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Contactor Basics

A contactor is a large switch controlled by an electromagnet under software control. It connects the battery pack from connections outside the pack (to the motor inverter, chargers, etc.). There are two contactors in the Model S inside the battery pack – one for positive and one for the negative. This is a clever safety design, because if one fails shorted, software can still disconnect the battery from the outside.

Here’s an example of a contactor that is very similar in style and size to the ones used in the S85.


Switching the Contactor

Ideally for the longest life, the contactors should be switched when there is no current flowing through it. The Supercharger, AC/DC charger, DC/DC inverter, and motor inverter all are controlled by software, such that normally the contactors are not switched when power is flowing through them. It is designed to handle switching high-current and voltages, but this is not ideal.

When powering the motor, it was reported in Tesla’s 10Q that it can draw a peak of 1000 amps (on the S85). From the Ludicrous release notes, it can handle 1300 amps (on a P85/P90) and 1500 amps with the Ludicrous upgrade. When Supercharging, you could have 300 amps at 400 V (120kW) flowing through each contactor.

I expect there are conditions where the contactors purposely drop out while high-power is flowing. For example, during a car crash (shock) or if a ground fault is detected (a safety issue) software may release the contractors while power is flowing through it. A software bug might also connect or disconnect while power is flowing. The contactors can handle this, but high frequency repetitive occurrences under load will shorten the life of this part.


The contactors are mounted inside the pack itself, again for safety reasons. It’s located at the end of the battery towards the back of the car. This is adjacent to the external power connection.

Contactor Problems

There have been a few reports of a contactor failing. If failed in the open position, then the car cannot be driven or charged. If shorted, it may be possible to drive the car, but likely Tesla’s software detects this condition and will not let the car charge or drive as half of this critical safety system has failed.  A shorted condition occurs when the contacts weld shut. It may not be detectable until software releases the contactor and detects it didn’t release.

Some of the messages that may appear after a contactor failure occurs:

  • 12 volt battery failure (it is not the 12 volt battery)
  • Your car needs service (true)
  • Pull over safely (If the contactor fails opens while driving)

We do not know the reason for the failures. It may be a batch of contactors was manufactured poorly. Defects may be very hard to detect until real-world usage. The plating on the contacts might be too thin or the spring has the wrong tension. It could even be a software bug on some older software version disconnected the contactor at the wrong point too often.

Just like the many other parts that fail (in any brand car), I don’t expect we will ever hear what the exact issue was. The fact Tesla tells us the part failed and/or is being replaced before it fails is better than most car companies.


If one contactor fails, I expect Tesla replaces both parts. While the contactors are expensive, the labor is far more. In the past, the battery is removed and returned to the factory for the repair. Tesla may replace the battery with a temporary battery during this time. Once fixed, the battery is returned to the service center and exchanged for the temporary battery. At the end of 2014 Tesla rolled out a new process to select service centers to make this repair at the service center, avoiding the need to sent the battery back to the factory.

Part Details

From the disassembly of an 85 kWh “D” battery pack (by wk057), the contactor is a TE Connectivity part 2138922-1, made in Mexico. It is likely Tesla sources this part from multiple vendors, so other parts may be used in different cars made at different times.  While I’ve been unable to get the exact specifications on this part, a similar TE contactor that handles only 500 amps up to 900 VDC costs $120 each.

The new Ludicrous option upgrades the contactor to a new design. This contactor uses inconel, a high temperature superalloy, instead of steel to work with very high currents that produce a lot of heat.

One thought on “Contactors

  1. How does Tesla’s Ludicrous Mode Work? says:

    […] our friends at TeslaTap, current flowing from the battery passes through a contactor which is a large switch controlled […]

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