Voltage Drop

swooshdave · Aug 11, 2021

So I have had my CWL from https://www.improvingclassicmotorcycles.com for a while. The charging light goes off at pretty high rpm and I finally put a voltmeter on the bike to watch it. So I'm seeing the light go off at around 13.8v. According to Graham at https://www.improvingclassicmotorcycles.com it should be 12.6v. I confirmed with the headlight off it does indeed turn the CWL off at 12.6v and Graham told me to check for voltage drop. The voltmeter is just temporary but it was useful.

I've heard of voltage drop before but this is the first time I've had to really look into it.

I'm hoping because when I turn on the headlight that I can quickly narrow down the issue but I suspect that because the power to the headlight goes through a few spots I'm going to need to poke around for a while.

This video was very helpful (if not slightly annoying) in talking and demonstrating in simple enough terms that I could understand what exactly voltage drop is.

Tornado · Aug 11, 2021

I've got a similar issue with the SparkBright Eclipse CWL....showing low volt warnings at various rpms with no improvement moving the Earth lead to the battery +ve terminal rather than main frame ground point. Negative lead comes off the ign switch feed so the LED isn't on all the time when parked up (though the maker states this is not a problem as the current draw is tiny...would take >a few months before battery charge was needed).
Will see what you determine is your issue and check my bike ('cuz I'm a lazy bugger) ;-)

mikeinidaho · Aug 11, 2021

I would be more impressed if he bolted that starter onto an engine and measured the voltage drop while it's actually cranking an engine.

jbruney · Aug 11, 2021

Internal corrosion in a wire had my goat last year.... speaking of voltage drop. I was lucky though in finding the issue quickly as it affected my EI.

swooshdave · Aug 11, 2021

mikeinidaho said:
I would be more impressed if he bolted that starter onto an engine and measured the voltage drop while it's actually cranking an engine.

Having the starter in the car wouldn’t have made any difference in demonstrating the concept. Plus it’s harder to show the components all at once while installed.

RoadScholar · Aug 11, 2021

I may be missing something, so please correct me if my observations are not right.

A fully charged battery should have a voltage reading between 12.7 and, when fresh
off a charger, 13.1 a faulty battery may show these figures, but they will degrade in, usually, in about 24 hours or less. If I read the OP correctly the charge warning light shouldn't come on at all. with a good battery. When the charging system is at full operational voltage the reading should be between 13.8 and 14.2. If I'm right the charge warning light is functioning properly.

If your harness is original you may well have a number of high resistance connections and brittle wires with a number of internal breaks causing the surviving strands to carry more current which will hasten their demise and cause additional higher resistance figures. Add high resistance connections to high resistance wires and the electrical system is producing heat that it was not designed for as well as the voltage drops you already suspect. E=IR is a useful tool to see this mathematically.

Best.

swooshdave · Aug 11, 2021

RoadScholar said:
I may be missing something, so please correct me if my observations are not right.

A fully charged battery should have a voltage reading between 12.7 and, when fresh
off a charger, 13.1 a faulty battery may show these figures, but they will degrade in, usually, in about 24 hours or less. If I read the OP correctly the charge warning light shouldn't come on at all. with a good battery. When the charging system is at full operational voltage the reading should be between 13.8 and 14.2. If I'm right the charge warning light is functioning properly.

If your harness is original you may well have a number of high resistance connections and brittle wires with a number of internal breaks causing the surviving strands to carry more current which will hasten their demise and cause additional higher resistance figures. Add high resistance connections to high resistance wires and the electrical system is producing heat that it was not designed for as well as the voltage drops you already suspect. E=IR is a useful tool to see this mathematically.

Best.

You are discounting the voltage of a running bike at idle with the headlight on which can easily drop below 12.7v.

I also said this aftermarket CWL is set to turn off at 12.6v and that it works correctly with the headlight off. The voltage drop with the headlight on causes it to misread the voltage.

Wiring harness was custom made by me and modified since then. Any issues with the wiring are 100% my fault. But we don’t learn if we always get it right the first time.

RoadScholar · Aug 11, 2021

swooshdave said:
You are discounting the voltage of a running bike at idle with the headlight on which can easily drop below 12.7v.

I also said this aftermarket CWL is set to turn off at 12.6v and that it works correctly with the headlight off. The voltage drop with the headlight on causes it to misread the voltage.

Yes, thanks, the voltage at idle with headlight and taillight on would be considerably less than static, switch off, battery voltage.

I would think that the CWL light going off at 12.6 isn't telling you if the battery is getting a proper level of voltage/current to keep from draining?

What voltage do you measure with a fully charged battery on the bench and then installed with key off? What voltage do you measure when the engine is at 2500+ RPM, lights on?

Best.

swooshdave · Aug 11, 2021

RoadScholar said:
Yes, thanks, the voltage at idle with headlight and taillight on would be considerably less than static, switch off, battery voltage.

I would think that the CWL light going off at 12.6 isn't telling you if the battery is getting a proper level of voltage/current to keep from draining?

What voltage do you measure with a fully charged battery on the bench and then installed with key off? What voltage do you measure when the engine is at 2500+ RPM, lights on?

Best.

The light going off at 12.6v indicates that it’s charging because anything above the static charge level is assumed to be charging.

How do you think a CWL should work?

RoadScholar · Aug 11, 2021

swooshdave said:
How do you think a CWL should work?

I think a charge light should be on when system voltage is below 12.6 to 12.8, it should go off when system voltage is above 13.7, give or take. My thinking is that the alternator is regulator limited to 14.2 and should attain that level at some RPM when ignition and head/tail lights are on.

My point is that a charge warning light should go out when system voltage is above maximum static battery voltage. At idle with lights on, as you mentioned, system voltage will go below 12.7/12.9, that, in my experience, is when the charge warning light should come on to tell you that the battery is draining, a normal condition, but aberant when the engine is at or above 2500 RPM.

Coming on at 12.6 and going off at 13.8 I could live with. If you installed a ground (earth) loop from headlight to engine to battery to tail light, that's good. If you have a halogen headlight bulb some wire up differently. If you Google the bulb you should be able to find out what the spades are expecting (hi, low, ground).

Best.

swooshdave · Aug 12, 2021

RoadScholar said:
I think a charge light should be on when system voltage is below 12.6 to 12.8, it should go off when system voltage is above 13.7, give or take. My thinking is that the alternator is regulator limited to 14.2 and should attain that level at some RPM when ignition and head/tail lights are on.

My point is that a charge warning light should go out when system voltage is above maximum static battery voltage. At idle with lights on, as you mentioned, system voltage will go below 12.7/12.9, that, in my experience, is when the charge warning light should come on to tell you that the battery is draining, a normal condition, but aberant when the engine is at or above 2500 RPM.

Coming on at 12.6 and going off at 13.8 I could live with. If you installed a ground (earth) loop from headlight to engine to battery to tail light, that's good. If you have a halogen headlight bulb some wire up differently. If you Google the bulb you should be able to find out what the spades are expecting (hi, low, ground).

Best.

Yes, adding an overcharge indicator would be a nice to have. Luckily failed regulators are far less common than failing to charge.

I would put it a bit higher than 13.8v. More like 14.5 or even 15v. At those levels you are pretty much in trouble. You’d need a different indication than just coming on.

I had a failed regulator on my Guzzi and was mighty puckered as the voltmeter was 16v+! I was a ways from home so the high beam went on. Still ended up replacing the battery (and obviously the regulator).

MexicoMike · Aug 12, 2021

THE VIDEO DOES NOT SHOW A VOLTAGE DROP TEST!!!!

It shows what some people THINK a voltage drop test is when they hear the term. Rather than try to explain it, I searched and found a correct explanation/demonstration/use of a voltage drop test: https://us.autologic.com/news/how-to-perform-a-voltage-drop-test

I have been using voltage drop tests for many MANY years. It is probably the best electrical test there is for diagnosing such problems. TBF, I quit watching the video about 5 minutes in. If it somehow got around to actually explaining the CORRECT procedure for a voltage drop test, unlike that first section, then the video is about 30 minutes longer than it should be!

Tornado · Aug 12, 2021

MexicoMike said:
THE VIDEO DOES NOT SHOW A VOLTAGE DROP TEST!!!!

It shows what some people THINK a voltage drop test is when they hear the term. Rather than try to explain it, I searched and found a correct explanation/demonstration/use of a voltage drop test: https://us.autologic.com/news/how-to-perform-a-voltage-drop-test

I have been using voltage drop tests for many MANY years. It is probably the best electrical test there is for diagnosing such problems. TBF, I quit watching the video about 5 minutes in. If it somehow got around to actually explaining the CORRECT procedure for a voltage drop test, unlike that first section, then the video is about 30 minutes longer than it should be!

It did get around to proper voltage drop testing past the 20 min mark or so. Guy was showing all the standard tests most laymen do and how these can be misleading. Sure, not the best presentation style but got there eventually.

cliffa · Aug 12, 2021

Tornado said:
I've got a similar issue with the SparkBright Eclipse CWL....showing low volt warnings at various rpms with no improvement moving the Earth lead to the battery +ve terminal rather than main frame ground point. Negative lead comes off the ign switch feed so the LED isn't on all the time when parked up (though the maker states this is not a problem as the current draw is tiny...would take >a few months before battery charge was needed).
Will see what you determine is your issue and check my bike ('cuz I'm a lazy bugger) ;-)

In my experience you get more accurate colour changes when connecting the Eclipse straight to the battery, plus an advantage is you get a constantly flashing red LED, so it may make potential thieves have second thoughts.

MexicoMike · Aug 12, 2021

Tornado said:
It did get around to proper voltage drop testing past the 20 min mark or so. Guy was showing all the standard tests most laymen do and how these can be misleading. Sure, not the best presentation style but got there eventually.

Thanks for the information; I guess I should have kept watching.

My expectation was that the video would simply be an explanation of how to perform a voltage drop test so after the initial few minutes I incorrectly concluded the guy didn't know what he was talking about. Maybe my wife is right when she points out that I need more patience! OTOH, 30+ minutes is mighty long video when five minutes to explain an actual voltage drop test would be sufficient. Oops...I guess I'm being impatient again.

kommando · Aug 12, 2021

MexicoMike said:
My expectation was that the video would simply be an explanation of how to perform a voltage drop test so after the initial few minutes I incorrectly concluded the guy didn't know what he was talking about.

I was always taught that for a presentation its always best to start with a summary of what will be covered, if only to cut down on questions being asked before you get to the bit that covers the answer. So like a new bulletin you cover the headlines first and then cover each headline in more detail.

maylar · Aug 12, 2021

<sigh>

In simplest terms, a voltage drop is the difference between 2 voltages that should be the same. The voltage at your battery terminals should be the same as the voltage at the CWL sensing wires. The ignition switch, wires and connections between those 2 points have resistance, and when current flows through a resistance there will be a voltage "drop". You can check that by measuring the voltage difference between the battery terminal and the CWL sense wire, with headlight on. Ideally it should be maybe 1/10th of a volt.

Be aware however that the CWL is an intelligent device. If you connect it to a variable power supply the bulb will never turn off (it will start blinking at 15 volts however). That's because the CWL needs to see some AC ripple from the charging system before it decides that your alternator is working. So, even if your battery is 13 volts at idle, the CWL will turn on because the alternator is not charging it.

Derek Wilson · Aug 12, 2021

I have a digital voltmeter mounted on my bike full time. It uses the sensed voltage line to power it and is wired originally for negative ground. It also has a constant power line, as it has a clock feature as well, which I like. Because my bike is still positive ground, I use a relay to trigger it so that I can switch the positive earth. The negative power to the voltmeter comes straight from the battery, which mostly negates any affect of voltage drop. When checked against my HF multimeter, the voltmeter is 0.2 volts optimistic, which is fine by me. So if anything, I am getting negative voltage drop - LOL.

Back when I had a 3 phase alternator and an incandescent headlight bulb, I used to run a harness kit that powered the headlight straight from the battery, switching beams with relays. This really helped the headlight brightness, by eliminating any voltage drop through the original harness. When I put in the LED headlight, it was no longer of any benefit, and I took it out.

FWIW

swooshdave · Aug 12, 2021

With the fairing on it's impossible to get to many of the places I need to test. I did try some testing last night and at least between the battery and fuse panel there doesn't appear to be any issue. I need to test at the headlight next.

I am testing without the bike running so far. It's just too $%^& loud to start up at 9pm.

Schwany · Aug 12, 2021

I have that same Harbor Freight bike stand.

If you took that CWL smart device off the bike, your electrical problem would go away. Particularly since you may not have one.

Voltage Drop

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