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).

contactor schematic

Introduction to Contactors

There are two main types of contactors: Normally Open (NO) and Normally Closed (NC). A NO contactor does not allow current to flow when the actuation circuit is not powered. However, an NC contactor connects the external circuit when the contactor actuation coil is not energized. Therefore, we use EV contactors as NO to switch off the circuit in case of loss of actuation circuit supply.

contactor current rating vs time

How to select EV Contactors

Contactor selection is an iterative process that includes consideration of system specifications as well as failure scenarios. Most of the contactor manufacturers share necessary information in datasheets. Thus, a wise approach would be making a comparison table to cover basic specifications like thermal and mechanical initially.

Contactor Faults

Contactors are the only moving part in a battery pack and although simple there are a number of contactor faults that will stop the operation of the battery pack.

The faults are typically:

  • permanently closed
  • permanently open
  • overheating

There are a number of reasons for contactor failure:

  • Poor contactor design
  • Manufacturing quality issues
  • Incorrect sizing and selection
    • electrical
      • LV and HV side need to be considered
    • thermal sizing
    • mechanical vibration and shock loads
  • Incorrect design of associated busbars and fixings
  • Incorrect control


There are a number of companies that make contactors, some are electro-mechanical, solid state and a mix of both technologies.

Pre-Charge Resistor

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:

  1. Close the main negative contactor
  2. Close a contactor with a resistor in series
  3. Close the main positive contactor

A simplified schematic shows the basic principle.

Contactor Mass

This suggests we have a base mass of 150g and then an increment of 1g/A, based on the equation and looking at the graph this is a first reasonable relationship to use for a pack mass estimating tool.

However, the steps in mass does suggest frame sizes.