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There are some comments on the question Are there any modern diesel engines that require electricity to continue running? suggesting that disconnecting the battery while the car is running can damage electrical components.

In the 1960's to 1980's alternators had voltage regulators, once the car was running the battery was just a load, like a light bulb. I have memories of needing to drive more cars then we had working batteries for. Put the positive terminal in an insulated container so it does not touch anything and two people drive two cars, with one battery.

How could disconnecting it be any more harmful then turn the headlights on or off?

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Removing the battery with the car running is a bad idea. Rather than re-writing something which exists on the interwebz, I pulled this from Troubleshooters.com under the section called Automotive Troubleshooting:

Your battery does more than just provide electricity. It also shorts AC spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, endangering every semiconductor circuit in your car. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and even the car's stereo.

Even if your computers and stereo remain intact, in a great many cases removing the battery burns out the diodes in the alternator, necessitating a new alternator. If disconnecting the battery interferes with the voltage regulator's control voltage input, it's possible for the alternator voltage to go way over the top (I've heard some say hundreds of volts), frying everything.

Even the initial premise was wrong. If you disconnect the battery and the car conks out, you don't know if it conked out due to insufficient alternator current, or whether the resulting transients caused your ECU (the car's computer, which controls fuel mixture, timing, and much more) to spit out bad data, shutting down the car.

Nobody should EVER run your engine without a battery.

And yet when you tell them not to, they'll roll their eyes. "I'm a professional. I do this every day. It's fine!" They'll sound so authoritative. So commanding. So in charge. So intimidating. But they're wrong.

The problem, of course, is that disconnecting the battery doesn't always damage something. It does it only sometimes. Less experienced jump start professionals and automotive technicians figure if they got away with it a few times, it must be OK.

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    Really? That's how I test an alternator. (Fire it up, disconnect the neg. battery cable.) :( – 3Dave Apr 4 '18 at 18:08
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If the car has electronic components, like ECUs, central commanding modules, etc, doing that "may" fry them...or not. Sometimes the ECU turns off the engine if the battery is non present. Same reason the makers discourages checking spark against the engine metals.

I had a Daewoo Nexia some years ago, and it didn't liked that, no matter how much I revved it up to make the alternator try to supply enough current, it just stalled.

However, it can be done in older cars because they have rather simple electric systems, no computers, no electronic modules stuff, just alternator, battery, coil and distributor with contacts and capacitor :) Nothing to damage.

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The answer is a bit complex here:

As Paulster2 has mentioned, the battery does help protect from transients, but as the alternator has its own regulator and the computer and audio system have their own as well, this should become useful just if there is something else wrong. Overall this is not really a problem and the article is from some guy promoting his ebook online...

The issue has to do with with the fact that the alternator is usually not designed to handle a 100% load:

When the engine revs higher, you trigger the spark coil(s) more often, and as you open the throttle, the compressed air/fuel mix is more dense, necessitating more energy to trigger the spark, or when you use the car's audio system power demand increases, or use the windows, etc.

All of these create an extra demand for energy and, during the normal conditions the alternator provides more than enough, but if you suddenly accelerate, open the window and crank the sound up at the same time, it may not and the design expects the battery to pick up the slack since those are temporary conditions.

Not having a battery will create an extra load on the alternator's voltage regulator.

Typically, alternators die from bearing failure, but the second cause of death is regulators. Running the car a little bit without a battery is not a big deal, but running it for long period, even with a bad battery, will wear out the regulator as it will not be able to cool itself sufficiently and will eventually break.

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  • Here's the rub: the alternator's regulator can only react so fast to a massive step change in load, such as when you unhook the battery while the car is running. (Especially if it's a flat battery -- if you are going to unhook the battery on a running car, make sure it's fully charged first!) The resulting transient is termed a load dump event, and is one of the most severe and stressful electrical fault conditions a car's low voltage system can experience. – ThreePhaseEel Mar 10 at 11:38
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If the engine's already running, disconnecting the battery will still keep the engine running.
If you're trying to start a car without a battery or with a very dead battery, it won't start (even with push start in a manual car) because there's no electrical supply (spark) to the combustion chamber.

The battery is a backup for extra electricity use in car (other than for engine cranking) when alternator is under heavy load, eg. hard acceleration or high power audio systems, which in turn gets charged by the alternator when it generates some extra electricity.

This electricity is a pulsating DC current which needs to be smoothed before fed to transducers eg. battery. Normally this 'smoothing' is done by a condenser & a voltage regulator. The battery, being similar in working technique (ie. anode & cathode plates being separated by a dielectric, creating a phase delay between charge & discharge currents) acts as an additional condenser, providing extra buffer in times of (unexpected) fluctations higher than what the rectifier is designed to handle eg. during lightning strikes or static electricity buildup (& sudden discharges) when travelling at high speeds (when air ionizes to form plasma, especially in moist/humid warm weather). Removing the battery under these conditions will effectively remove the contingency.

In modern cars filled with microelectronics, a good working battery is a reliable source of 12V power supply, to offer the contemporary driving experience. Without the battery, the driver would be taken back to the days before EFI & power assisted options like steering, braking, clutches, power windows/mirrors, etc.
Have you seen Massey Ferguson tractors that could be cranked by hand?

If the alternator is generating sufficient current to keep the engine & related electrics running, removing the battery for a short time wouldn't blow the house down.
However, the risk increases with time.
Given the above contingencies, having even a dead battery is better than no battery.

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  • RE "This electricity is a pulsating DC current" are you sure? I thought alternators delivered AC not DC. – James Jenkins Mar 9 at 12:19
  • The fluctuating magnetic field in the alternator produces AC in the coils, which is then rectified by the bridge rectifier before being smoothed by the condenser. – Zimba Mar 10 at 13:18

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