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What happens if I disconnect the battery while my car is running?

I’ve read a number of responses and the consensus seems to be that while the car can technically keep running, disconnecting the battery will most likely fry the electronics, but I’d like to hear a more authoritative answer (both on the effects and their explanations).


. It doesn’ t seem unreasonable that one could loose a connection while driving and hitting a bump…and no cars have blown up as a result. So, I will say, nothing. But cars handle throwing the tran in reverse differently. So…To be sure, do it with you own car and let us know.

The battery of a car puts out about 12.6 volts. In order to charge the battery, the output of the alternator is over 14 volts. If you disconnect the battery, there will be a spike in the voltage from the alternator which will then send more that 14 volts to all the electronics in the car. This may damage these electronic components.
Even in the old days before electronics, when cars had a generator, disconnecting the battery would cause a voltage spike which could damage the ignition points and the radio if it was a transistorized unit and happened to be on.

Your battery does more than just provide electrical current. It also shorts AC, spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, potentially destroying every semiconductor circuit in your vehicle. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and the vehicle’s stereo as well. I’ve never been struck by a bus but I’ve never stepped out in front of one either. Some mechanics still do this little feat and some are lucky and some…not so much.

In the old days of generators and magnetos their would be no problems. In a current car the charging circuits will be unstable. Meaning spikes in voltage and current. Circuit boards don’t handle either well. Damage will vary but ignition, fuel injection, and dash pods are good prospects for damage.

Possibly it will keep running, maybe it will stall. What it will do is put a lot of stress on the alternator and possibly damage the vehicle’s electronics, as others have said.

So, if you jump start a car with a jumper battery to a car battery with one poorly connected terminal because of corrosion, and the car starts? Does that result in all the “circuit boards” failing when you disconnect the jumper battery ? I don 't think it’s a yes or no answer… This may have happened a significant number of times and I don’t see all these cars needing new electronics. Or, if a battery fails suddenly and looses internal connection, would that result in loss of cars electronics ? Just asking.

The problem is the alternator. As the alternator’s rotor goes round and round, each time a pole (winding) is passed, a burst of electrical power in the form of voltage and current gets generated. It works sort of like what would happen if you took a can and three tennis balls and removed one of them, then shook the can back and forth. The analogy of the tennis balls bouncing off the ends of the can, a similar effect is what produces the burst of electricity in the alternator.

So the alternator generates electricity in a series of electrical bursts, fits if you will, rather than what would be more desirable, a more or less constant voltage and current output. If you hooked an o-scope up to the output of an alternator (sans battery), you’d see this happening on the display, and it wouldn’t be pretty. There’d be large voltage and current spikes everywhere.

The car battery sort of smooths the alternator output, making it more or less constant, suppressing the alternator’s electrical fits of bursts of power. Most electrical power supplies – for example the one in the computer you are using – use a capacitor for this same reason. And a car battery represents one huge capacitor. It’s there anyway, so the car engineers just decided to use the battery as the capacitor for taming the alternator.

Running the engine w/out the battery connected, the risk is that the alternator’s voltage and current spikes could damage anything connected to the alternator output, which is pretty much everything, including the car’s computer circuitry and ignition components. All or most of those components have their own capacitors for protection against power spikes, but those capacitors have limits to how much they can take. Without the car battery, its a much iff-ier situation.

In my car, if the battery is disconnected it stalls immediately. No ECM, no fuel pump, nothing.

@nw2012 … your car may have a built in safety protection circuit which detects a battery disconnection event and immediately cuts off the alternator output and turns off the engine. I think I heard that one of the latest Ford econoboxes has 70 separate computers in it alone. 70 computers in one car. That’s a lot. And the car designers don’t want an easy to do DIY’er accident like disconnecting the battery while the engine is running to destroy all 70 computer chips at once. Without some form of protection, disconnecting the battery would be so expensive to repair, it might just total the car.

As others have said with the battery in the circuit, the voltage is fairly constant i.e. the ripple is low and any spikes are damped. When the battery is out of the circuit, the AC ripple is a lot higher; the voltage regulator has a harder job holding the voltage at the correct point; and any voltage spikes generated by switching solenoids or running electric motors will ride on top of the steady state voltage. Usually the electronics can tolerate the supply voltage instability but sometimes the spikes bleed through to the computer programs causing erroneous actions and program crashes. In most cars you have the fuel pump buzzing away with all sorts of electrical noise.

@Dagosa…if you jump start a vehicle you need to have a battery in the vehicle being started. That battery acts as a “damper”. If you removed the battery and then tried to jump start the vehicle with just jumper cables to vehicle cables…damage will probably result before the vehicle ever starts. The resistance created by trying to jump start a vehicle in that manner would be through the roof.

I found my car battery dead. A jump didn’t work completely. As a quick solution, my battery was removed, another vehicle battery installed, and then the car started just fine. While it’s running, the battery was removed, the engine kept running, and my battery was reinstalled. I believe the car did not get damaged as it’s an antique 1983 Chevy with a carburetor and none of that computer controlled garbage. The battery recharged just fine as it was drained due to me leaving the dome lights on for a week.

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FYI, You responded to 5 year old post. Note the text in the upper right corner of the post before yours - Dec '13. That means it was posted in December of 2013.

@Andrew_Waterworth Andrew as the banner says , Welcome to Car Talk community . I fail to see just what you have added to this old thread that is of any use If you are applying for a Moderator position I am sorry to inform you that we already have a very intelligent and articulate person by the name Carolyn to handle that job.

06 elantra battery died when dome light was left on. No jump box, cables, or charger around so took battery from late 70s early 80s ford pickup hooked up to mine started no problem then removed truck battery and connected mine while running. Now no dash lights blinkers all blink fast now front and rear. No power windows no radio no brake lights no rear running lights. Highs and lows r fine dome and map lights fine reverse lights good PRND321 on dash light up but no speedo or tach lights. Did i fry half my shit? Fry as in irreparable? Also i was taking fuse box out under hood to check for some kind of burn halo mark underneath and the ratchet touched both terminals baaahaha… WAM! felt great. Although nothing seemed to happen from that just figured id put it out there.

And here’s a real world example for why you shouldn’t remove a battery while your car is running!