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Can someone explain why the modern car radio, even in a Prius, momentarily shuts off when the car is started? Is it taking up that much current that it had to be disconnected? Does it happen to AC too?

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Yes, because the starter motor draws so much current (up as high as 200 Amps for some cars), any non-essential electrical devices are usually shut off until the engine starts, to ensure the starter can do its job.

You might think it doesn't make a difference, but in reality, starting a modern engine really takes it out of a battery. If you ever want to test this, see how fast you drain your battery trying to start an engine with no fuel. It doesn't take many attempts...

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It isn't a deliberate design feature. It's just a consequence of the fact that when the starter begins to crank the car, the battery voltage will dip significantly, maybe as low as 9 or 10 volts instead of 12. The car radio circuits (which these days are mostly digital signal processing done by computer-type circuitry, not the "simple" analog electronic circuits you would find in a 1960s or 1970s vintage radio) can't handle that, and the audio output gets interrupted.

It is easy to "see" the voltage drop if you start the car at night with the lights on. The headlights will go dim while the starter is cranking.

If you really want to pay a few hundred dollars more for ICE that doesn't do this, you can probably buy it - but it's not a cost effective option for standard car audio equipment.

Incidentally, the same thing would also affect the car's ECU computer, except that it is designed to deal with the voltage drop - if it couldn't, the car would be impossible to start! Before electronic ignition, there was a mechanical switch connected to the starter switch which effectively set the ignition circuits to work on 6V not 12V while the starter was cranking - and that had the side effect that if you left the ignition for a long time on with the engine stalled, you could fry the ignition electrical system by overheating it with 12V when it was only expecting 6!

  • It happens in the hybrid Prius too even though the engine is not starting. I guess the circuitry that powers all the fancy USB converts are also affected by the same voltage drop. – ilya1725 Jun 16 at 14:49
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    This is not all quite right, it is in fact a deliberate design decision. When the key is not in the Start position, the ignition switch only activates a subset of the cars electronics package, these are only the things absolutely necessary for starting the car. Although the voltage drop when the start is cranking is pretty significant (9-10V), this is not typically low enough to cause electronics to stop working. ECU's, Radios etc have voltage regulators that can withstand the drop. Things like ignition coils and injectors require longer activate time though which the ECU accounts for. – H. Daun Jun 16 at 20:11
  • @H.Daun I can clearly see the radio going off. It is even more apparent with iPhone connected with CarPlay because it disconnects. – ilya1725 Jun 17 at 17:39
  • @ilya1725, sorry I was referring to some of the stuff alephzero had mentioned. It definitely will be turning off, I was just explaining that it is an intentional thing rather than just due to voltage drop etc. – H. Daun Jun 17 at 21:07
  • @H.Daun. Got it. Do you know why it is even done in a hybrid? The engine itself won't start anyway (most of the time). – ilya1725 Jun 17 at 21:58

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