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A lot of people've been there. I also have done this (unfortunately). But why does this happen? Procedure is very simple: turn the ignition on, engage 2nd gear, push the clutch in, get up to speed, throw the clutch. But why does the car start this way? This works even if the battery is completely dead. But where does the spark come from then? Even if battery is dead enough so it's unable to power the starter motor and turn the engine over but still has juice to run the fuel pump and power the spark plugs -- that's what I was thinking. Is it true? Thanks

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The thing is, it won't do it if the battery is completely dead. Here's what's going on ...

When you go to push start the car with a severely weakened battery (or a bad starter or whatever), when the engine starts to turn over, there's enough power to the alternator to create a field within the alternator, which in turn gives you enough power to power the ignition and other things to get it running. The only way a push start would work with a dead battery is if the vehicle was built with a generator and not an alternator. If there's no power from the battery, there's no field generated in the alternator, which means your vehicle won't start. I found this out the hard way once. I thought I was smart in my 2003 Honda Civic and tried this. Got it 1/2 way down the block before I realized it wasn't going to start. I had to drive my truck down there to give it a jump.

I don't know what the minimum power requirement is from the battery for a push start to work, but would suggest it's probably around the 8-9vdc arena. This would be far less than what is needed to actually get the starter to kick the engine over, but should give it enough power to get the energy flowing through the alternator to power things when it starts moving.

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  • I push started a few OLD cars; standard transmissions and generators ; generators were worthless at cranking or pushing RPM. So , because the generators were no help , there was always enough in a "dead" battery for the low power needed for ignition ( only needed for a matter of seconds) . Fuel pumps were mechanical.
    – blacksmith37
    Commented Jan 29, 2022 at 22:16
  • The only problem I remember was ice and snow; the wheels could slip and not turn on ice.
    – blacksmith37
    Commented Jan 29, 2022 at 22:22
  • A generator provides nothing during push starting The same for alternators until the engine runs. This means the battery is powering the EFI system. If everything is turned off (heater blower, radio, ac, internal/external lights, etc) with ignition on, virtually all car batteries have enough remaining capacity to supply the ignition system if not the entire EFI system for a push start.
    – F Dryer
    Commented Jan 31, 2022 at 4:12
  • @FDryer - You really need check out how generators work. They have permanent magnets which don't require a power source to get them working. And batteries can easily be down below the point where the EFI system will not function correctly. If it is below this point (as stated, probably below 8-9vdc), the car isn't going to start. Before you downvote me out of spite, you should really check your facts.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 31, 2022 at 11:34
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    Apology accepted. Let's move on. Since generators are no longer used and every engine with EFI uses alternators, push starting a vehicle relies on the battery. Your approximation of 8-9 volts may be the absolute minimum to keep the EFI system running.
    – F Dryer
    Commented Feb 5, 2022 at 19:39
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What you are saying is correct, the battery doesn’t have enough charge to supply the required current for the starter motor, since it could require something like 100A to make it turn the engine over.

Unless the battery is totally flat, then there will likely be enough current to power the essential components of the engine, like the fuel pump and ignition system, without the battery voltage dropping too low.

By manually turning the engine over by pushing the vehicle you enable it to start, and then once running, the alternator can power everything and charge the battery.

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