Voltmeter sensitivity

[60Hurts]

All,

I installed a digital voltmeter on my motorcycle and it works fine. However, the volts run from about 12.8vdc to 14.0vdc depending on RPM and can bounce around a lot. Instead of "instantaneous" voltage, I just need a suitably averaged voltage (over a few seconds, I'd guess) to keep an eye on so I know that the charging system is working.

Is there something that I can add to the circuit or input wiring like a capacitor to slow down the VM reaction time? If a capacitor (or other components) will do this how do I calculate the proper values?

Thank you.

[IanB]

You could put an RC filter on the input to the voltmeter. For example, as shown here:

https://en.wikipedia.org/wiki/RC_time_constant



Note that the resistance R will be in series with the voltmeter so you want the resistance to be small enough that it does not affect the voltage very much. Perhaps make R less than 1% of the voltmeter input impedance.

Then, with a value of R chosen, choose a value of C to give a time constant in the order of 5 seconds or so.

You can play around with the numbers a lot. Experiment to see what seems good.

[60Hurts]

That is exactly what I was looking for.  Also, I have used LTSpice for other projects and this circuit is simple enough to model to find the component values needed. There is already a delta of 0.5v low between the dash voltmeter reading and the voltage at the battery (what actually matters) that I have been mentally adding it to the readout. If that delta goes up to 1.0v then the mental math would even be easier so NBD. Thanks!

[Rick Law]

Assuming your battery is lead acid (SLA) and not Li-Ion as some automotive batteries are these days, and assuming motorcycle battery works like batteries in cars...

You may not want to slow it down too much.  Between 12.x Volt to 14.x Volt, the number is giving you some useful info. 

These numbers are approximate, but the pattern should be similar:
14.5v, the generator is charging your battery
14.5v to >13.6v battery being charged
13.6v, the generator just finished charging and disengaged itself (to save fuel)
13.6v to 12.6v, the motorcycle/car's battery is in discharge mode.  It is using the battery for electrical stuff with generator disengaged to save fuel.

At somewhere below 12.6v, it would re-engage the generator to re-charge the battery, meter showing 14.5v again.  Below 12.0v is basically an empty battery.  Some power need to be reserved so there is power to run the starter motor if the engine got turned off.  Thus, recharging typically starts before it drops below12.0v.  How much sooner will depend on design.

Personally, I would not want to slow it to >0.5 second.  The whole charge/discharge duration is in minutes, but I would like to see the rate at which it changes (14.5v to 13.6v).  If it isn't moving at around my typical rate, I know either the battery or generator is not performing typically.

Thus taking me to this other point: The battery is kind of like a very big capacitor and should be holding the voltage to fairly steady.  Your seeing it jumping around with changing RPM may be just showing you: "now the engine is fast enough to begin charging" vs "RPM too low to really generate charge voltage."  That may be information you don't want to average-out.

Lastly, it is ok to see 10-ish volt during start when starter motor is going. Before the motorcycle/car engine is started (and not being started), anything below 11v is bad news.  Being planning funeral for your battery. 

EDIT:  (Adding this:)

I was trying to let you infer it.  But on second thought, I should be more direct:

Since the battery is like a real big giant capacitor already, the voltage reading should be rather steady.  Adding a small capacitor is not going to change the picture much.  If it is jumping around, likely it is jumping between charging/not-charging as a result of the changing RPM.

Take it out on a steady drive (therefore, steady RPM, steady everything), it should charge for a while (14.5v, for multiple seconds or even minutes), discharge for a while (13.6v to 12v for multiple seconds or even minutes), than back to charge again.  If the voltage is still jumping around at sub-second rate while your RPM and everything else are steady, something MAY be wrong -- or not..

Get a "behavior profile" first then go from there.

[SteveThackery]

I've never known a motorcycle behave like that. Mine sit around 14.4V all the time (to be fair I haven't measured the voltage on all my motorcycles.

You've described a "smart" charger behaviour. Maybe some modern bikes work like that? It would be interesting to find out how many do. I'm assuming the OP's bike might be such an example.

It might also impact on the possibility of fitting a LiFePO4 battery to it.

[PGPG]

I've never known a motorcycle behave like that.

I've also never heard of such behavior.
May be hybrid cars behave like that but standard... I don't suppose.
For me in the description it would be:
13.6 - zero energy balance - alternator efficiency equal to momentary consumption (in a car - lights, air conditioning, heated windows...)
below 13.6 - engine not started yet or temporarily negative energy balance.

In 70s I have installed in my fathers car a battery voltage detector with 3 LEDs (red,yellow,green).

[CaptDon]

Good golly Miss Molly, some of the answers go on and on. Yes, the action of your voltmeter can be expected on 'some' motorcycles. They can have a charging system that becomes an 'all or nothing' at the point of full charge. Meaning the charging alternates between full alternator output available at the current RPM and as the voltage rises to the set point the charge 'cuts off' as opposed to 'cutting back' then repeats as the voltage sags. My Suzuki Titan 500 operated that way and you could detect that action in the headlight at night. Perhaps your charging system 'should' cut back but do to a malfunction in the regulator it is behaving like my old Suzuki. At any rate, the RC circuit suggested will give a more average reading and you can easily tailor the response time to your liking. One poster mentioned a capacitor won't work. A capacitor by itself won't be of help BUT an RC circuit will indeed give you the result you are looking for. As mentioned the resistor should be a small fraction of the meters input impedance. A suggested starting point could be a 1K resistor and a 220uf@25vdc capacitor. Make sure to connect the capacitor correctly with respect to polarity. Cheers mate! Ride safe!!

[BeBuLamar]

May be an ammeter to see which way the current flows?

[60Hurts]

Thanks for all the thoughtful replies. Instead of answering each in a reply I will give a summary and more background. The bike is old, a 2007 Suzuki V-Strom DL650.

• The charging system is a permanent magnet generator with a new (used) stator (old one died at 95K miles) and Flintstone era shunt Regulator/Rectifier (R/R) with a lead acid battery. Battery is charging just fine after repair.
• The generator outputs whatever power it can generate at a given RPM with peak power at about 5000 RPM, i.e. not an alternator. The R/R dissipates any excess power necessary to hold the DC voltage to ~14vdc, assuming RPMs are high enough.
• Newer bikes have better voltage control using a series R/R which electronically "disconnects" the AC stator to control DC voltage whereas the shunt R/R shorts it to ground to control voltage (wears out the stator insulation).
• Battery discharge or barely charging at idle (1300 RPM) is normal. While freeway cruising at 5.5K RPM output is +13.8 volts and stable. I mostly ride winding back roads (aka "twisties") so RPM can vary from 2000-10,000 RPM (on a good day ;-).

All that said, I think the RC filter on the input to the voltmeter will get me what I want, less sensitive voltmeter reading. It will be cheap and easy to try so worth a shot.  Thanks again for all the help.



   

[CaptDon]

Sounds like Suzuki didn't change the circuit much since my 1975 TT-500. The regulator on the Titan used an SCR to dead short the output of the stator which like yours was excited by permanent magnets. When the battery was weak you could really notice the regulator action in the headlight!

[60Hurts]

Okay, I thought I'd take a crack at figuring out what capacitor I need for my RC filter. After thinking about this for a bit I realized that I probably don't need to add a resistor, just the capacitor, to the circuit. Attached is a schematic for reference. V1 is the battery, R1 is the resistance on the positive wire feeding the VM (unknown), C1 is the capacitor needed to get a time constant of around 5 seconds (TBD), R2 is the input resistance of the voltmeter. I modeled the voltmeter as a light-emitting diode D1, ignoring any electronic components.

The reason I don't think I need to add a resistor is that the negative wire branch feeding the voltmeter is a straight shot whereas the positive wire goes through an ATO 15A fuse and two contact switches (headlight cutout switch on startup and Hi/Lo beam selector) thus adding resistance compared to the negative branch. The headlight cutout switch is known to fry because it was under spec'd to carry the current for two H4 60W headlights. The headlights are currently on a direct-to-battery feed and the cutout switch only operates a relay now (a few 10's milliamps) instead of carrying the whole load. The right headlight connector is used to operated the relay and free left headlight connector is where the voltmeter is plugged in.

With key on, engine not running, I measured the battery voltage V1=12.0vdc and voltmeter volts at D1=11.3vdc so there is a 0.7v difference between the display and actual voltage at the battery.  I also measure the input resistance of the voltmeter R2=138K ohms.

I need to infer R1 to calculate C1. I did a search and digital voltmeters typically draw about 100mA or less (~50mA if good). Since this is a cheap one (a few dollars) I assume that the VM current is 100mA. I also assume that the current through R2 VM input resistor is negligible so the current through R1 is ~100mA. Using V1=I*R1, (12.0-11.3)=(0.10A)*R1 then R1=7 ohms. For an RC time constant of 5 seconds and using T=R1*C1, I get 5=7*C1 or C1=714.3 mF.

I don't have a background in electronics (hence the post to "Beginners" forum) so I don't know if this even makes sense. The 7 ohm resistance on the positive feed wire does seem a bit high. But that is a lumped average resistance for two contact switches, ATO fuse, wire and connector losses. Also, when I installed the headlight relay to take the load off the headlight cutout switch at ~75K miles I refurbed the switch and the contacts were already burned. I cleaned them up and they work fine for the operating the headlight relay but maybe that is why the implied resistance for R1 is so high.

Feel free to fire away at my assumptions and analysis (I won't be insulted) but I am hoping to get some feedback to learn.


 

[PGPG]

I don't know if this even makes sense.

No. It not makes sense.
Typical 1000uF/25V capacitor will have about 10mm diameter and 20mm length and it is only 1mF while you need 714mF!

If voltmeter input impedance is 138k and it draws about 100mA than you have 100mA*138k=13800V. Almost 14kV is certainly danger.

You can not assume to not use R1. Resistances in your circuit are thousand times smaller than you need to get time constant as you need with capacitor smaller than a brick.

[IanB]

I think you have a problem here, because (a) the voltmeter should not be reading lower than the battery voltage, and (b) the voltmeter should not be drawing any current (should be microamps at most).

Where I think the problem lies is that the voltmeter is drawing power to operate, to light up the display. This will totally distort the reading unless you have a very low resistance connection to the battery. Also, it will prevent you from putting any filtering on the input. The filtering would have to be inside the voltmeter, not outside it.

Unless, by any chance, the voltmeter has three terminals as in (power), (voltage), (ground), I do not think you can do what you want.

The (power) terminal can take the 100 mA to operate the display, but the (voltage) terminal should draw essentially no current at all. The (voltage) terminal (if it exists) is where the RC filter should go.

[Lstkartagi]

It might just be EMI with a low cost digital display too [EMI somehow injecting into the meter circuit other than the directly through the input terminals].  For example, see: https://www.triumphrat.net/threads/multimeter-giving-erratic-readings-on-battery.974738/

[CaptDon]

You could certainly try 1000uf@25vdc right at the input leads to the meter. Is it a two wire meter or is it three wires, + / - / sense? The 1000uf cap would also clean up any of the noise that Suzuki's weird regulator circuit would generate. I think I posted years ago here on EEV about the stator burning out from being shorted by the regulator circuit. The basic answer was NO because there is a limited amount of excitation from the magnets. The voltage would attempt to keep rising the faster you cut the lines of force, however the available number of lines of force remains the same so the total available power output can only rise and begin to taper off do to winding resistance and to the stator's increasing magnetic inefficiency at higher RPM's.

[iMo]

Your voltmeter has to have at least 3 wires you have to use:
GND .. Ground
Power+ .. the Power for the voltmeter itself like +12V
Inp+ .. the voltmeter's Input for the measurement (with the 138k you have measured against ground).

In case of the Inp+ is the 138k and your meter displays XX.X Volts and there is the 1k resistor wired in series with Inp+ you will see a drop by aprox 0.1V.

A capacitor of 2200uF will filter the Battery's voltage jumps as you may see below.

[IanB]

With key on, engine not running, I measured the battery voltage V1=12.0vdc and voltmeter volts at D1=11.3vdc so there is a 0.7v difference between the display and actual voltage at the battery.  I also measure the input resistance of the voltmeter R2=138K ohms.

Something doesn't seem right here. If the input resistance of the voltmeter were really 138 kΩ in operation, then the current would be about 12 V / 138 kΩ = 0.087 mA.

For a current of 0.087 mA to drop 0.7 V in the wire, the wire resistance would have to be 0.7 V / 0.087 mA = 8.1 kΩ

This seems very unlikely. More likely, you cannot measure the input resistance of the voltmeter using a multimeter, since it is an electronic device.

I think you really need to see if the voltmeter has three terminals. If it has a separate sense terminal, you need to connect that separately to the battery to avoid having current flowing through that wire.

[60Hurts]

Thanks for all the comments and help. Here are my replies.

No. It not makes sense.

Now clear to me but I am learning.

I think you have a problem here, because (a) the voltmeter should not be reading lower than the battery voltage, and (b) the voltmeter should not be drawing any current (should be microamps at most).

I have installed voltmeters on multiple bikes over 15 years and there is always a voltage drop compared to the battery voltage on past bikes or using different VMs. I just mentally add 0.5v, 0.7v and 1.0v or whatever to infer the battery voltage on past bikes. The delta doesn't really matter for picking up a change in normal patterns at idle, starting out, cruising etc. If normal idle volts are around 12.6v at the battery with 0.7v delta (vm displays=11.9v) but suddenly battery volts drop to around 12.1v (vm=11.4v) then I need to investigate.

Where I think the problem lies is that the voltmeter is drawing power to operate, to light up the display. This will totally distort the reading unless you have a very low resistance connection to the battery. Also, it will prevent you from putting any filtering on the input. The filtering would have to be inside the voltmeter, not outside it.

Yes this is the key idea that I was missing. I always thought the voltage drop was just due to the connectors, contact switches, fuse and wire resistances in the path to the VM. As discussed, the voltage delta isn't a problem for my use case.

Unless, by any chance, the voltmeter has three terminals as in (power), (voltage), (ground), I do not think you can do what you want.

I just took apart an old moto voltmeter in my junk box and it only has two terminals. The one installed now is definitely a 3 terminal because it has red/black/yellow input wires. Red/Black to power the device and Yellow for the sensed voltage so this looks promising. I'm guessing most voltmeters sold for bikes are two terminal type. When I wired it I just jumped the yellow/red terminals.

The (power) terminal can take the 100 mA to operate the display, but the (voltage) terminal should draw essentially no current at all. The (voltage) terminal (if it exists) is where the RC filter should go.

Got it and thanks!

It might just be EMI with a low cost digital display too [EMI somehow injecting into the meter circuit other than the directly through the input terminals].

Thanks for the comment and interesting link.  I'll see how it goes after separating the vm power from the sensed power and put the filter only on the sensed circuit. TBH, I was starting to think that the VM might even be defective or not suitable for a moto.

You could certainly try 1000uf@25vdc right at the input leads to the meter. Is it a two wire meter or is it three wires, + / - / sense? The 1000uf cap would also clean up any of the noise that Suzuki's weird regulator circuit would generate.

Yes, the current VM is a three terminal but I jumped sensor and power terminals which won't work. I'm going to rewire and add the input filter.

I think I posted years ago here on EEV about the stator burning out from being shorted by the regulator circuit. The basic answer was NO because there is a limited amount of excitation from the magnets. The voltage would attempt to keep rising the faster you cut the lines of force, however the available number of lines of force remains the same so the total available power output can only rise and begin to taper off do to winding resistance and to the stator's increasing magnetic inefficiency at higher RPM's.

Yes, see my earlier background post. I mentioned that this is an old-school, Flintstone era PMG with shunt R/R. Modern bikes use a series R/R that "disconnects" the stator to control voltage instead of shunting excess power to ground. The series R/R raises the average volts on the stator but lowers the average current which spares the stator insulation. Higher voltage is not a problem because the cause of insulation failure (resistive heating) is minimized.

Your voltmeter has to have at least 3 wires you have to use:
GND .. Ground
Power+ .. the Power for the voltmeter itself like +12V
Inp+ .. the voltmeter's Input for the measurement (with the 138k you have measured against ground).

Yes, TIL. I learned I have to separate the power from the sensed voltage and put the filter on the sensed circuit only.

In case of the Inp+ is the 138k and your meter displays XX.X Volts and there is the 1k resistor wired in series with Inp+ you will see a drop by aprox 0.1V. A capacitor of 2200uF will filter the Battery's voltage jumps as you may see below.

This is great and even with a plot! Thanks.

Something doesn't seem right here.

Quite a bit actually ;-)  Please disregards my previous attempt.  Your comment about separating the sensed input from the VM power was the key.

This seems very unlikely. More likely, you cannot measure the input resistance of the voltmeter using a multimeter, since it is an electronic device.

Yes, TIL. However, I know that VMs have large input resistors and I figured a static read on input ohms might help me figure out the cap needed to filter but my approach was all wrong.

I think you really need to see if the voltmeter has three terminals. If it has a separate sense terminal, you need to connect that separately to the battery to avoid having current flowing through that wire.

Three terminals confirmed and I am going to proceed with rewiring to separate the VM power from the sensed input and put the RC filter on the sensed input only. Thanks again for all your help, Ian.

[SteveThackery]

I have installed voltmeters on multiple bikes over 15 years and there is always a voltage drop compared to the battery voltage on past bikes or using different VMs. I just mentally add 0.5v, 0.7v and 1.0v or whatever to infer the battery voltage on past bikes.

I'm still having a real problem understanding this. What could be dropping that 0.5V (or 0.7V, 1.0V)?  It can't possibly be due to the current drawn by the voltmeter's LED display - the resistance of the wiring would need to be unrealistically high.

Maybe some parts of the wiring loom do drop that kind of voltage - the lighting circuit, probably; maybe the ECU and its connected devices.  Perhaps there are other things that draw enough current to drop 0.5V to 1.0V over the loom.

Obviously it all depends on where you connect the voltmeter. If it is on the end of a circuit carrying a lot of current, perhaps it might read low. But you say that a voltmeter reading of 0.5v to 1.0V below the battery voltage is routine on all your bikes? I dunno - that bit doesn't feel right to me.

How are you measuring this discrepancy between the battery voltage and the voltmeter voltage? Are you using a multimeter across the battery and comparing it with what the voltmeter is displaying? If so, I'd be asking questions about the calibration of the multimeter.

Have to connected your multimeter AND the voltmeter both directly across the battery terminals at the same time? That would at least allow you compare their readings under identical conditions.

If I am wrong and voltage drops of 0.5V to 1.0V across the loom are indeed normal, then the right way to cure it is to run the voltmeter's sense wire and ground wire right back to the battery terminals. If you are doing that, then you might as well run the +12V wire back as well. Remember to put an inline fuse in the wires going to the positive terminal.

Three terminals confirmed and I am going to proceed with rewiring to separate the VM power from the sensed input and put the RC filter on the sensed input only. Thanks again for all your help, Ian.

There are two issues here. Yes, separating the VM power from the sensed input is the right thing to do if you want to filter the sensed voltage. But it won't cure the permanent voltage difference between the battery and the VM unless the VM operating current itself is causing that 0.5V to 1.0V voltage drop (which it won't be).

You really need to run all three wires back to the battery. When you have done that, I don't think you will even need to filter the VM sense input.

[60Hurts]

I'm still having a real problem understanding this. What could be dropping that 0.5V (or 0.7V, 1.0V)?  It can't possibly be due to the current drawn by the voltmeter's LED display - the resistance of the wiring would need to be unrealistically high. Maybe some parts of the wiring loom do drop that kind of voltage - the lighting circuit, probably; maybe the ECU and its connected devices.  Perhaps there are other things that draw enough current to drop 0.5V to 1.0V over the loom.

Yes, that has always been a bit of a mystery but you are right, depending on what circuit the VM is on there other things drawing juice with resistances in the path. The circuit the VM is powered by now is the sensed circuit and that line also powers the headlight relay (a few mA) and routes through two contact switches and a fuse and and unknown number of harness connectors. On these older bike most of the connectors are not sealed so corrosion and resistance as they age is common.

Obviously it all depends on where you connect the voltmeter. If it is on the end of a circuit carrying a lot of current, perhaps it might read low. But you say that a voltmeter reading of 0.5v to 1.0V below the battery voltage is routine on all your bikes? I dunno - that bit doesn't feel right to me.

Those deltas were from past bikes and/or different voltmeters. Also, the 1.0v drop was probably actually 0.9v and I just rounded up to make my mental math easier. More importantly and as discussed, this isn't some electronic experiment or device, it is a motorcycle. I just need to know if the charging volts drop and the battery stops charging by monitoring for changes in normal patterns at idle, starting out, cruising, etc. The delta does not really matter to me and this 17 year old bike with nearly 100K miles is on its 3rd stator and second R/R. I caught and replaced them all before the battery died using this method.

How are you measuring this discrepancy between the battery voltage and the voltmeter voltage? Are you using a multimeter across the battery and comparing it with what the voltmeter is displaying? If so, I'd be asking questions about the calibration of the multimeter.

Yes, measured at the battery. Two DVMs one is a UT61E (bought on recommendations from EEVB) the other (my garage gauge) is a cheaper unknown brand but works fine and gives the same result.

You really need to run all three wires back to the battery. When you have done that, I don't think you will even need to filter the VM sense input.

Running wires back to the battery is a bit of a pain on a moto. I was thinking the same thing -- that separating the VM power from the sensed voltage might obviate the need for a filter. I am curious enough that I will probably measure the resistance of the wire (& etc.) between the VM and battery just to see what it really is.

Thanks for your comments.

[SteveThackery]

Running wires back to the battery is a bit of a pain on a moto.

Well, it's a lot easier than on a car, where you have to get up behind the dashboard and find a route through to the engine/battery compartment. Once you've got the tank off it shouldn't be too bad.

I was thinking the same thing -- that separating the VM power from the sensed voltage might obviate the need for a filter.

Actually that's not what I meant - sorry for being unclear. I meant running the three wires back to the battery might remove the need for a filter.

[Rick Law]

I've never known a motorcycle behave like that.
... ...

I was at my BMW dealer servicing my 328i  battery (around early 2000).  The Service manager and I had a long chat about batteries because my battery was replaced while out of town and was not a BMW OEM and that would void the car warranty unless remedied.  He convinced me to remove my merely weeks-old Diehard battery with a BMW one.  Perhaps because we were sitting at a table near their bike display, some point during the conversation, he pointed at the bikes and said "even those bikes has the same battery management system..."

As noted in the beginning of my reply, I pointed out I was assuming the bike battery would be like my car battery.  That even by now so few bikes do that is a surprise to me as electronics are not as expensive as it was in early 2000.  It does increase fuel economy and the longevity of the battery.  However, it was a pain in the backside that the car's battery management system has be "told" when a new battery is in place and reset the charge profile.

[CaptDon]

Installing a non-OEM battery will void the warranty. Probably another reason I would never consider buying a beamer!! Over engineered and under implemented like the stupid 500HP small cubic inch engines that all seized a piston in the bore or spun the bottom end bearings. They tried to weasel out of that one also!!!! We tested your oil and it wasn't the proper grade.....Well it was the dealer who did the oil changes!!!

[MrAl]

All,

I installed a digital voltmeter on my motorcycle and it works fine. However, the volts run from about 12.8vdc to 14.0vdc depending on RPM and can bounce around a lot. Instead of "instantaneous" voltage, I just need a suitably averaged voltage (over a few seconds, I'd guess) to keep an eye on so I know that the charging system is working.

Is there something that I can add to the circuit or input wiring like a capacitor to slow down the VM reaction time? If a capacitor (or other components) will do this how do I calculate the proper values?

Thank you.

Hello there,

I've done this same thing with my automobiles in the past and I caught a bad alternator on the way home one day from the store.  I was able to get a new alternator without going through the hassle of a dead battery too.

Anyway, one of the things I always did was wire my voltmeter right up to the battery.  That way I get no-nonsense battery voltage, not after-the-fact-with-some-other-unknown-load voltage.  The only catch is you have to also install a switch to turn it off because it will draw from the battery, and even though it's a small amount it is still very undesirable.
The one I use now plugs into the cigar lighter, so I just unplug it when I get home.

I've used analog meters too, and they work pretty nice.  They have a tendency to average the voltage but they can be very fast too.

A low pass RC filter is also an averaging filter.  You can use more than one stage too.  However, the series resistor has to be big enough to limit the charge and discharge of the capacitor or else it won't do much.  If you really need low resistance, you might have to use a lower value resistor and a larger value cap.

An interesting possibility is that the 0.7 volt drop you are seeing now could be due to the way the meters respond to a changing voltage.  With an averaging RC filter that may improve.