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| Illustration of IR Measurment |
The Fundamental variation in insulation resistance (IR) readings of Lithium Batteries are due to below reasons namely,
- Battery Pack Design and Construction
- Voltage Level
- Environmental Conditions
- Contamination or Aging
- Measurement Path and Method
- 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;
- SAE J1766 (Society of Automotive Engineers),
- ISO 6469-3 (Electrically Propelled Road Vehicles – Safety Specifications),
- IEC 61851-23 (Electric Vehicle Conductive Charging System),
- 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.
