You've really only got to fire one plug one time to start a car that's in a fundamentally startable condition. A battery that doesn't have enough juice to turn a starter motor (which requires a lot of amps) often still has more than enough to fire the plugs if you can turn the engine over some other way.

Several times in my life I've had to roll-start a (manual) car with some regularity[0]. You don't have to getting it going to even a brisk walking pace if it's a small (< 2 liter) four cylinder engine. If you clutch out and back in pretty quickly, it'll start if it'll fire. If the battery is too dead to run the ECU and/or plugs, you're pretty well dicked unless you can find somebody to jump you.

Now, neither of these applies to an F-150, since you can't get one with either a manual or a four-banger, but practical experience indicates that for at least some cars, it's got to take less than 9000 joules to start the engine.

In rough numbers, if you have a 1500 kg car moving 1 m/sec, you've only got 750 joules to work with. In all cases, I wasn't trying to do this in the dead of winter.

[0] On 2 different cars:

1) I left the headlights on two days in a row and discharged the battery pretty deeply. I got it back after a couple days of driving my short commute.

2) I had an intermittently bad connection between the battery lug and the wire running to the starter motor. I finally figured it out after it did it at night and I could see the sparks with the hood open. That was after 2 or 3 months of bump-starting the car at least once a week :-)