[anupmathur]

Originally Posted by headers

Guess I'm still sure about cars not being able to run with battery positive terminals disconnected!

Like I said, expensive cars might not, if they have much monitoring equipment, but normal mid budget cars do.
Best for you to seek advice elsewhere. Battery chaps or car mechanics will tell you and can show you!

Originally Posted by headers

this is from the link you provided. Guess I'm still sure about cars not being able to run with battery positive terminals disconnected!

Headers, you have not said which car you tried with.
If your engine is dying out on disconnecting the battery terminal, maybe the alternator is nearing end of life:

So, how can you tell if the alternator is failing without taking it apart and doing some measuring inside the alternator? It's really pretty simple. You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.

Go to your car mechanic or battery chap and say you want to have your alternator checked/tested. Here is one of the ways they test:
He will start the car, disconnect the positive terminal from the battery and measure the voltage at the alternator. That will be close to 14 volts or a little over, even with lights etc.. turned on.
Then he will turn off the engine (or maybe not!) and reconnect the battery terminal.

[headers]

Originally Posted by anupmathur

He will start the car, disconnect the positive terminal from the battery and measure the voltage at the alternator. That will be close to 14 volts or a little over, even with lights etc.. turned on.
Then he will turn off the engine (or maybe not!) and reconnect the battery terminal.

Hey, I agree to the process of measuring the effectiveness of the Alternator etc etc, but the check across alternator terminals is done without disconnecting the battery.

The Spark Plugs get their juice from the Battery. Thats why even if your alternator fails, you can drive the car till the battery is dead.

Older Diesel engines do NOT require a battery once started [Eg. 137D, Amby diesels etc ] but most modern diesels too require the battery to be connected for the car to run!

And in petrol cars, the battery completes the circuit!

Cheers

[anupmathur]

Originally Posted by headers

The Spark Plugs get their juice from the Battery. Thats why even if your alternator fails, you can drive the car till the battery is dead.

And in petrol cars, the battery completes the circuit!

OK, I call it quits here. Wish when you do learn the truth you'll let me know!

[SirAlec]

It will run but you have to keep someone inside the cabin and asked him to press the accelerator paddle to keep a constant high rpm. not sure about high end cars though.

[anupmathur]

Originally Posted by SirAlec

It will run but you have to keep someone inside the cabin and asked him to press the accelerator paddle to keep a constant high rpm.

If none of the lights etc are on, even idle rpm is OK for the alternator to keep the engine running.

[esteem_lover]

Originally Posted by anupmathur

Go to your car mechanic or battery chap and say you want to have your alternator checked/tested. Here is one of the ways they test:
He will start the car, disconnect the positive terminal from the battery and measure the voltage at the alternator. That will be close to 14 volts or a little over, even with lights etc.. turned on.
Then he will turn off the engine (or maybe not!) and reconnect the battery terminal.

I have had my alternator checked many times, but never with the battery disconnected.

[star_aqua]

Originally Posted by esteem_lover

I have had my alternator checked many times, but never with the battery disconnected.

with anupmathur. Nothing wrong with checking alternator with the battery disconnected. thats the actual method to check the voltage developed by the alternator at idle rpm.
and why cant an engine run without a battery? its the problem with your alternator which doesn't supply enough voltage at low idle.
I have done push start of my M800 when the battery was completely dead which could not even able to glow a small LED. How did it start with no juice from the battery??

[esteem_lover]

Originally Posted by star_aqua

I have done push start of my M800 when the battery was completely dead which could not even able to glow a small LED. How did it start with no juice from the battery??

Did you try push starting without the 'dead' battery ?

[star_aqua]

Originally Posted by esteem_lover

Did you try push starting without the 'dead' battery ?

Nope. i did not remove the battery. What difference will it make?

[adheesh]

To test the alternator, start the engine and disconnect the positive terminal of the battery. The engine should still run if the alternator is fine. If the engine stalls, do the above thing once again with the engine raised at about 1200rpm .If the engine still stalls, there is some problem with the alternator/wiring.

Also the engine MAY not pushstart since many fuel systems now a days have a idle circuit solonoid which needs to be open for the idle circuit of the fuel delivery system to get fuel. The solonoid valve remains open when there is current supplied to it

I hope this solves all confusion

Best Regards,
Adheesh Parelkar

[hrag]

Originally Posted by headers

Please try removing the battery after starting the vehicle and running it, especially a petrol car. It will NOT run!

I agree with headers. I have seen this happen in my Indica. After starting, the battery was disconnected - the engine ran for a few minutes and then died.

[star_aqua]

Originally Posted by hrag

I agree with headers. I have seen this happen in my Indica. After starting, the battery was disconnected - the engine ran for a few minutes and then died.

Hrishi, its again a alternator problem. your engine could able to run for few minutes after removing the battery. why? The capacitor/condensor coil will eventually die because of inadequite supply from the alternator. if your alternator could able to supply enough voltage and current at low idle, then your engine would never die. The alternators are usually designed to develop enough voltage at idle speed. but as they get older, the operating speed will shift to higher rpm. I have seen quite different behaviour with different alternators. the ideal one will work at idle speed and when there is any electrical load, it will directly taken from the alternator which in turn loads the engine as they become more rigid and your engine will feel that extra load. This you can also observe in Palio 1.3MultiJet.
Say if your alternator working speed is shifted to higher rpm, then your battery acts as the source only till the alternator voltage goes higher then the battery's. These issues will not affect the normal running of the engine, hence we do not notice it.

[hrag]

Vijay, how will the alternator of a petrol car supply the load demanded by the ignition system, fuel injection system and the fuel pump? All these work off the battery, right?

The diesel Indica is a different story.

[SirAlec]

Originally Posted by anupmathur

If none of the lights etc are on, even idle rpm is OK for the alternator to keep the engine running.

Probably your idle rpm were set too high than normal.

[star_aqua]

Originally Posted by hrag

Vijay, how will the alternator of a petrol car supply the load demanded by the ignition system, fuel injection system and the fuel pump? All these work off the battery, right?

The diesel Indica is a different story.

It again depends on the rating of your alternator. Do you think the amount of energy required to turn/run the alternator at a constant speed is allways same either you draw current out of it or not? The alternator(Source) becomes rigid when there is any load(sink) connected to it. and the rigidness depends on the load. its electrical property of the alternator and no electronics invloved. Thus the additional load on the engine. Please remember Turning your headlights ON will consume fuel. how? ofcource negligible, but it does due to the above said reason.