Why did residual voltages available on the load side after contactors open in a high-voltage (HV) battery pack?

Load side voltages available at high voltage battery packs after contactors/relays open are due to residual voltages because of link capacitors in the inverter circuitry of the EV vehicles.

What are Residual Voltages?

Due to the capacitance of the stored charge in the link capacitors, the voltage appeared on the load side even though the contactors remained open.

To discharge those voltages, some active and passive methods are employed by OEMs to drain the charges accumulated at capacitors.

Active Discharge: Using separate Contactors along with resistors to drain the charge.

Passive Discharge: Using only resistors, which are connected between DC High-voltage Buses, acts as a passive discharge to drain the voltages.

As per industry standards, which are followed worldwide, the maximum allowed residual voltage at the load side is between <60V DC, and for the safer side, some OEMs like TATA use <30VDC as a safe limit.

As per Ohm's Law, since the resistor we choose has very high resistance, only a small, minuscule current flows through it and doesn't have much impact while the contactors are closed.

But when the contactors are open, the resistors will act as a load to drain the charge buildup at the DC link capacitor.

In all modern electric vehicles, active residual voltage draining methods are employed to drain the charge accumulated in the DC link capacitors as soon as possible.

The minimum time for which the charges are drained will be between ( 5-10Seconds ).

Example Calculation of time required to drain the available Voltage at the DC link Capacitor:

The voltage across a discharging capacitor at any given time (t) can be calculated using the formula:

V(t)=V0​×e−t/τ

The formula for the RC time constant is:

$\tau =R\times C$

Let R = 1 MegaOhm (1,000,000 Ohms) and C = 1 mF (0.001 Farads).

These are often much smaller in real systems. 

For example: $\tau =1000000\Omega \times 0.001F=1000seconds$

•  Say the available voltage at the DC link capacitor is $100V$:

• We want to solve for $t$.

  Step 1: Isolate the exponential term

  $$\frac{60}{100}=e^{-\frac{t}{1000}}$$$$0.6=e^{-\frac{t}{1000}}$$

  Step 2: Take natural logarithm on both sides

  $$\ln (0.6)=\ln \left(e^{-\frac{t}{1000}}\right)$$$$\ln (0.6)=-\frac{t}{1000}$$

  Step 3: Calculate ln⁡(0.6)

  $$\ln (0.6)\approx -0.5108$$

  Step 4: Solve for tt

  $$-0.5108=-\frac{t}{1000}$$

  Multiply both sides by $-1000$:

  $$t=0.5108\times 1000=510.8$$

  Final answer: 

                                                                            t ≈ 511 ( 8.5 min)

• Say the available voltage at the DC link capacitor is 800V:

We want to solve for $t$.

Step 1: Isolate the exponential term

 Divide both sides by 800:

$$\frac{60}{800}=e^{-\frac{t}{1000}}$$$$0.075=e^{-\frac{t}{1000}}$$

Step 2: Take the natural logarithm on both sides.

$$\ln (0.075)=\ln \left(e^{-\frac{t}{1000}}\right)$$$$\ln (0.075)=-\frac{t}{1000}$$

Step 3: Calculate ln⁡(0.075);

$$\ln (0.075)\approx -2.59027$$

Step 4: Solve for t;

$$-2.59027=-\frac{t}{1000}$$

Multiply both sides by $-1000$:

$$t=2.59027\times 1000=2590.27$$

Final answer:

$$\boxed{{\displaystyle t\approx 2590.27}}$$

t ≈ 2590.27 ( 43 Min)

We can refer to ISO 6469-3 or ECE R100 for standards on residual voltage.

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 behaviour 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 systems 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 measurement, click below...

How Insulation Resistance in EV Vehicles Is Measured and Why?.

Let's say, for a 400V Battery EV Vehicle.

HV+ to chassis: ~ +200 V

HV− to chassis: ~ −200 V

HV+ to HV−: 400 V

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

Conclusion : 

Measuring approximately half of the HV battery voltage between either HV+ or HV− and chassis is normal behaviour 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.