Electrical Ramblings                       
original article 05/93 R. Kwas, revisions on-going, updated 7/2015


Voltmeter vs. Amp Meter
Battery State of Charge
(Bad) Arguments Against Amp Meters
Reference Information
Amp Meter in a 122
Amp Meter in an 1800
The Adel Clamp
Industrial Current Shunt
Amp/Volt Meter in the Snow-Weasel


Background:  At the risk of explaining the obvious, an Amp meter, or Ammeter, is a measuring device that indicates Amps or electrical current (GEE!).  It may be installed in any circuit to show the magnitude of current flowing to the load (load being the general descriptive for the device which does the work--light bulb, electric motor, radio--you get the point.  Isn't electricity a splendid thing?) 

When referring to the Amp meter in a car, its' electrical location, and therefore its information to us, is very specific, but sometimes the source of some confusion which needs to be clarified.  To aid in this, I present my simplified (almost) copyrighted "H" diagram which shows the correct way to connect a vehicular Amp meter, and which applies with few exceptions, to just about every vehicle on the road: 

General "H" Diagram

For a specific wiring diagram showing AMP Meter connection in a Volvo 122 or 1800, see Reference Information below. 

If you follow the four different current paths around, you essentially have all possible (main) paths that occur. Path (1) Starting--straightforward--the actual magnitude of this current is not really of interest (unless we are in the business of working on starters) and quite high, so high in fact, that we would certainly damage a typical +/-60 Amp meter.  We will not measure it, since we will know when it flows by other means.  (2) Loads--again we will lump a number of electrical items into this category (both Battery and Ignition powered) and again we are not really interested in seeing a precise indication of the magnitude of these currents--we will know in short order that the Ignition or Radio or the Blower are, or are not working.

(3)(4) Charge/Discharge--these are the currents of interest!  In the two paths shown, we will have both the currents going from the Battery to our Loads. We will call this Discharge current, and this current becomes of concern to us because it serves to relieve the battery of its charge and ability to get us started (if we're parked, motor off) or tells us our alternator is not making it! (motor running).  Current in the other direction, Charging current, is an indication of energy going back into the battery from our source, the Charging System...and it should be of interest to us, whenever the meter is in the negative range, because that's when the Discharge current is exceeding the Charging current!  This means the Charging System is not supplying all of the Loads as it should, and current is being drawn from the Battery to make up the difference...perhaps we are running too many loads, or brushes are worn in the Generator, or a diode in the Alternator has failed open, or the Fanbelt is slipping...whatever the cause, we want to know when it happens! 

Note that installing an Amp Meter in a vintage Volvo requires relocating the "Load Wire" from terminal 30 of Starter (the common tie-point for Battery connections) to Voltage Regulator in order to put it on the same side of Amp Meter as the Charging System, so that the meter does not indicate Load current (see also: Reference Information )


Discussion of Voltmeter vs. Amp Meter:  This is often discussed using various correct and incorrect information and assumptions.  Here is a somewhat objective presentation to help the reader decide, with no incorrect information or assumptions...examples of these are shown below in:  Additional!  

Amp Meter: 
In general, Current is a Result of Voltage difference between two points.  An Amp Meter therefore displays a Resultant.  That is, it displays currents which result from the difference in Voltage between Battery and the voltage sensing of Regulator, which controls the Charging System. 

Advantages/Disadvantages:  An Amp Meter shows Battery Charge and Discharge currents as explained above.  Typical automotive meters have the necessary shunt internal, so must be connected with heavy gauge wiring which is more expensive than small gauge wiring.

Typical action of an Amp meter, is that indicator goes to a slight negative (Discharge), as Ignition is turned ON, and before Engine is started.  What is being indicated at that time is the Ignition System load being supplied by Battery.  As we crank Starter (this current will not be shown!) and finally Start, current indication jumps high into the positive region (Charging) after Starting, as Charging System replaces Starting power load which caused Battery Voltage to drop.  This was sensed by voltage sensing of Regulator, which called for output of Charging System by enabling Generator/Alternator Field.  As Battery Charge (and voltage) comes up, charging Current indicated decreases, eventually dropping to just into the positive (trickle charge), and we can expect it to remain there for the remainder of trip. 

Amp meter, showing typical range of indicator.  Pointer in the negative when engine is running,
 indicates no charging, and driver should take notice!

Unusual Conditions indicated by Amp Meter:

Low Charging Current ...slipping belt (possibly accompanied by squealing, but not always), possible Charging System or Battery problem.

Continuous High Charging Current for no good reason (like if we didn't drain Battery inadvertently)...if we drained Battery, and got a Jump to get started, we would expect a higher than normal Charge current indication possible Regulator or Battery problem. 

Continuous High Discharging Current.  Charging System off-line, lost fan-belt, or some other problem.  This condition needs checking ASAP!


Automotive Amp Meter Internal and Function.  Looking closer at an automotive Amp meter:  

Internal detail of Automotive Amp Meter.  Main current path at Green.  Miniscule measuring current at Blue.

When in the circuit to measure current, most of the current flows through an Internal Shunt resistor of low resistance, but this path has been designed for and perfectly suitable for the expected high current.  As current flows, it develops a small voltage across this resistance (as Ohms Law states it must!), this small voltage which develops being representative of the current flowing, and enough to drive a highly sensitive D'Arsonval (moving Coil) center zero meter movement, also contained within the meter package.  This is a theoretical explanation only...I don't know if VDO has cheapened the design to do it simpler and cheaper (and undoubtedly less accurate)...having taken apart some cheaper meters, I do know other manufacturers have, by simply using the significant magnetic field associated with current flow, to directly drive a magnetic indicator needle...kinda like a compass (cheap, effective, but not so accurate)...once again:  You get what you pay for!  



In general, the Voltmeter on the other hand, displays the input parameter which causes the Resultant (Current).  It can be connected anywhere in the system to indicate the Voltage there, but typically it is connected somewhere near the Battery, so its' indication is not subject to voltage drops.

Since only miniscule current is flowing through a Voltmeter, it may be connected using small gauge wire, and it may be connected wherever we want to measure voltage (which should be at the Battery terminals or electrically nearby).  ...but since Vehicle Voltage buss is pulled up or down as a function of a number of factors, the information supplied by the voltmeter, is quite limited, since the 100 to 0 percent charge condition actually happens within a mere 8 tenths of a volt span (see chart below, special thanks to Mr. Yoder of Interstate Battery)! 

Even with an expanded scale voltage meter (many automotive voltmeters are, see below) by the time we notice a low voltage condition we are already in trouble. This, opposed to a current meter which as soon as it shows discharge while the motor is running signals a charging system problem--thankfully while we still have an almost fully charged battery to get home or to safety on.  Furthermore, the only absolute voltage reading that is truly indicative of the "state of the battery" is the "open circuit voltage," that is, before the alternator is charging or the loads are discharging the battery. When the motor is running, the voltmeter indicates the vehicle's system or bus voltage which will be pulled up or down as a function of mean battery current.

Battery State of Charge (see also Related Links belowState of Charge):  

Open Circuit Voltage Percent Charge
12.68 100
12.45 75
12.24 50
12.06 25
11.89 0

(70įF) Source: Battery Service Manual, Copyright 1987 Courtesy Battery Council International

Expanded Scale Voltmeter with 0 to 100% charge levels marked.  Indicator will be above 12.7V,
 while Charging System has an output and is pulling vehicle power bus up. 

Typical action of a Voltmeter:  PLACEHOLDER FOR CONTENT  but basically if the Voltage indicated rises after engine is started, that's a good sign of Charging System putting out, pulling up system voltage, and (probably) charging Battery...if it shows no rise after engine is Started, that's a bad sign!

Unusual Conditions indicated by a Voltmeter: 

Voltage below Normal operating range ...low state of Battery Charge, or a high number of Loads have been turned ON, and this is pulling Bus voltage down.  Try turning OFF some of the major Loads to see if this results in Buss voltage coming back up...this could be an indication of Charging System problem!     

High Voltage indication.  This can be cause by a faulty Regulator (or something as simple as a poor chassis connection to the regulator), any voltage drop developed across this poor connection is added onto the voltage reference within the regulator...this will result in overcharging Battery. 

Automotive Voltmeter Internal and Function:  PLACEHOLDER FOR CONTENT



I believe the reasoning behind the trend of installing voltmeters instead of current meters is because voltmeters are simpler/cheaper to incorporate into the automotive electrical system (automotive design changes are often driven by cost!).  They can be connected with modest wire to almost anywhere in the system.  I suggest, therefore, for those drivers cursed with a voltmeter (...it does beat an idiot light only*) to make this simple modification which will make the instrument a bit more useful:  With a fully charged battery and the motor not running, observe and mark the "low normal" voltage with a small dot of white paint or tape on the meter glass.  Start the motor and after a minute or so, after the voltage settles out, mark the "high normal" voltage in the same way.  Later, when the indication deviates from these levels, it will stand out, as you (hopefully occasionally!) monitor your gauges, clueing you into a possible abnormal condition.  By making this simple modification, you have essentially turned the voltmeter into a 3 zone instrument (Under, Normal, Over), and like the 2 zone ammeter (Discharge, Charge) have made it, I believe, somewhat more informative.

[Non-objective comment:  For my money, I like to, and recommend installing an ammeter when doing my alternator retrofit/upgrade ]



Other People's (Bad) Arguments Against Amp Meters:  

[ ...some postings I found searching for "ammeter vs. voltmeter for car"...wow, there's some bad info out there!  My stupid comments, Highlighted.]

from:  http://www.hotrodders.com/forum/ammeter-voltmeter-46577.html

General Motors puts Volt Meters in their cars...........not Amp gauges. If the volt meter dies..........the car will still run.  NOT SO with a Amp gauge.  [FALSE!  A car would certainly run, since Charging System is on the same side as Ignition (Reference "H" Diagram above) and depending on the exact manner in which Amp Meter fails, it might even continue charging, just the indicator part might not work...for instance if the needle were to fall off its pivot.]

In looking around, I repeatedly saw the statement that Amp Meters are somehow less safe than Voltmeters, with anecdotal stories of failed Amp meters having started under-dashboard fires. 

Reiterating, yes, a typical automotive Amp meter requires heavy wires to and from because of the high current routed through it, but to say this makes it more susceptible to causing under-dash fires is a stretch!  There are plenty of other wires under the dash which are powered and capable of delivering fire starting energy...for instance, the Ignition Switch supply wire (which is also unfused and coming directly from Battery).  If a continuous high Charge Current (something wrong with Battery?) or Discharge Current (Failed Charging System?) is indicated, ignoring it will lead to further problems (see:  Phil's posting below).  Fact is, Amp Meter wires are no more susceptible than any other powered wires under dash for starting fires!  Good wiring practice such as routing, support with Adel clamps (See Reference Information:  The Adel Clamp) and properly protecting wiring and harness with sleeving and from sharp edges prevent these!   

...not like this:

If behind your dashboard looks like this, you don't need dangerous Amp Meter wiring to start an electrical fire! 
This is not of a Volvo, but might be a picture after an explosion in a spaghetti factory!


Fly Away!  ...and if Amp meters are so "unsafe" why do small aircraft have Amp meters?  Answer:  Because they give better information, that's why!  If Voltmeters gave better information, we would expect to see them installed in aircraft where performance and usefulness come before cost.  It should be noted that in aircraft, the shunt resistor, and so the high current, is typically "remote" (but not always, see below!).  That is, by removing Shunt resistor from within the instrument and relocating it in the engine compartment, heavy gauge wire (and weight, also important in aircraft!) is saved, and the Indicator can simply consist of the sensitive D'Arsonval meter movement located at the instrument panel (and be wired with small gauge wiring, because the large gauge wiring is so scary to some!).  A remote shunt type could certainly also be fitted in a vehicle, and that is in fact what I have in the Snow-Weasel, see Reference Information:    Amp/Volt Meter in the Snow-Weasel)

Here is an example of a high-end instrument which might be fitted in aircraft.  [My Highlights.]  It is pricey and certainly beyond what we might consider for a car, but included here for reference information.  It is a dual instrument indicating both Voltage and Current to give the most information(!), it allows internal or external (remote) shunt options, and has special Overvoltage and Discharge monitoring circuitry and Indicators for each.  Both of these are important unusual conditions when engine is running, which a pilot needs to know about as soon as they occur, so the instrument will enunciate these problem-conditions with lights to attract the pilot's attention!

The high-end avionic metering option. 

In Industry, Amp meters typically are also often the remote shunt type.  Similar to the aircraft application, this means that the shunt and related high current wiring is short and stays in the technical department, while indicators are wired with modest wire and located in the control room, or wherever necessary.  (See Reference Information:  Industrial Current Shunt) 


More Misconceptions found at GT40 forum:  I will address these individually:  http://www.gt40s.com/forum/gt40-tech-fueling-electrics-engine-cooling/27678-voltmeter-ammeter-more-useful.html  [My stupid comments included, highlighted.  Ron]

Anthony:  "ammeters were more important when charging was provided by a generator rather than an alternator but I could be mistaken."

[Yeah, he's mistaken...what does it matter if the Charging System is Generator or Alternator based?  It doesn't!  The point of a gauge is to be able to monitor the most important parameters of the vehicle electrical system...and those are:  1.  Health of Charging Sys, no matter if Generator or Alternator based, and... 2.  If Battery is getting charged or discharged.  An Amp meter does both, directly!  A Voltmeter does the former indirectly, and the later only marginally (both needing lots of interpretation)! ]


Randy V.:  "Ammeters / Amp meters have all been long discontinued from any production car due to the potential source of fire..."

[Randy has clearly been taken in by the common explanation...what about all the other scary wires under the dash, and what about that terribly flammable gasoline in the tank and under the hood?  This is the pseudo-technical (but BS) reason so often repeated, but with zero supporting information...the real reason they were discontinued is cost of installation. ]


Phil's Posting: 
Poor Phil...he understands several things just plain wrong, and unfortunately, it looks like he was fooled by two wrongs!:  "I once installed an ammeter in a car I had in the 60's, and everything was fine for a while [...I'm not so sure about that! ] until the car would not start, ammeter showed plenty of charge, almost 60 amps at times.
The problem was the alternator which had a bad field coil and although it was putting out lots of current [HUH???  I fail to see how it could be putting out "lots of current" with a "bad field coil"...if an Alternator puts out without a working field coil, why does it nesd a field coil at all? ], it couldn't overcome the voltage (As Brett mentioned Pressure) stored in the battery. The result was an 11 volt charging voltage [11Volt "charging voltage" is NO charging Voltage...that Alt was not putting out...Period! ] that a voltmeter would have shown. [True, but your Amp meter was also showing it, I just suspect it was wired backwards. ]
Out went the ammeter and a lot of wiring that I discovered was cooked (8 Guage to boot). [Then you knew where those 60A were going...and not from the Alt because it wasn't putting out...but from the Battery, running everything, and burning up the wiring in the process...again, it sounds like his Amp meter was hooked up backwards, and what Phil thought was "plenty of charge" and that everything was OK, was really plenty of Discharge current from the Battery! ]
One question you had was the fact that ammeters have a negative side, this would show a reversal of current flow such as lots of draw with insufficient current coming from the alternator [Or NONE, as in your case! ]. A voltmeter will also show this as a low voltage condition.  [But only after Battery is well on the way to being dead.  Then you have nothing...no Charge Sys, and no reserve in the Battery...all you can do is park and walk! ]
Since the voltmeter measures system potential it can be installed in any 12V supplied area in the system, and will measure the potential (pressure) as long as the source has no resistance such as an ignition coil supply etc."  [No matter which kind of meter is installed, Amp or Volt, connecting it up correctly and in the right place electrically, then checking out its' function, is required! ]


Link:  What happened to the ammeter?  by Scott J.:  http://blog.ks3j.net/?p=1024  [Don't just take my word for it!]


Related Links:

Most recent (spirited) Ampmeter vs. Voltmeter discussion on Brickboard   https://www.brickboard.com/RWD/volvo/1629993/1800/wiring_aftermarket_ammeter.html 

AmpMeter discussion:  http://www.brickboard.com/RWD/index.htm?id=1213119

Another Amp vs. Voltmeter discussion:  https://www.brickboard.com/RWD/volvo/1620133/444-544/voltclock_ammeter.html

State of Charge:  http://all-about-lead-acid-batteries.capnfatz.com/all-about-lead-acid-batteries/lead-acid-battery-maintenance/battery-state-of-charge-using-a-voltmeter/

SwEm Technical Article Battery Notes 

Link to good additional Battery info:  http://www.uuhome.de/william.darden/


Amp vs. Voltmeter discussion 120-130


I usually agree with your good information, its based on experience, sound engineering practices and principles...but this time without recognizing the advantage of an Ampmeter, you have simply condemned it...the discussion for/against Amp/Voltmeters is not that straightforward, and the points which you have stated as disadvantages against Ampmeters are not valid IMO, and I disagree with what you state is an advantage of the Voltmeter...

1. "ALL the current has to pass through the ammeter to give valid readings.
Bad wiring = FIRE." Yes an Ampmeter must be connected with heavy gauge wire, but wire doesn't generally "go bad" by itself...wires typically fail and get hot if they are overburdened or allowed to vibrate or abrade on sharp corners which compromises the insulation (=improper installation), so using 600V insulation (double thickness compared to the standard 300V) AND sleeving on AMPmeter wiring which runs through the firewall is definitely in order (in any car wiring!)...there is an additional risk in that there is now a wire essentially connected to the bat (and therefore capable of high currents) running behind the dashboard, but so what, there plenty more of those! Replacement bat. cables also have a "fusible link" which limits current which can be passed through this cable...and the AMPmeter makes a good place to connect addition loads in a vintage Volvo so as not to put additional current through the original wiring. Finally, I would say that there is no more risk for having a fire in Ampmeter wiring than having a fire in any other wire...

2. "Voltmeter gives better info and can be hooked up easily about anywhere in the system." I have to disagree with this statement also - A Voltmeter indicates the system voltage, and depending on a number of factors (like state of charge of bat., loads turned ON, temperature), this is pulled up and down quite a bit, so if the point of installing a meter in the first place is to be able to monitor the (A) health of the charging system and (B)state of charge of bat., it doesn't do either one very well...(A)state of the charging system is not easily determined (because of this variation depending on loads), and (B) neither is the state of charge of the bat (from 10% charge to 90% charge occurs within 0.75V!, temp dependent)...I fail to see where this can be considered to be "better info". It does have the advantage of being able to be connected with a small gauge wire.

Ampmeter on the other hand, is split into two areas, charge and discharge...interpreting its indication is much simpler...if the indication is ever in the discharge area while engine is running, charging system is not putting out adequately to keep up with loads...not much interpretation necessary...

More here: http://www.sw-em.com/elecramb.htm

The bottom line is that either one is head and shoulders above an "idiot light" only.



Reference Information: 

Example of the proper electrical location for an Amp Meter in a 122 (similar to a 544!)

Extract from the 122 Wiring Diagram showing correct electrical location of AMP Meter at Red.  Note relocation of one black wire from terminal 30 of Starter to Voltage Regulator B+ terminal at Blue, to assure the correct electrical location of Amp Meter per "H" diagram.  Charging System shown in Green block, is the original equipment Generator based system, OR could also be Alternator based. 

Amp meter in an 1800:

Extract from 1800 Wiring Diagram showing correct electrical location of AMP Meter at Red.  Relocation of one black wire from terminal 30 of Starter to Voltage Regulator B+ terminal at Green, is also necessary here to assure the correct electrical location of Amp Meter per "H" diagram.

The Adel Clamp ...for when you need to "secure a harness right!" :

Industrial Current Shunt

Note Ratings shown:  200A is current maximum which can be passed through this shunt (by way of suitable heavy gauge wires connected to big bolts on top), and at this current, 50mV is voltage dropped across the shunt's resistance.  A voltmeter (which draws very little current) is used to measure this current, by connecting it to the small screws on the side, using modest gauge wiring, and this can be a long distance removed (and fused for safety if that is called for).  The assembly is gold plated to minimize corrosion and maintain its' accuracy over time.  Further, construction-wise, it looks like this shunt uses standardized end-blocks into which conductors are soldered to make shunts of varying values...this particular configuration has two of those soldered-in plates, but there's absolutely no magic going on here, just good engineering and practices! 

...as a mathematical exercise, we can use Ohm Law to calculate the Resistance of this hunk of metal.  R = V/I , .050/200 =  .25milliOhms (not very much at at, so it doesn't influence the current we are measuring, only sniffing away a miniscule amount with which we can indicated it!)


Amp/Volt Meter in the Snow-Weasel:

The Snow Weasel actually has an Amp/Volt Meter of my own design.  Located on a little black plastic panel, located where ashtray used to be, which collects all additional switches and indicators to prevent under-dash clutter, and which somewhat matches the panel of the adjacent Blaupunkt radio.  It is based on a small, ruggedized, center-zero, military meter movement which I harvested from some military communication gear. 

In the Amps indicating position of the associated (position locked) DPDT switch, it uses the vehicle wiring as a Remote Shunt (only small gauge wiring runs to the dashboard), and is of course connected in the standard "H" configuration.

In the Volts indicating position, negative terminal of the meter is connected to chassis, a 402K Ohm resistor limits current into the positive terminal of the sensitive meter to give a usable reading, and a micro-fuse protects things.  The I/V selector switch is typically left in the I (Current) position (and the position locking feature will keep it there without a deliberate switching action to change it).  Since the meter is wired upstream of the Ignition Switch, if left in the Volts position, it would keep on indicating even after engine (and Charging System) was off, acting as a (miniscule) load, but nonetheless slowly discharging Battery.  This does not occur in the Current indicating position...only Charge/Discharge current flows! 

Snow Weasel Electrical System Modified "H" Diagram

Here's what it looks like:

Snow Weasel dashboard. NEED PIC WITH BETTER DETAIL of meter, less of shoe!!


This article is Copyright © 1993-2016.  Ronald Kwas.  The terms Volvo, VDO and are used for reference only.  I have no affiliation with these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 


B A C K ! . . .to Tech Articles Index Page