Voltage drop

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sphen
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Voltage drop

#1

Post by sphen »

Not sure if this is normal and I’m just paranoid-

After rebuilding my project bike (and removing a load of accessories and wires), when I turn the key on (no engine running) my battery warning flashes and the in dash volt meter shows around 10v.

I’m measuring north of 13v on the battery (off) and it drop ls to about 12.5v when the key comes on.

No issues starting.

Is that normal or do I have something pulling to much? I replaced all the lights with repro incandecants (head tail and signals) from 4into1. Wondering if I should Have gotten LEDs.
Project bike "Dixie" - 1986 GL1200 Aspencade being rebuilt from ground up to be a naked 'wing. Build progress thread here: viewtopic.php?f=30&t=71959

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tlbranth
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Re: Voltage drop

#2

Post by tlbranth »

I'm confused. 10v or 12.5v with key on?
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sphen
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Re: Voltage drop

#3

Post by sphen »

12.5 across battery
10 indicated dashboard dial.

What I think may be happening is the wire that feeds the dial is on a busy circuit and is showing a low voltage. I may need to find another isolated fuse or run a new one from the battery to get a purer reading.
Project bike "Dixie" - 1986 GL1200 Aspencade being rebuilt from ground up to be a naked 'wing. Build progress thread here: viewtopic.php?f=30&t=71959

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Whiskerfish
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Re: Voltage drop

#4

Post by Whiskerfish »

The stock gauge on my 78 shows about a volt and a half low as compared to Meter readings directly from the battery. Surface charge is what you read when there is zero load. Loosing a half a volt or more is not uncommon with the key on.
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redglbx
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Re: Voltage drop

#5

Post by redglbx »

Sphen, as whisker said the OE gauge reading is poor at best, my 81 cbx is the same way and basically is just nice to look at but really not accurate. I think you are good to go, a fully charged 12v battery should have 2.2volts per cell and with 6 cells should be 13.2v in a perfect world, 12.5 -12.8 with the ignition on is fine, so just go for a ride and enjoy your bike !
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1980 CBX , in the que, to fix the ignorant heavy handed owner
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Sidecar Bob
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Re: Voltage drop

#6

Post by Sidecar Bob »

"Voltage drop" is a specific electronic term referring to the relationship between the supply voltage and the voltages across circuit elements that are connected in series. When I saw the thread title I thought you were probably using the term incorrectly but maybe not.

The first thing to do is check the accuracy of the instrument panel voltmeter by connecting your multimeter across it.
Assuming it is reasonably close to accurate I'll be surprised if the problem doesn't turn out to be the result of the resistances in all of the connections between the battery and the instrument panel adding up to enough to cause a series voltage drop. There are enough connectors and wire joints in a bike's wiring that this can add up to a significant total without any one connection having enough to be concerned about.
I'd even go so far as to predict that most of the losses are probably in the ground half of the circuit because other loads use the same wires and the voltage dropped is the product of the current multiplied by the resistance.

I have a similar situation on my winter bike. The voltmeter I installed in its fairing indicated up to 2V less than the voltage at the battery and it varied depending on what is turned on (handlebar heaters, stereo, sidecar headlight). I measured the voltage between battery positive and fairing voltmeter positive and between battery negative and voltmeter negative and found that most of the loss was in the negative (ground) portion. Fixing it properly would require taking the harness out of the bike, opening it up and re-doing all of the connections and maybe replacing the main green & black wires with heavier ones. I may do that eventually but for now I made a couple of simple improvements to the ground circuit and the meter in the fairing now reads about 1V low with everything turned on. I can live with that as long as I know.
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redglbx
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Re: Voltage drop

#7

Post by redglbx »

On my 76 I ran a volt meter off of the wiring to the headlight and found it reads about 1v low to reading directly off the battery, just not worth worrying about, it’ll let me know if there is a problem which is all I need.

I also added one to my 77 cb750k because the voltage regulator wasn’t reliably regulating and left me walking twice. No more as I just buy the mosfet regulators from Ricks, fairly expensive but they just work ! Very well !

Doesn’t fix your volt meter error but if your voltage is good at the battery just note the difference and go ride it !
Red 1976 oe owner
1976 LTD restored
1980 CBX , in the que, to fix the ignorant heavy handed owner
1981 CBX
1977 CB750 K7
2014 FJR OE owner, sold
1980 GL1100
1984 GL1200 naked
1969 CL350, in the que
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