95 votes
Accepted

Why are there so many relays used in a car, instead of transistors?

Relays are much more stable temperature-wise: a sealed relay has essentially the same characteristics at -30°C and +70°C, both temperatures being common for cars. A transistor works quite differently ...
Dmitry Grigoryev's user avatar
81 votes
Accepted

Will radar/lidar still work when every car is equipped with them?

You'd be surprised. This is actually topic of ongoing research, and of several PhD dissertations. The question which radar waveforms and algorithms can be used to mitigate interference is a long-...
Marcus Müller's user avatar
73 votes

Why are car batteries still so heavy?

So, obvious answer first: why do batteries still weigh 20kg? Because they're still the same lead-acid batteries. Simple as that. No other technology came near the low cost per Ampere (and ampere-...
Marcus Müller's user avatar
65 votes
Accepted

Why are car batteries still so heavy?

So, now after the answer to your literal question to your real question, that you sadly didn't ask Battery technology has moved so far in the last 100 years. The lead-acid starter battery became ...
Marcus Müller's user avatar
48 votes

Why are there so many relays used in a car, instead of transistors?

A car is (still, in the modern era) an electrically harsh environment. The "12V supply" is typically 13.5 to 15 V, and may occasionally spike to 80V or more. There may also be some high-frequency junk ...
Ecnerwal's user avatar
  • 5,614
45 votes
Accepted

12 V car battery short circuit with a wrench that has resistance of 0.5 ohms, how much current?

A wrench does not have a resistance of 0.5 ohms, it's way lower. Your basic multimeter cannot measure resistances to better than an ohm or so, the resistance of leads, and the unreliability of ...
Neil_UK's user avatar
  • 165k
42 votes

Why do car blinkers flash at twice the speed when a blinker light has burned out

The current when two lamps (front and rear) are on is higher than when one is burnt out. This heats up the bimetallic strip to a higher temperature causing it to bend more. The strip keeps bending ...
RussellH's user avatar
  • 14.7k
36 votes

Why do car blinkers flash at twice the speed when a blinker light has burned out

Modern electronic flasher units flash at a higher rate as a design feature in order to alert the user to a dead 'bulb'. What was once a side effect is now a design feature'.
ATCSVOL's user avatar
  • 795
34 votes

How can the contact points of jumper cables handle the hundreds of amps required to start a car?

Hint: resistance is not just a matter of cross section, but also of length. Would you expect the jumper cables to work if they were say, 10 meters long? What about 100 meters? You got my point. Even ...
Vladimir Cravero's user avatar
31 votes

Why don't we use the 7805 for car phone chargers?

Because it gets too hot. A linear regulator like the 7805 reduces voltage by turning that excess voltage into heat. So if you feed it 12V from the car to produce 5V@1A, it is turning 7V@1A worth of ...
DKNguyen's user avatar
  • 56.4k
22 votes

Is "automotive" grade always better?

It means that the component underwent testing under specific Automotive standards and passed. It doesn't mean that the non-Automotive component would not pass as well, it just wasn't tested. If it is ...
Davide Andrea's user avatar
22 votes
Accepted

Does using a car’s electricity use up more fuel?

No. Using electricity from the alternator makes the car consume more fuel. The battery is only a "buffer" for the electrical energy generated by the alternator, so we can ignore it for our ...
Jonathan S.'s user avatar
  • 17.8k
20 votes

Why are car batteries still so heavy?

The latest batteries are much lighter and cost less over a vehicle lifetime than ones of yore. But they do not use LA (lead acid) chemistry. A LiFePO4 (Lithium Ferro Phosphate) battery will do what ...
Russell McMahon's user avatar
  • 150k
19 votes

Why are car batteries still so heavy?

Lithium starter batteries exist, primarily for racing or other performance or luxury applications where the weight savings or bragging rights are worth the cost. As others have noted, however, the ...
J...'s user avatar
  • 1,281
19 votes

How can the contact points of jumper cables handle the hundreds of amps required to start a car?

Have you noticed how the clamps get hot after a jump-start? That means power is lost, it ends up in those clamps. You're right the connection isn't ideal so power is lost but not so much that ...
Bimpelrekkie's user avatar
  • 80.8k
19 votes
Accepted

For how long should I charge my dead car battery without damaging it?

A typical Pb-Acid car battery is charged with around 13.8V. So 13V will be fine. A battery trickle charger is a voltage source, meaning that as the battery voltage rises, the charger puts less ...
Aaron's user avatar
  • 7,572
18 votes
Accepted

Why aren't starter circuits protected?

There's only any point in trying to protect a circuit with a fuse if there is enough headroom between the operating current, and the fault current, to ensure that the fuse won't blow in normal ...
Neil_UK's user avatar
  • 165k
18 votes
Accepted

Is there a technical reason why sensors goes from 0.5 to 4.5V volts?

Fail-safe. For a 0-5 V signal, how do you determine if a wire is loose and you measure 0 V or if the sensor has bottomed out? If your sensor is open-collector with a 5 V pull up and you read 5 V ...
winny's user avatar
  • 14.4k
17 votes
Accepted

Automotive ground shift phenomenon

My question is this: is this phenomenon something that truly exists, or is it simply an internal "old wives tale" with little to no basis? Well, do the math. If you sink let's say 100 A into a steel ...
Marcus Müller's user avatar
17 votes

Why aren't starter circuits protected?

Having a power fuse blow on the circuit that is going to the engine starter can result in a serious safety issue, especially in marine systems where the inability to start the engine can mean limited ...
JRaef's user avatar
  • 3,641
17 votes
Accepted

Sophisticated order to connect the poles for a jump start of the car battery

Positives first (any order.) Negative pole to battery negative pole (only on one battery.) Negative pole to chassis of the car for the remaining battery. Disconnect the negative pole from the chassis ...
JRE's user avatar
  • 70.9k
17 votes

What does an EV "wall socket charging cable" do?

It is for safety. It can't be omitted. The box tells the car how much current is available at maximum so the car knows how much is the maximum current it can draw. The box also turns on mains voltage ...
Justme's user avatar
  • 144k
17 votes
Accepted

Windmills generate electricity by the spinning blades. Why can't EV vehicles recharge their batteries from their spinning wheels? Is that too hard?

They do. It's called regenerative braking. The only time you can practically recover energy from the wheels is during braking. Taking energy from them slows them down. If you do it while they are ...
GodJihyo's user avatar
  • 22k
17 votes

Why do car blinkers flash at twice the speed when a blinker light has burned out

While all previous replies are correct, a clearer (and certainly more entertaining) explanation is this video from Technology Connections. Bottomline: Properties of bimetallic flashers present slight ...
Daniel Melendrez's user avatar
15 votes

Can a 12V LiFePO4 battery be charged using a car battery?

If you just hook the LiFePo up to a car battery, you'll get a lot of smoke from both the cables and the batteries involved, and it can also cause the LiFePo to ignite or explode. The problem is that ...
Jonathan S.'s user avatar
  • 17.8k
15 votes

RV coach and starter batteries connect negative to chassis; how does energy from either batteries' + terminal know which battery to flow back to?

If charge leaves a battery via its positive terminal (using +ve charge convention here, don't worry about electrons, the sign only confuses things), then an equal amount of charge has to enter that ...
Neil_UK's user avatar
  • 165k
15 votes

Help finding a circuit for discharging a high voltage capacitor

Forgive me for addressing the problem instead of your intended solution. A 4.7 kOhm power resistor draws negligible power compared to the motor driver (70 W vs ~ 20 kW peak), and does so only when the ...
Davide Andrea's user avatar
14 votes

Why are there so many relays used in a car, instead of transistors?

The voltage in a car is only about 12V, which means that even moderately powered components can draw large currents. The dash in my car is illuminated by four 12V 2W bulbs. They draw a current of ...
Robert Stiffler's user avatar
14 votes

Why are car batteries still so heavy?

The answer is very simple: Because we haven't found anything better. A car battery needs to hold its charge over a long period of time, be able to deliver a huge current and fit into a small space. ...
Andrew's user avatar
  • 6,912

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