When the battery pack contactors are closed onto a motor and inverter there will be an inrush of current into the inverter capacitor. This very high current is at a minimum likely to age the contactors, it could permanently damage the contactors.
Therefore, when we closed the contactors on the battery pack we do this in three steps:
- Close the main negative contactor
- Close a contactor with a resistor in series
- Close the main positive contactor
A simplified schematic shows the basic principle.
The time taken to pre-charge the capacitors in the HV system will depend on the resistance in the total circuit, the voltage of the battery pack and the capacitance in the system. Using a few equations we can calculate and plot a few of these terms.
The voltage of the inverter capacitor starts at 0V by definition and increases until it is within 10V of the pack voltage.
The current starts high and as expected decays to zero.
The values in the calculator have been preset with values to get you started. As you can see this is an 800V pack and we have used some typical values. We used these values to create the above fixed graphs.
When selecting the pre-charge resistor it is worth calculating how many times it might be cycled in quick succession. Such an event might occur if there is another fault on the system and in this case the user might request the close contactors a number of times. The pre-charge resistor failing due to over-heating then needs to be at least considered.
References
- Andrew Schneer, Brian Munari, How to Design a Precharge Circuit for Hybrid and Electric Vehicle Applications, Sensata Technologies
- Claire Chang and Tilden Chen, Why Pre-Charge Circuits are Necessary in High-Voltage Systems, Texas Instruments
- Murat Kubilay Ozguc, Eymen Ipek, Kadir Aras and Koray Erhan, Comprehensive Analysis of Pre-Charge Sequence in Automotive Battery Systems, Transactions on Environment and Electrical Engineering
- Precharge Calculator, Sensata Technologies
Contactors
In a battery the contactors are a switch that can be operated by the control system. They are essentially a relay. These contactors are designed to be able to break (switch off) the circuit under full load (maximum current and at maximum system voltage).
Using a resistor to pre-charge the bus is a conceptually simple solution but does have some downsides.
The pre-charge current dissipates power in the resistor. Each successive pre-charge adds more power so if the resistor has not cooled between operations then the temperature will rise. Frequent pre-charge operations will cause the temperature of the resistor to increase, potentially to the point where the resistor overheats and fails.
Specifying a resistor that can dissipate enough heat means a costly and bulky component and heat sinking arrangements. So usually there is a time limit imposed between successive operations.
In ‘normal’ usage the time limit is not a problem. But in some more unusual use cases it can result in a very noticeable delay. One obvious case is when the driver continuously turns the ignition on and off. Others can be HV fault situations where the vehicle system can continue to drive but needs to turn the HV bus off when the vehicle is stationary.
A switch mode power supply could be used to pre-charge the bus from say, the 12v supply. This solution has the benefits of eliminating the costly power resistor with its contactor, reducing heat dissipation and potentially providing a faster bus pre-charge.