someone told me once that if you have a bad alternator that if you have the car running and you disconnect the battery that the car will shut off,,,is this true?

 

yes, if it works the car will stay running

 

Let's assume a bad alternator has no output. Disconnecting the battery would kill the engine because there would be no power to run the ignition, injectors, ECU, or fuel pump.

If the alternator still had some output, but not up to spec, the engine may still run.

So, the answer to your question is ... Maybe.

It would be best to get an alternator tested. A lot of places will do this for free in hopes of selling you a replacement.

By the way, disconnecting the battery on a running engine is a big no-no. The battery acts as a noise and spike supressor. All those somewhat fragile electronic components in your car may not like it and just might ask you for $$$$.

 

pulling the batt cable on a computerized car can blow your computer and any other electronics or lights that happen to be on. they don't like to run on ac voltage your alt puts out. just thought I'd let you know. get a volt meter and see if the voltage is over 12.6. thats normal batt voltage with engine off. if alt is working it should be 13 volts or above. good luck.
PS. Most parts stores now will do an alt check for free.