, typically displaying values in megaohms (MΩ).

Healthy systems show IR values significantly above the minimum requirements, often >1500 to 4000 kΩ (1.5 to 5 MΩ), as seen through diagnostic tools like CANALYZER & PEAK CAN with monitoring softwares.

There are 3 measuring methods in IR measurment, Which are;

1. BMS Internal IR Measurement: 

BMS applies a known voltage through a high-value resistor to the chassis from either the positive or negative terminal and measures the resulting current

2. Insulation Resistance Tester (Megger or Megohmmeter) :

By Connecting the megohmmeter Positive terminal to ground or Negative terminal to ground and applying a DC test voltages 500V or 1000V DC will show us the resistance value either in KΩ or MΩ depending upon the fault condition. Higher resistance mean no leakage of power and IR is OK.

The Physical method with known applied voltage to get the IR Value is same for all the batteries and there wont be any changes in IR Value shown for healthy systems of any OEM batteries and battery types.

3. Midpoint Method

This method identifies whether the leakage of power is stronger toward the positive or negative side of the HV line.

Please refer below link to know more.....

How Insulation Resistance in EV Vehicles measured and Why?.

It uses Voltage divider circuit to find the midpoint Voltage and once any variation in the branches of HV line then it informs the VCU to disconnect the main battery pack through opening of HV Contactors.

Conclusion: 

Insulation resistance is not a fixed value it is a function of physical build quality, environmental conditions, pack size, and testing method. It should typically be >1 MΩ or 10000KΩ  for safety.

I hope the above explanation about IR measurment and types cleared why different pack have different IR Values only through diagnoistic and monitoring softwares.

Why there is a measurable HV voltage between the Battery Terminals to Chassis?

 

HV voltage between the Battery Terminals to Chassis

The voltage between the HV battery positive terminal and chassis ground measures approximately half of the total HV battery voltage and this behavior is expected due to the high resistance grounding method often used in isolated high voltage systems, such as in electric vehicles (EVs) and industrial battery systems.

Typically a voltage divider circuit is used  in the insulation resistance measurement to scale down the high voltage present in an electric vehicle's HV system, allowing safe measurement and assessment of insulation resistance.

HV battery system in EV vehicles are generally floating ( not directly grounded ) . so, both the positive and negative HV lines are connected to chassis ground via high-value resistors (e.g, 500kΩ to 10MΩ).These resistors are sometimes called symmetrical voltage dividers or bleeder resistors.

How it Works: 

The high voltage (HV) is applied to the first resistor, and the second resistor is connected to chassis ground.

A measurement point or Test point is placed at the junction between the two resistors, where a lower voltage proportional to the HV battery voltage is obtained.

This scaled down voltage is used by the insulation monitoring device to assess the insulation resistance without exposing the measurement circuitry to full HV.

To read more on Insulation measurment click below...

How Insulation Resistance in EV Vehicles measured and Why?.

Lets say, for 400V Battery EV Vehicle.

HV+ to chassis: ~ +400 V

HV− to chassis: ~ −400 V

HV+ to HV−: 800 V

This is normal and expected behavior in a floating HV battery connected to an EV Vehicle.

Conclusion : 

Measuring approximately half of the HV battery voltage between either HV+ or HV− and chassis is normal behavior in electric vehicles (EVs) that use a floating high-voltage system.

Intentional and compliant with international standards such as:

ISO 6469-3:2018 - Electric shock protection

ISO 21498-1/2 - HV power supply systems

UNECE R100 - Legal compliance for EV safety in many countries

These standards explicitly allow such floating systems with high-resistance paths to chassis and require insulation monitoring (IMD) to detect loss of isolation. The presence of a measurable voltage to chassis confirms the system is working correctly as long as it's symmetric and monitored.

The above design ensures Electrical safety,Improved fault detection and Compliance with global EV regulations.

How Insulation Resistance in EV Vehicles measured and Why?.

Measuring Insulation Resistance in EV Vehicles

                                    

EV high-voltage systems are designed to be extremely safe, and insulation integrity is a cornerstone of this safety. Regulations such as ISO 6469-3 and IEC 61557-8 set stringent requirements for isolation resistance in EVs. The use of IMDs and such resistor networks is a direct implementation of these safety standards to

1. Prevent electric shock: By ensuring that a single fault does not create a dangerous path for current through a person touching the vehicle.

What are the resistance acting upon a car.

Resistance acting upon a Car

Air Resistance (Aerodynamic Drag)

• This is the force exerted by the air opposing the car's motion as it moves forward.
• It depends on factors like the car's speed (drag increases with the square of velocity), its shape (aerodynamic design reduces drag), frontal area, and air density.
• For example, a sleek sports car experiences less air resistance than a boxy truck due to its streamlined shape.
• Mathematically, drag force can be expressed as:
  $F_d = \frac{1}{2} \rho v^2 C_d A$, where:
• $\rho$ = air density,
• $v$ = velocity,
• $C_d$ = drag coefficient,
• $A$ = frontal area.

Rolling Resistance

• This is the force resisting the motion of the car's tires as they roll over a surface.
• It arises mainly from the deformation of the tires and friction between the tires and the road.
• Factors affecting rolling resistance include tire material, pressure, road surface, and vehicle weight.
• It’s typically much smaller than air resistance at high speeds but significant at lower speeds.
• The rolling resistance force can be approximated as:
  $F_r = C_r m g$, where:
• $C_r$ = rolling resistance coefficient,
• $m$ = mass of the car,
• $g$ = gravitational acceleration (9.8 m/s²).

Gravitational Resistance (Gradient Resistance)

• This occurs when a car is moving up an incline, as gravity pulls it downward.
• The force depends on the slope angle and the car’s weight.
• On a flat surface, this component is zero, but on a hill, it can significantly resist forward motion.
• It’s calculated as:
• $F_g = m g \sin(\theta)$, where:
• $\theta$ = angle of the incline.

Note: 

At low speeds (e.g., city driving), rolling resistance dominates.

At high speeds (e.g., highway driving), air resistance becomes the primary opposing force.

Car manufacturers reduce these forces through aerodynamic designs, low-rolling-resistance tires, and lightweight materials to improve fuel efficiency and performance.

What is VECU received BMS derate flag? Where and What to check.

 

VECU received BMS Derate Flag

The Battery Management System (BMS) is responsible for monitoring and controlling the battery pack to ensure safe and efficient operation in any ev vehicle.

 When it detects conditions that may affect battery health or vehicle performance, it can send a derate flag to the Vehicle Electronic Control Unit (VECU) over the Controller Area Network (CAN) bus.

This derate flag instructs the vehicle to reduce power output, limiting acceleration, regenerative braking, or other power-intensive functions. The derate condition is temporary and will be lifted once the issue that triggered it is resolved.

 Possible reasons for the BMS derate flag to be triggered:

High battery temperature: The BMS may derate the power output to prevent the battery from overheating.

Low battery voltage: The BMS may derate the power output to prevent the battery from being over-discharged.

High current: The BMS may derate the power output to prevent the battery from being damaged by excessive current draw.

Cell imbalance: The BMS may derate the power output to prevent further imbalance between the battery cells.

Other BMS faults: The BMS may derate the power output due to other faults detected by the BMS, such as a sensor failure or a communication error.

 

What will the VCEU do after receiving derate flag CAN messages from BMS?

Reduce the power output of the vehicle: This may involve limiting the motor power, disabling certain features, or even reducing the vehicle's speed.

Display a warning message in the Cluster: This will inform the driver of the issue and the reduced performance of the vehicle.

Store a diagnostic trouble code (DTC): This will help technicians diagnose the issue.

BMS Derate Flag is a critical signal that protects the battery from damage. By diagnosing the exact cause (temperature, voltage, current, or internal faults) we can troubleshoot the problem within the EV Vehicle.