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Car battery disconnecting

If I disconnect the negative post of the car battery to the car while it’s running and it dies out what will be the problem!??? Could it be the altenator and how can I be sure!??? Thanx!!!

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Okay thanks! How can I find out if the alternator isn’t working and needs replacing???

Measure the voltage at the battery while the engine is running.


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That’s the problem the car doesn’t start it needs to be jumped. It started with the headlights not turning on. Only the high beams. Then battery draining now it seems it killed the battery!?

Then disconnect the battery from the vehicle and recharge the battery.

Reconnect the battery, start the engine, and measure the alternator voltage output at the battery.


Ok thank you

My understanding is that you shouldn’t do this because you risk damage to various electronic components.

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I shouldn’t do what!?

Shouldn’t do what ? Mr. Lion is answering the question that you asked about removing a battery cable while engine is running.

Charge you battery and then if you are in the US Autozone will check your battery and alternator for free.


What if the alternator isn’t working, and the parts store is fifteen miles away?

You make to the parts store to have the alternator tested, but can’t get the car started because the battery is dead.

Do you buy a new battery to get the car started to find out if the alternator is bad? And if it is, do you change out the alternator in the parking lot?

Most parts stores don’t allow that.

It’s better to find out the problem where the vehicle sits.


Running the engine w/the battery disconnected can result in expensive problems to the car’s sensitive electronics. The battery provides the buffer needed to remove voltage spikes from the alternator’s output, so they don’t damage stuff like the engine computer (ecm), radio, instrument panel, and the like.

If the alternator is working properly, the battery voltage will measure about 12.6 volts before the first start of the day, and will increase to 13.5-15.5 volts immediately after starting the engine. An inexpensive DVM is the only tool the diy’er needs to make this test.