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Today, my car didn't start. I think that the crank was turning over because I could hear the loud sound a car makes just before starting up but the car was not starting. Anyways, I called a mechanic. He changed the battery and the car works now. Coming to the point, I initially thought that since the crank was turning, there was no problem with the battery (like a weak battery) since I presume that turning over the crank requires a lot of energy. But in this case, it looks like there are parts in a car that requires even more energy than the starter motor, in the starting process of a car. So my question is, could you present a generic comparison of energy required per car part? Especially, which parts in starting process require more energy than the starter?

EDIT: Or am I completely out of the ballpark by thinking that there are parts that require more energy than the starter?

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For starting an engine, the battery should have a minimum voltage of 9V and above. if that is not available, then starter will be able to crank the engine, albeit weakly but would not be able to start the car. Low battery voltage can happen due to a defective battery or because of quiescent current consumption that drains the battery. Generally Battery is required only for starting the vehicle and powering the components when engine is not running. in all other cases, it is the alternator that provides the electrical energy

  • I would say you are wrong on several accounts here. You will need more than 9V to start a vehicle (in most vehicles). Once the battery gets low enough, even if the power is enough to energize the starter, there may not be enough to actually fire the ignition. The battery is used as a buffer (like a capacitor) for electrical spikes. This protects the electronic components of the vehicle from damage. While the engine is running, the battery supplies the power and the alternator provides the energy back to the battery. Yes, the engine will run without the battery, but it's not good. – Pᴀᴜʟsᴛᴇʀ2 Oct 28 '15 at 18:54
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The part of starting a car that takes the most energy is overcoming the compression inside the cylinders. When the engine is running, the fuel/air mix in each cylinder is compressed by the firing stroke of another cylinder, but when starting, the starter motor has to do it all.

If you think that the engine was turning over, but it was not starting, it is quite likely that it was not turning fast enough (as the battery didn't have enough charge) for the engine to start running by itself.

  • Also, if the battery has a low charge, the voltage drop from running the starter can affect the ignition system so it cannot provide the voltage needed for creating sparks on the spark plugs. – Allman May 20 '15 at 9:42
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When a starter is energized and cranking the engine, it will pull more power than any other electrically operated part on the vehicle. You say you think the car was cranking over ... I'd submit that maybe what you thought you heard was not what was actually happening.

  • On an undercharged battery, the massive current draw from the starter motor can easily cause voltage to drop too low for spark. – Leliel Oct 28 '15 at 2:17

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