What is that clicking noise when you try to start a car with a dead battery?
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Imagine if you have a super bright light – brighter than any flashlight.
Now you have a switch to turn this light on and off.
This light can use a lot of energy because it’s so bright, but a little touch can turn it on and off because of the switch.
There’s a device called a relay – which works on that exact principal. A relay is a light switch, with a tiny electric motor that turns it on and off. That tiny motor only needs to be strong enough to flip the switch.
A relay makes a distinctive clicking sound anytime it flips the switch. This is a malfunctioning relay that keeps switching:
[https://www.youtube.com/watch?v=Xk378Vpna4A](https://www.youtube.com/watch?v=Xk378Vpna4A&t=30s)
If a car battery is weak – sometimes there’s not enough electricity for the relay to flip the switch so it keeps trying.
If the car battery dies – the computer that flips the switches loses it’s memory. Then it doesn’t remember if the flip is switched on or off. Now the computer might send a signal to everything telling it to flip off, and then send a new signal telling it to flip on.
Sometimes when jumping a car – the starter will take all of the electricity – and the computer loses it’s memory over and over which can cause the process to keep repeating.
Similiar to relays, as u/rc3105 stated – cars also have solenoids. These operate on a similar principal where you have a small linear electric motor that can be used to do things like shift gears in a transmission or lock torque converters.
Sometimes there’s lots of things we don’t think about that we don’t think about much. I had a car with a 6 disk CD changer. If the battery is low and the car is jumped – you can hear the radio making a bunch of noise as it forgets everything. The radio then goes through a routine where it checks each of the 6 internal slots to see if it has a CD.
I’m a used car dealer, and my career works by using my eyes and ears to make quick gambles. There auctions where we get cars are like this:
[https://www.youtube.com/watch?v=TdwRiZdl3dM](https://www.youtube.com/watch?v=TdwRiZdl3dM)
Someone drives the car through a lane in order to get it sold to a dealer. We can walk around the car to look for damage and also try to listen very carefully in a very noisy environment. In seconds we create a mental estimate of how much the car will cost to repair, what we could sell the car for, and then we decide on a good price to buy it for.
As a result I have to be very aware of any sounds the car makes. If I’m confident the car sounds good and is running smoothly – I can try and offer a higher price to have a higher chance of getting it. If the car makes a noise – I have to think of what the problem is and how much it costs to fix that.
We gamble thousands of dollars just based on noise. If I don’t hear a problem – then I have to pay to fix it. Some people are scared by noise and avoid those cars. If I can diagnose the problem by sound, and I’m confident it’s a cheap fix – then I can place a higher bid on that car than another person who might be more risk-averse.
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