[ferongr]

>then I needed to find a brake controller that can work with the higher voltage (14.4v vs the normal 12v)

Put a voltmeter on the battery terminals of a regular car at 2000rpm and note the voltage. You'd be surpised (the alternator can produce as high as 15V on some cars).

[AlotOfReading]

Automotive transients can be wild. I did a bringup with a board that had specified 100+v range specified for transients and finicky quality requirements on the output. The power supplies took up most of the (very large) board.

[mixdup]

14v is not a transient, if your voltage was 12v with the car running, there's something wrong with the charging system (DC-to-DC in an EV, alternator/generator in an ICE)

13-14v is normal in all 12v automotive systems as the charging voltage

[nandomrumber]

If I recall correctly, a fully charged lead acid battery has an open circuit voltage of 13.6V.

So the alternator has to put out at least something higher than if it’s planning on recharging the battery after 500 to 700 amps have been pulled from it for a few seconds to start the engine.

[numpad0]

Yeah, max CV charging voltage is ~14V, max charging C rate is ~0.2C, open circuit voltage at 100% is that 13.x range. And lead acids like to stay at 100% unlike Li-ion which likes 50% +/-30%, so "12V" ICE cars just use a bus voltage of 13-14V and wire the battery there. At any given moment, the car's "12V" bus voltage MUST be above 13.x and below 14.4(absolute max).

It's a bit perplexing that those lead acid systems are referred to as "12V" systems when that figure is effectively the 0% voltage, whereas 3.7V for single Li-ion cell is the 50% voltage.

e: also, ICE transients can be in kV range, coming from ignition mechanisms. I've heard that you can literally measure engine RPM by selecting 1/dt on an oscilloscope and dividing that by cylinder count.

[K0balt]

The nominal range for automotive systems is 10-16v. If you are designing anything for automotive use that doesn’t work reliably in that range, you are manufacturing problems for people.

[imp0cat]

This. Most cars nowadays come with the so-called "smart" alternators that vary voltage wildly depending on the current driving conditions.

One minute you might be accelerating and the onboard voltage drops as the battery supplies most of electricity. Then, as you reach the crest of a hill and start engine-braking, the car frantically tries to convert all the available kinetic energy to electricity, raising the onboard voltage to quickly charge the battery.

[mindslight]

nit: Some vehicles can use a two stage charging system where if the ECU is not trying to charge the battery and the power draw is otherwise low, the voltage sits in a lower range rather than constantly float charging the battery. This can surprise you if you're trying to diagnose a battery issue!

[hatsunearu]

Yeah, this is normal. When the battery suddenly disconnects (for example of the lugs pop off) the alternator's momentum will send a massive, long-standing transient on the bus up to 100V. This is called a load dump.

[jeffreygoesto]

Saw up to 800A on units like the FSD for the short time until the caps were full. Slow starting a SoC is a software problem, slow starting the Cs and keeping the impedance low at the same time a non-trivial hardware problem.

[nandomrumber]

I typically fault anything above 15.6V as “that’s a bit high, your alternator might be on its way out” when working on automotive / caravan / camper van appliances and accessories.

[jeffreygoesto]

For static voltage sure. For short term resilience against static electricity these units typically are specced to endure 2kV on each pin.