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Red 2022 Late XC40, Yellow 2017 BRZ
1,171 Posts
Discussion Starter · #1 ·
Nice article on Jalopnik today.

The contactor allows the main battery power to the vehicle to be shut off when the car is in an accident, being worked on, or simply not being driven. It’s an important safety measure that helps prevent electrical fires and/or electric shocks, and allows the main traction electronics to be de-energized when not in use. Contactor is a fancy word for what is essentially a big switch that controls the flow of current from the battery pack. A contactor is turned on by supplying voltage to a coil. This coil acts as an electromagnet, moving a larger set of contacts, which allows current to flow out of the high-voltage battery. Switch the coil back off, and the contacts spring apart, breaking the circuit and disconnecting the battery. It’s just like a big relay, basically.

The need for such a device poses a problem: How do you energize this contactor in order to connect the main traction battery to the rest of the car’s electronics? An easy way to do this is to use a nice, reliable 12 volt battery. Fancier solutions would work too — such as modern Li-Ion 12V batteries — those are already shipping in some cars. But with lead-acid car batteries already proven and rated to last in an automotive environment, and available all over the world, why reinvent the wheel? Or the battery, for that matter.

The other main area where the 12-volt battery helps is in all the ancillary systems in a modern car. Things like blower fans, electric windows, headlights, and infotainment systems have all historically run on 12 volts. They all work great as-is. Engineering an electric drivetrain is strenuous enough, so it makes sense from a cost and durability standpoint to use existing, proven designs for systems that can largely carry over.
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