Control and driving contactors is important for safety, lifetime and energy consumption. The system level view of the control is covered by another post.
Contactor Control is another one of those areas where responsibility is divided between a vehicle’s system level control and the battery management control. From the systems engineering perspective;
The battery management should decide if it can open or close the contactors
The vehicle system should decide when the contactors are requested to open or close
Driving Contactors
Shuangbing Dong [1] covers the driving of contactors in the Texas Instrument white paper on this subject. For safety the driver circuit should control the high and low side of the coil.
The coil will always be energized and cannot be shut off when short-circuit failure happens if only the high- or low-side switch is used. The failure is in line with a short circuit to the battery on the high-side switch and short circuit to ground on the low-side switch.
A large current flows through the coil and cannot be switched off, thus the coil might be damaged due to high power dissipation.
The driver control profile has three sections [3].
- Pickup phase Current reaches maximum and contactor closed during the phase
- Hold phase Keep smaller current to maintain contactor closed efficiently
- Fast decay phase Current fast decay with voltage suppression to open contactor quickly
- Faster release time
- Longer life time
This post has been built based on the support of and sponsorship from: Quarto Technical Services, Eatron Technologies and About:Energy
Control
Shuangbing Dong [1] also shows the control sequence required to limit the inrush current associated with the large capacitance in the inverter.
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.
We should note that before we even begin to close the pre-charge contactors a number of safety checks need to be made (as mentioned in Contactor Control and by XBattery [2]). These include:
- Temperature
- HV Isolation
- Current flow
These signals would be part of a number of functions within the Battery Management System (BMS) that would together give a status and ok to connect.
References
- Driving High-Voltage Contactors in EV and HEVs, Technical White Paper, Texas Instruments
- The Role of Contactors in Battery Energy Storage Systems (BESS) and Battery Management Systems (BMS), XBattery
- High-voltage contactor control, David Dong, Texas Instruments
See also: Contactors, Contactor Weld Detection