Battery Voltage vs Fuse block voltage

[jdross440]

With a digital volt meter I am showing 13.8 volts at battery (engine running at idle and above) but only 12.4 volts at each fuse input including the Auxiliary terminal. The onboard voltmeter reads even less. I had wired in a digital volt gauge that agrees to what I am seeing at the fuses. It goes as low as 11.3 volts. I had replaced my stator and rectifier and wired the rectifier output directly to the battery. 

Could the voltage drop be caused by the key switch?  Anyone else have this problem and found a solution. I would like for the fuse block and components downstream to see the real voltage available.

[saddlebum]

2 minutes ago, jdross440 said:

With a digital volt meter I am showing 13.8 volts at battery (engine running at idle and above) but only 12.4 volts at each fuse input including the Auxiliary terminal. The onboard voltmeter reads even less. I had wired in a digital volt gauge that agrees to what I am seeing at the fuses. It goes as low as 11.3 volts. I had replaced my stator and rectifier and wired the rectifier output directly to the battery. 

Could the voltage drop be caused by the key switch?  Anyone else have this problem and found a solution. I would like for the fuse block and components downstream to see the real voltage available.

Are you testing from the same ground point

[jdross440]

Yes. Using the negative battery terminal.  I get the same readings if I use the negative terminal on the fuse strip. It's the difference between positive battery terminal and the positive terminal on the fuse strip. Also the same readings if tested using the inputs to each fuse.

 

Edited May 17, 2021 by jdross440
More info.

[Marcarl]

Check the readings between the positive post and the negative post, then keep your one lead on the negative post and move the positive to the clamp on the post, if that reads well, move down the system with your positive lead, keeping the negative on the negative post.

[saddlebum]

37 minutes ago, Marcarl said:

Check the readings between the positive post and the negative post, then keep your one lead on the negative post and move the positive to the clamp on the post, if that reads well, move down the system with your positive lead, keeping the negative on the negative post.

After following Marcarl's steps then start again only this time leave the positive test lead connected on the battery and test along all your ground connections. If you see a sudden drop following Marcal's directions you have a bad connection or wire on the pos side.

If you find a sudden drop in voltage following my directions then you have a bad connection or wire on the ground side.

Also when you say "the same readings if tested using the inputs to each fuse." are saying the voltage before and after the fuse is the same or is it different? If the reading is  different before the fuse from what you get after the fuse I would say the fuse connections are probably corroded and/or dirty. This is specially true if you still have the old glass fuses. the fuse retaining clips on these not only get corroded but they often get very weak making for a very  poor connection to the fuse

Edited May 18, 2021 by saddlebum

[Flyinfool]

An easier way to test for the voltage drop.

Attach the positive voltmeter lead to the battery positive. Use the negative lead to probe each connection as you follow the power from the battery positive on out. This way the meter will read the actual voltage drop so you do not have to do all the math. a reading of zero is perfect but not realistic. Wire has resistance and every connection will have some resistance, resistance causes voltage drop, the goal is to have the lowest number possible. As you move out from the battery and find a connection that has a big voltage drop, fix it and then continue on.

[cowpuc]

On 5/18/2021 at 7:42 PM, Flyinfool said:

An easier way to test for the voltage drop.

Attach the positive voltmeter lead to the battery positive. Use the negative lead to probe each connection as you follow the power from the battery positive on out. This way the meter will read the actual voltage drop so you do not have to do all the math. a reading of zero is perfect but not realistic. Wire has resistance and every connection will have some resistance, resistance causes voltage drop, the goal is to have the lowest number possible. As you move out from the battery and find a connection that has a big voltage drop, fix it and then continue on.

@Flyinfool,,, GURU ALERT!! Fool who is no fool,, so what is happening here? Is the meter reading the difference of what is available in voltage at the source (battery) and what is available at the device?? Trying to put it in laymens terms so I can attempt to say what I mean = lets say you have 12vdc at the battery but, because of resistance/volt drop, at the device you  only have 11.5vdc.. Will the meter then show .5 or a half of volt dropage on its readout?  

[saddlebum]

On 5/20/2021 at 10:34 PM, cowpuc said:

@Flyinfool,,, GURU ALERT!! Fool who is no fool,, so what is happening here? Is the meter reading the difference of what is available in voltage at the source (battery) and what is available at the device?? Trying to put it in laymens terms so I can attempt to say what I mean = lets say you have 12vdc at the battery but, because of resistance/volt drop, at the device you  only have 11.5vdc.. Will the meter then show .5 or a half of volt dropage on its readout?  

Not exactly. When bridging a bad section the voltmeter acts as a bypass or detour. The worse the connection  or the higher the load placed on the circuit the higher the reading because more voltage is taking the detour. What you want to see is no volts at all in this type of test. Say you touch one  test lead to the Batt post and one lead to the clamp or terminal on that post. With the circuit powered or loaded up and you see no voltage  you know you have a perfect connection at the battery terminal.  If there is a reading you have a bad connection and the higher the reading the worse the connection. Keep in mind the bigger the load on the circuit the higher the reading as well and the easier to find the bad connection. I sometimes add an old head lamp to the circuit to make this test more effective. An old fashioned test light used in place of the voltmeter will work to. No light circuit is good. Any light at all circuit is not good and the worse it is the brighter the light. Here again you would have to clean or repair the bad connection or faulty wire.

Edited June 9, 2021 by saddlebum

[Flyinfool]

The amount of current that will flow thru the meter is so small that it does not matter. Most digital meters are above 20KΩ per volt and you are measuring a connection that is a fraction of an ohm or if it is a really bad connection it might get up to a couple of ohms. But as Saddlebum mentioned it does require that the circuit is actually turned on and working and drawing current thru the connections.

[saddlebum]

On 5/20/2021 at 11:56 PM, saddlebum said:

Not exactly. When bridging a bad section the voltmeter acts as a bypass or detour. The worse the connection  or the higher the load placed on the circuit the higher the reading because more voltage is taking the detour. What you want to see is no volts at all in this type of test. Say you touch one  test lead to the Batt post and one lead to the clamp or terminal on that post. With the circuit powered or loaded up and you see no voltage  you know you have a perfect connection at the battery terminal.  If there is a reading you have a bad connection and the higher the reading the worse the connection. Keep in mind the bigger the load on the circuit the higher the reading as well and the easier to find the bad connection. I sometimes add an old head lamp to the circuit to make this test more effective. An old fashioned test light used in place of the voltmeter will work to. No light circuit is good. Any light at all circuit is not good and the worse it is the brighter the light. Here again you would have to clean or repair the bad connection or faulty wire.

Also I find an ohmmeter used to check for resistance in a circuit not as effective to check for bad or faulty connections because as stated by @Flyinfoolan ohmmeter draws so very little currant. I have seen a circuit with a corroded wire test good by an ohmmeter because the green powder bridging the gap had enough conductivity to fool the ohmmeter into showing a good reading.