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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3593
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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When I take my DMM positive lead and put on negative battery

Customer Question

When I take my DMM positive lead and put on negative battery terminal and take the negative lead and put on the negative battery cable in my car, im getting -12v, is this normal?
Submitted: 1 year ago.
Category: Electrical
Expert:  Kevin replied 1 year ago.

Hello and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.

1) You have reverse polarity. Thus the reason why you are showing a negative (-12VDC).

Reverse the meter leads on the battery posts and it will show a positive (+12VDC) which is the correct reading on a car battery.

2) The red meter probe is placed to the battery positive terminal (red wire) and the black meter probe is placed to the battery negative terminal (black wire). Make sure you insert the black meter probe wire into the meter COM terminal hole. The red meter probe will insert into the meter hole that shows (V) and it is typically colored also as red.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

Otherwise, don’t forget to rate me before you log Off.


Customer: replied 1 year ago.
I think you misunderstood me, I'm not testing the battery, but from the batteries negative post to the negative battery cable, and getting voltage...
Expert:  Kevin replied 1 year ago.

If measuring, negative to negative it will show a (-) reading on a DVM. Your 12 volt measurement is correct, it is just the polarity is reversed since measuring from neg to neg.

Customer: replied 1 year ago.
Okay, now I was also wondering, if I wanted to know if somewhere in my system I have a "bad" ground from inside the engine compartment, is there a way to do it?
Customer: replied 1 year ago.
I have a feeling there is, because when I plug my phone charger into my auxiliary (12v) sockets, I get static interference on my radio
Expert:  Kevin replied 1 year ago.

1) The negative of the battery is ground which is also grounded to the vehicle frame.

2) Yes, if a ground loop exists due to a loose ground wire or a loose ground termination, this can cause static.

3) Set the DVM to read ohms (resistance) and probe the battery’s negative stud and ground connection on the accessory. If you have a reading less than five ohms, the ground is OK. If greater then 5 ohms, then a faulty ground connection, most likely at the AUX Socket connection/termination.