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I work at an auction site where we have to move cars from one spot to another. Often, we get a car with a dead battery that we have to jump.
I've noticed that if the jumper cables are removed a second or two after the car has started, it dies. To avoid that, we rev the engine to charge the battery.
It's because of a couple things: The battery is very dead and the alternator cannot put out enough power at idle RPMs to run the whole system and charge a battery. This leads to a lack of power which may kill one or more systems most likely the spark "ignition" system since this takes the most power and is necessary to run the car.
In Fact you can remove the battery all together making sure the positive cable is insulated and run the car that way too. At Least once you get it started.
Andre Borie has a good point. removing the battery can potentially cause damage to some of the more delicate electronics in the car. This is due to the battery acting like a big capacitor smoothing out some of the larger spikes in the electrical system. These spikes can originate from many different sources: the alternator, power surges from the coil packs, or even a wire crossing something like a spark plug wire causing some induction.
kmarsh below has a good point here is a video on how a self starting alternator works and if the car has been sitting too long the alternator can go flat too. Next time you think your alternator died try jumping it and forcing it on.
The majority of cars built in the last 10 years have what is known as a smart charge system, i.e. the alternator only puts out what the consumers demand. With a totally flat vehicle and a donor car connected the suspect alternator sees a fully charged battery with an existing alternator output so does not turn on.
I have fooled these systems by starting the flat vehicle and then turning on the interior blower to max before disconnecting the donor. This sudden rise in current drain usually has the effect of kicking the smart charge system into action.
Vehicle behavior depends on the two types of alternators:
A vehicle with a self-exciting alternator (aka one-wire alternator) will run without battery power. This describes my Honda and most Chevys. These vehicles can be jump-started without a battery and will continue to run once jumper cables are removed.
A non self-exciting alternator requires 12V power on one lead to stay energized and in operation. Vehicles of this type (and a dead or disconnected battery) will jump-start off of another vehicle, then die the moment the jumpers are removed. This describes my old Dodge. These vehicles will die without a battery once jumper cables are removed.
Both types of alternator have an internal exciter circuit. The difference is how that circuit is powered. The self exciting alternator has a built-in voltage regulator that stabilizes and feeds power to the exciter. The non-self-exciting alternator has (or has externally associated) a voltage regulator which relies on the battery to complete the exciter circuit to the alternator's external exciter lead, and regulate exciter voltage.
Many cars are very slowly draining the battery at idle. Alternators often produce sufficient voltage or current only in the higher power band. They charge the battery during the times that your engine is in the higher rpms during actual driving. This depends on the car model, the engine size (which indirectly influences the starter motor, battery size and alternator needed to charge the battery), running electrical consumers such as lights, A/C, but also the outside temperature etc. Therefore a car which has a completely flat battery might, depending on the rpm and the above-mentioned consumers, not be able to sustain the necessary voltage and and/or current in order to run the ignition system and/or engine management electronics. That is why if there is an issue with your alternator or battery and the battery light comes on at idle due to low voltage, it might turn off at higher rpm as the voltage might return to a sufficient level to charge the battery.
