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Car dies after jump starting: Different types of alternators; self exciting vs non self exciting

September 22, 2017

So you go to jump start a car, and soon after you disconnect the jumper cables the car dies, what gives? Is it a bad battery? Bad alternator?

If you do a quick google search you will think you have a bad alternator, but often this isn’t the case, and its not the battery either.

The answer is more complex than that, because not all alternators work the same way. There are two types of alternators, self exciting, and non-self exciting.

Self exciting alternators are able to run the full demand of the cars electrical system stand alone from the battery.

Non self exciting alternators depend on a number of possible inputs such as a 12v supply from the battery, a signal from the ignition and for some vehicles an input from a regulator.

The non self exciting type is particularly¬†problematic on vehicles with “intelligent” or “smart” alternators. These intelligent systems actively sense on a timed interval the demands of the system and adjust alternator output as required. The problem when you jump start this kind of system is that it reads the current from the other vehicle (or battery pack) and assumes there is plenty of power and therefore does not excite the alternator.

It may seem counter intuitive, but one of the most effective methods for exciting a non self exciting alternator is to increase current. The increase in current will trigger the regulator to demand (read excite) more output from the alternator. The best way to do this is to turn the fan blower to high and turn your low beam lights on.

Once excited, a non self exciting alternator will act like a self exciting alternator does; able to self sustain the full demand of the vehicles system. Therefore able to run the car after the jumper cables are removed.



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