I am electrically challenged, hence this question. Once a vehicle has started and is runnung, does the battery play any role in anything anymore? Can a weak battery cause poor engine performance? Does removing the battery from a running vehicle change anything? Thanks.

 

If the alternator is working...don't need battery after it starts.

 

Thanks, BPofMD. So if I were to disconnect the battery from a running vehicle, the vehicle would continue to run? Would the lights, radio, etc. still function as long as the engine is runing even with the battery removed?

 

Do not disconnect the battery on a running vehicle

People do it all the time, but they risk ruining the electronics in the vehicle. In most older vehicles, this may the the electronic ignition or the radio. On the newer vehicles it could be any of the engine computers, or the body computers.

The alternator produces alternating or A/C current like is in your house. It has diodes in it to convert the A/C to DC, but this DC is noisy and choppy. The battery is like a storage tank, and absorbs and cleans up the voltage from the alternator.

The alternator is also capable of producing 100's of volts dc. The regulator keeps this in check, letting it produce only 14.5vdc, but it can fail, and it's job is not as important if the battery is hooked up to the alternator. If the regulator failed, the battery will absorb most of the excess voltage, and the battery will start overheating and boiling, rather than the voltage running sky high and ruining the vehicle.

I have been there and done that is why I am so against pulling the battery cable off while the engine is running. I did that, and the vehicle ran for about 30 seconds more, and then quit, never to start again till I replaced the ignition module.

 

You don't want to let that happen. See below.

100% correct. Disconnecting the battery can trick the regulator into thinking the voltage is really low, so it turns the field on all the time and the alternator output skyrockets. This fries electronics, which explains Franklin's fried ignition module. The battery also acts to filter the output of the alternator. The alternator is essentially a 3-phase rectifier and does nothing on its own to clean up the signal other than rectification.

 

Thanks for the info, fellas. So the engine would continue to run, but at great risk to electrical/electronic components, the failure of which could, in turn, kill the engine. I wasn't intending to try this...I was prompted to ask about this in response to another question as to whether an old/tired battery could cause poor idling. Thanks again.

 

Yes. A bad battery can internally short and pull voltage away from the charging system. The ECM wants 13-14v and expects that it gets it, so the sensors are more or less calibrated for this voltage. When the voltage is lower, less significant changes show to computer as more significant then they are. This is why the imports are gaining 2-4mpg by adding more ground points, the sensors are receiving more accurate voltage so the ECM has more accurate information.