What is that clicking noise when you try to start a car with a dead battery?
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A regular gasoline powered car has an electric starter to start the engine turning. The battery powers the starter for a few seconds until the spark plugs firing begins to take effect.
The starter is an electric motor with two main parts. The motor itself which of course spins to turn a gear, which turns a gear on the engine, and a solenoid to engage that gear with the engine.
But this starter gear isn’t always engaged. Normal engine speeds would spin the starter so fast it would be destroyed.
So, the gear slides into place only at the beginning of cranking the engine when you’re holding the key in the start position. Basically the way gears are changed in a manual transmission. Instead of making you pull a lever to shift the starter gear into position there is a solenoid on the side of the starter which moves it.
A solenoid is an electromagnet that attracts a piece of metal when you feed electricity to the magnet. Give it 12 volts and the starter gear is slammed into place just before the starter motor starts turning.
Normally that sound is drowned out by the sound of the starter motor and then in a second by the engine running. When you release the key from the start position the solenoid loses power and a spring pulls the gear back to disengage it from the engine.
When the battery is almost dead there’s not enough power to spin the starter and that click you hear is the solenoid pushing the starter gear into place. Clack! But no starter spinning or engine running noise so you can hear the gear shift position forward and then the spring pull it back when you release they key.
If you keep trying to crank it what little energy is still in the battery is wasted and each successive clack is quieter as the battery gets weaker. Eventually it won’t even click anymore.
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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