Reading 12V from battery post to cable
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Freshman User
Joined: Oct 2009
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From: Grand Junction, CO
Reading 12V from battery post to cable
Truck: '88 F150 with 5sp 302
If I take the positve lead off the battery and put a voltmeter between it and the positive battery post, I read 12V. Is this normal?
80skeys
If I take the positve lead off the battery and put a voltmeter between it and the positive battery post, I read 12V. Is this normal?
80skeys
Elder User
Joined: Nov 2006
Posts: 973
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From: Texas
Yep, it wouldnt if you took both the pos, and neg grounds of the battery. But with the ground still on it forms a circuit through the volt meter.
Be Careful! you can blow the fuse or even burn up your volt-meter if the vehicular is drawing to much amperage. I'v done this before, haha.
Be Careful! you can blow the fuse or even burn up your volt-meter if the vehicular is drawing to much amperage. I'v done this before, haha.
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Freshman User
Joined: Oct 2009
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From: Grand Junction, CO
Wouldn't the expected voltage be 0 unless there's a drain or short somewhere. Remember I'm hooking the positive lead from the volt meter to the positive disconnected cable, and the negative lead from the volt meter to the battery positive post.
Post Fiend
Joined: Apr 2004
Posts: 6,473
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From: Lafayette, IN
I remembered exactly what you said. Again, it's meaningless for troubleshooting. If you think you have a drain somewhere, you can hook up an ammeter as you described (but do it on the negative side), one lead of the meter on the negative cable, the other meter lead on the negative battery post. Make sure you have everything off--interior lights and the underhood light, both of which can pull several amps and fry the fuse in your ammeter. You may have to pull the bulbs. Then, measure what the drain on the battery is in milliamps. Anything over about 50 mA is suspect. One by one, pull the fuses and re-check your meter. When you pull a fuse and get a large drop in current, that circuit is likely your problem.
Another thing to check is the battery itself. Pop the vent caps off, and this time measure the voltage between each adjacent pair of cells (just stick the leads right down in the acid). Working from one side, measure 1-2, 2-3, 3-4, etc. All should be right around 2.1V. If you have a short in the battery, one pair will probably read either very low or zero. You should also get the same voltage between 1 and 6 as you do between the two lead posts.
Jason
Another thing to check is the battery itself. Pop the vent caps off, and this time measure the voltage between each adjacent pair of cells (just stick the leads right down in the acid). Working from one side, measure 1-2, 2-3, 3-4, etc. All should be right around 2.1V. If you have a short in the battery, one pair will probably read either very low or zero. You should also get the same voltage between 1 and 6 as you do between the two lead posts.
Jason
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Freshman User
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From: Grand Junction, CO
think you have a drain somewhere, you can hook up an ammeter as you described (but do it on the negative side), one lead of the meter on the negative cable, the other meter lead on the negative battery post. Make sure you have everything off--interior lights and the underhood light, both of which can pull several amps and fry the fuse in your ammeter. You may have to pull the bulbs. Then, measure what the drain on the battery is in milliamps. Anything over about 50 mA is suspect. One by one, pull the fuses and re-check your meter. When you pull a fuse and get a large drop in current, that circuit is likely your problem.
Jason
Jason
Post Fiend
Joined: Mar 2002
Posts: 19,402
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From: Bonita Springs FL
If you are measuring voltage, you do not have to interrupt the circuit to measure it. You will get the exact same result just by touching the leads to both battery posts. The voltmeter itself draws a small amount of current, just enough to measure the voltage.
In other words, you can measure voltage in parallel with the circuit.
Amperage is a different animal. If you try to measure amperage in parallel, you will only see the current that the meter is drawing, you won't know how much current is being drawn by the circuit. You must measure amperage in series by interrupting the circuit and putting the meter into the circuit.
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