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I have been in discussion with a bmw mechanic that is claiming a cars DC electrical system cannot have a brown out. Their claim is that the system is set up in such a way that it is either on or off.

My understanding is that this is what you would want in an ideal system but that alternators go out and electrical systems don't always work as they are intended when not getting the correct voltage and electrical levels.

Anyone have some personal knowledge of car electronics? Maybe from inside the automotive industry?

Edit: Closed as off topic just like most things on stackexchange.

NDEthos
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    Have you ever tried to start the car with low battery? It tries, but it can't. Does it count as "electrical system brown-out"? – Eugene Sh. Dec 08 '21 at 22:44
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    Both you and the mechanic are free to define what brown-out means to each of you so you may have different answer and you are both right. How do you define a brown-out in a car, and how does the mechanic define it? – Justme Dec 08 '21 at 22:52
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    Turn on headlights. Turn the starter. Do the headlights dim while the starter is turning? Go show the mechanic.... – Kyle B Dec 08 '21 at 22:54
  • What he MIGHT mean (and not know it) is that the battery is 12V, but most computers are 3.3 or 5V. So long as the battery doesn't brown out below 5V, technically the digital stuff wouldn't much care. It also can live through a brownout via capacitors. It may also have a really fancy voltage regulator that can BOOST voltages when necessary. But those are unknowns to the person outside .. You'd have to be probing the cars computer circuitboards to see how it's all setup and determine if a brownout actually occurs. – Kyle B Dec 08 '21 at 22:57
  • Anecdotally, yes they can. I had a car with a dodgy alternator that would be mostly ok, except for if you're driving at night and needed the headlights. The headlights would dim, the headunit would reset, but I never got it to the point of the car dying from lack of power - but it was a car from the late 80s, and dead basic at that so that could be why. – Zac Faragher Dec 09 '21 at 06:54
  • @KyleB: AFAIRemember from my days in automotive electronics, it's considered a feature for non-critical electronics to black out when the voltage drops too far while cranking the engine. The rationale is simple: when a car has been stationary and the battery discharged a bit, you want maximum power available to start the engine. Once it runs, the alternator will provide enough power to restart the electronics. – MSalters Dec 09 '21 at 14:05
  • Digital systems will either work or they won't. If they have sufficient power they will work, if they don't then they won't. So in that sense, yes, either it works or it doesn't. Analog loads, though, like the starter, the wipers, lamps, etc, will certainly display a spectrum of operation from normal through weak to completely non-functional. – J... Dec 09 '21 at 14:25
  • @KyleB I reckon most of my radio runs on 5V (except the power amp). That always reboots on cranking. However that's on an old Ford, not a recent BMW – Chris H Dec 09 '21 at 16:40

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Yes they can.

Normally, the battery is between 9 and 16 V. Most vehicle electronics are designed to work optimally in this voltage range.

Of course there is a wider range that can occur and vehicle electronics will survive and work (perhaps in a limited way) in the wider range.

  1. Cold crank. This is when the battery may be 9 V to start the starter, but the surges of current will drop the instantaneous voltage as low as 3-5 V for some ms. You don't want the engine control module to reset in this condition, or it won't have rebooted quickly enough to actually start the engine.

  2. Double battery - some cold weather regions use a 24 V 'double battery' to jump start cars. Electronics will work well with this voltage.

  3. Load dump -- about 40 V for 100 ms when there is a disconnect of the battery cable and there is (was) a high charging current.

jp314
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Depends, the type of system makes a difference.

Cars with generators could have the battery too flat to start with the starter but enough to produce a spark so would start with a bump or starting handle. The headlights would be dim ie brownout, but the generator would eventually bring the voltage up.

Cars with alternators with a flat battery could also be started with a bump or push start but the alternator may or may not start charging - depending on the alternator and how low the battery voltage was.

So, yes some cars can have a brownout ie low voltage but newer cars or a particular make could easily design the system such that it won’t come on without sufficient voltage.

My car model is known to produce a lot of "fake" errors when the voltage gets a bit low - it likes a healthy battery.

Solar Mike
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  • Yes I concur with the last bit, my car had a dying battery and often produced engine system errors, power steering errors etc caused by the low voltage when cranking. Also had a tendency to reboot the sat-nav which was really annoying on my delivery driving job. The problems actually crept up slowly over an 18 month period. New battery... everything fine. – Rodney Dec 09 '21 at 08:01
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If you define a "brown out" as the supply voltage falling below some specified level, or to the point where things cease to function, absolutely.

Just have the guy disconnect the alternator, then go for a long drive.

TimWescott
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I was once driving city streets in heavy tropical rain.

the lights take 16A the wipers take 8A the ventilator takes 4A the engine fan takes 8A

The altenator couldn't keep up with that with the engine idling while I was stopped at traffic lights.

The voltage dropped to around 6V and the engine stalled while I was heading waing fro a traffic light on a up-hill slope

fortunately there was enough charge left to run the hazard blinkers. after the traffic behind me cleared I was able to roll-start in reverse rev the engine apply the necessary loads, I continued my journey in a lower gear and turned off the headlights while stopped waiting for traffic lights.

Jasen Слава Україні
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