[Sutripta]

To inform the driver of malfunctions in the charging circuit, modern vehicles have only an Ign. warning light. This is inadequate for the more informed driver. The two instruments available (usually as a retrofit) to the driver (not service technician) is the ammeter and the voltmeter. So a few questions.

How does the Ign. warning light work? What can we learn from it? How do we interpret it?
How do we interpret the results of the ammeter. And the voltmeter.
Why did older cars have an ammeter, and newer cars a voltmeter?
If we want to retrofit one in our vehicle, how do we go about doing it. Any special points one needs to be aware of?
If we have to choose between ammeter and voltmeter, which one should we choose and why?

Hoping to have an informed discussion (and no useless internet links!)

Regards
Sutripta

[Aroy]

Ammeter
This device tell you which way the current to the battery is flowing. Positive, means the battery is charging, negative, the battery is discharging. Of course it tell you nothing about the alternator current. For that you will need a separate ammeter connected to the alternator output.

Voltmeter
All it tells you is the voltage at the battery terminals. The major utility I have experienced is if the voltage regulator malfunctions, the voltage may shoot up to 16V or more. This phenomenon is limited to older DC Dynamos in say Ambassador, which had a user changeable Voltage and Current regulator (vibrating relay type). Modern alternators have an inbuilt solid state regulator, which rarely fails, and when it does the alternator does not generate any thing!

Interpretation
AMMETER
. If ammeter shows heavy charging continuously, then either the regulator is bad, else the battery is kaput.
. If ammeter shows discharge, then the alternator is not working.
. If the ammeter shows charging only at high RPM, and discharge at lower RPM, then either the alternator is defective, or else other accessories are drawing heavy current (or there is a major short circuit some where), requiring the alternator to work at higher RPM. Either way, time to check both the alternator, as well as the electricals.
VOLTMETER
. Without the engine working if the voltmeter shows, voltage < 12.5V, the battery needs attention.
. If after a few unsuccessful cranking of the engine the voltage drops, say below 12V, battery is discharged.
. If the voltmeter shows voltage less than 10V while cranking, the starter motor is drawing excessive current - either the motor is bad, or the battery has inadequate capacity (mainly a problem in cold weather)

[SS-Traveller]

Originally Posted by Sutripta

If we have to choose between ammeter and voltmeter, which one should we choose and why?

My vote goes to a voltmeter across the battery terminals.

Feedback about what the alternator is doing as well as battery health can be interpreted through the voltmeter. The detailed technical interpretations (and how to install one) are not for me to discuss!

[SirAlec]

I would choose neither as its not required. One of my last purchase is a Ritz, it does not even have a Temperature meter. It does have a hot warning light.

What i feel is a engine temperature meter is a must, especially where i live in the hills. They discarded it for aesthetic reasons maybe. As ritz has a single big dial like mini and a digital oddo.

If you are hell bound for it you can fix a voltmeter instead of Ammeter. Not for its usage or something, as Ammeter is usually connected in series with the main line and good ammeter is hard to find. If it fries your main circuit will open and whole car will be a dud. Volt meter is connected in parallel to the battery. Even if its fries main line will not be disturbed.

Now the technicality.
Ammeter is very useful as it tells you the actual health of the battery the current it delivers when used. I have seen 12.5 volts even in battery that does not have the power to crank.

PS: i am planning to fix a real time temp meter in Ritz.

[vina]

Originally Posted by SirAlec

...

If you are hell bound for it you can fix a voltmeter instead of Ammeter. Not for its usage or something, as Ammeter is usually connected in series with the main line and good ammeter is hard to find.

As any electrical designer will tell you - high current ammeters are not connected in series - they are connected in parallel with a known shunt impedance. The shunt impedance + ammeter is of course connected (overall setup) in series.

In fact things are murkier than this - almost all real ammeters I personally know off (that were not built 3 decades ago and/or that can measure large currents) are actually a R+voltmeter combination - the current flows through the R and voltmeter measures the voltage across that R.

In general it is very hard to "fry" the R in question.

Of course if you are totally paranoid, you can use a hall effect sensor.

Originally Posted by SirAlec

If it fries your main circuit will open and whole car will be a dud. Volt meter is connected in parallel to the battery. Even if its fries main line will not be disturbed.

What if the fried voltmeter causes a short circuit?

Originally Posted by SirAlec

Now the technicality.
Ammeter is very useful as it tells you the actual health of the battery the current it delivers when used. I have seen 12.5 volts even in battery that does not have the power to crank.

PS: i am planning to fix a real time temp meter in Ritz.

True health of the battery can't be gauged just by voltmeter/ammeter - though if you do have both of them available then internal resistance can be measured and if you keep track of it that is a pretty good indicator of battery health.

Battery charge state can be gauged to some extent by voltmeter. Lower voltage than nominal means low charge. Ammeter will of course give you a more direct estimate after the fact. i.e. once you are cranking and find that the battery has died on you.

If you want a DIY project though you can actually have an integrating ammeter - that will tell you how much charge went in overall - that is the max that can come out (this is one of the algorithms Li-ion battery controllers work - the o/p voltage is very unreliable)

[SirAlec]

Thanks for the insight. Thats what i learned in my high school physics, and that was long time ago.

any more insight or books, pdf on this new breed of Ammeter will be helpful.

Thanks again.

True health of the battery can't be gauged just by voltmeter/ammeter - though if you do have both of them available then internal resistance can be measured and if you keep track of it that is a pretty good indicator of battery health.

BTW best judge of that will be the time when you actually know you are in a dud car and its not cranking at all.

But temperature meter that i talked about is a dead serious issue, It actually will save the engine from cessation and hence will save money and time.

[Sutripta]

Great that some discussion on!

Think it is best if we go through some (very) basics. Then we can work out what expected behaviour should be, and therefore spot anomalies. Then we can correlate the anomalies with problems.

Also think it is best if explanation (initially at least) goes along lines of high school and college physics. This will also be of advantage to the large number of IT/ ITES personnel in this forum.

To start of with, the electrical system consists of a large number of loads, and two sources of power (battery and alternator. We shall discuss dynamos only if there is sufficient interest at the end of the thread). But instead of thinking that the loads are connected to the battery (through switches/ relays etc), and that the battery is charged by the alternator, I think it is conceptually clearer if we think that the loads are connected to a 'power bus', and that the power bus has two sources of power, the battery and the alternator, which are connected to it in parallel. The nominal voltage of this power bus is 14.4 volts, not 12. All loads, except self starter (and I suppose glow plugs) are designed to operate at 14.4 volts.

The self starter, though logically connected to this power bus, is physically handled separately because of its very high current consumption.

Immediate question should be how can one connect two sources in parallel, which is what we are doing when we are connecting them to the same bus. More in the next post.

Regards
Sutripta

[Sutripta]

Originally Posted by Aroy

Modern alternators have an inbuilt solid state regulator, which rarely fails, and when it does the alternator does not generate any thing!

I've seen more AVR failures than I can care to remember. And most were that the output voltage went uncontrolled!

Originally Posted by SS-Traveller

My vote goes to a voltmeter across the battery terminals.

And why did the older car designers choose an ammeter?

Originally Posted by SirAlec

I would choose neither as its not required.
I think we Bhpians can handle the information overload. The car manufacturers think that we morons will be confused by too much information. The fact that they save some money in the process is just collateral benefit.
One of my last purchase is a Ritz, it does not even have a Temperature meter. It does have a hot warning light.

What i feel is a engine temperature meter is a must, especially where i live in the hills. They discarded it for aesthetic reasons maybe. As ritz has a single big dial like mini and a digital oddo.
The Alto K10 VXi has a speedo - Tacho combination. And a LCD bargraph for fuel. No Temp gauge.
Temp is one of the inputs for the ECU. It would be very simple to display the temp on the fuel bargraph. Either on driver intervention, or when it needs attention. Question is why not. Anyway, we are way OT.

I have seen 12.5 volts even in battery that does not have the power to crank.
You are right. Why?

Originally Posted by vina

As any electrical designer will tell you - high current ammeters are not connected in series - they are connected in parallel with a known shunt impedance.

It will be interesting if we can work out why cars evolved differently.

Regards
Sutripta

[vina]

Originally Posted by SirAlec

Thanks for the insight. Thats what i learned in my high school physics, and that was long time ago.

any more insight or books, pdf on this new breed of Ammeter will be helpful.

Thanks again.



BTW best judge of that will be the time when you actually know you are in a dud car and its not cranking at all.

But temperature meter that i talked about is a dead serious issue, It actually will save the engine from cessation and hence will save money and time.

Actually I learnt about ammeters and voltmeters from my father's high school books - mine didn't really have any details and my sister's (she is younger) almost skipped the two !

The books did talk about ammeter being connected in series and voltmeter in parallel, but then there were sections about how to measure voltage with ammeter and current with voltmeter (also sections on how to measure 50V with a 5V capable voltmeter and how to measure 100A with a 1A capable ammeter). The rest is just breaking apart the things and looking for yourself .

Digital voltmeters are actually analog to digital converters - they compare the voltage directly with a reference and have very very high impedance. A digital ammeters is a precision resistors across which a voltmeter is connected.



Coming to temp. sensor - very good idea and as I wrote earlier you may want to put one on the battery as well. Very hot batteries and very cold batteries do not work very well no matter what their state of charge is. Though of course if engine is running cold/hot (and you know about that) you can change your driving style. With battery there isn't a lot you can do.

Originally Posted by Sutripta

Great that some discussion on!

Think it is best if we go through some (very) basics. Then we can work out what expected behaviour should be, and therefore spot anomalies. Then we can correlate the anomalies with problems.

This is what preventive maintenance is all about - keep looking for any unexpected or abrupt changes in behaviour.

Originally Posted by Sutripta

Also think it is best if explanation (initially at least) goes along lines of high school and college physics. This will also be of advantage to the large number of IT/ ITES personnel in this forum.

Dada, my explanations were actually from Physics textbooks of 11th grade from the early 1960s (written in Hindi).

But I agree we must keep it as simple as possible for as long as possible.

Originally Posted by Sutripta

To start of with, the electrical system consists of a large number of loads, and two sources of power (battery and alternator. We shall discuss dynamos only if there is sufficient interest at the end of the thread). But instead of thinking that the loads are connected to the battery (through switches/ relays etc), and that the battery is charged by the alternator, I think it is conceptually clearer if we think that the loads are connected to a 'power bus', and that the power bus has two sources of power, the battery and the alternator, which are connected to it in parallel. The nominal voltage of this power bus is 14.4 volts, not 12. All loads, except self starter (and I suppose glow plugs) are designed to operate at 14.4 volts.

Bus, relays, glow plugs ! How is this at a high school level ?

I don't know specifics of car eletricals so here's a question - is the battery directly connected to the "power bus" or is there a switch/relay mechanism in between?

also is there only one "power bus" or are there two or even more (perhaps different vehicles have it done differently)? From what I know car chassis is used as ground for all return currents (by the way, how does it work when chassis goes carbon/fibreglass)

If I were designing things I would define different "domains" - at least one for mission critical stuf - attached to the battery and another for non critical stuff (like ICE) directly connected to alternator with optional switch to battery, and perhaps more.

Originally Posted by Sutripta

The self starter, though logically connected to this power bus, is physically handled separately because of its very high current consumption.

Immediate question should be how can one connect two sources in parallel, which is what we are doing when we are connecting them to the same bus. More in the next post.

Regards
Sutripta

Originally Posted by Sutripta

I've seen more AVR failures than I can care to remember. And most were that the output voltage went uncontrolled!
...

It will be interesting if we can work out why cars evolved differently.

Regards
Sutripta

First paragraph:
chip designers usually don't have a clue how unreliable their products are. I remember talking to a technician with British MoD once - their nuclear missile submarines used vaccum tube electronics from the 60s well into late 90s - because it was "boot proof" i.e. you kick it like hell with army boots and it will still work.

Second paragraph:
Cars did NOT eveolve differently. This has nothing to do with cars. But let me ask you a different question (if you want to stick to the "ammeter is series" theology) - if you want to measure the o/p current of a local substation's transformer, do you seriously believe you will put the "ammeter" is series?

As people wrote on other threads - at start the electricals take currents as high as 300A - the small thin wire of the coil in the ammeter will be glowing red in no time if anything like that passed through it. In fact it will be glowing red if the nominal 50A passed through it.

[Aroy]

Originally Posted by vina

As people wrote on other threads - at start the electricals take currents as high as 300A - the small thin wire of the coil in the ammeter will be glowing red in no time if anything like that passed through it. In fact it will be glowing red if the nominal 50A passed through it.

Actually that 300A+ current passes through a separate cable - the starter cable which is thick enough. All other electricals, including ignition are supplied through a separate conductor which in most of the cars is routed through the ignition switch.

Coming back to the ammeter, in older cars the ammeter was wired in series after the ignition switch. It would work only if the ignition was on. The reason was primarily reliability and ruggedness, as a high current device had thicker components. A voltmeter across a shunt (it detects low currents), is much more delicate instrument, and could not withstand the vibration and/or temperatures of an automotive environment. With modern digital displays that is no longer valid, so we can easily install a Digital Meter for both voltage and current measurements. In fact if you really want to, you can install a series of small shunts, where ever you want to measure the current using digital meter; either one for each shunt, or simply switch between them.

[tsk1979]

Actually, a volt meter is often enough to tell the state of the Alternator. If with headlights and AC on you can get 13.5 on battery terminals, it means everything is fine. the moment it goes beyond 15.5 or below 12.8 or something, you know trouble is there somewhere.
A voltmeter always stays in my toolkit now. When TASS started recommending battery replacements, it was with a voltmeter I discovered that the alternator was not charging battery properly(low voltage)

[vina]

Originally Posted by tsk1979

Actually, a volt meter is often enough to tell the state of the Alternator. If with headlights and AC on you can get 13.5 on battery terminals, it means everything is fine. the moment it goes beyond 15.5 or below 12.8 or something, you know trouble is there somewhere.
A voltmeter always stays in my toolkit now. When TASS started recommending battery replacements, it was with a voltmeter I discovered that the alternator was not charging battery properly(low voltage)

tsk, this is a good observation.

The alternator o/p is regulated with load, so if something goes kaput, voltage regulation must go bad too - voltmeter would find out all such cases (which is most cases when alternator/regulator is not working properly)

Originally Posted by Aroy

Actually that 300A+ current passes through a separate cable - the starter cable which is thick enough. All other electricals, including ignition are supplied through a separate conductor which in most of the cars is routed through the ignition switch.

Coming back to the ammeter, in older cars the ammeter was wired in series after the ignition switch. It would work only if the ignition was on. The reason was primarily reliability and ruggedness, as a high current device had thicker components. A voltmeter across a shunt (it detects low currents), is much more delicate instrument, and could not withstand the vibration and/or temperatures of an automotive environment. With modern digital displays that is no longer valid, so we can easily install a Digital Meter for both voltage and current measurements. In fact if you really want to, you can install a series of small shunts, where ever you want to measure the current using digital meter; either one for each shunt, or simply switch between them.

While the part you wrote about starter cables being separate is almost certainly true - the last time I saw an ammeter that was really a series ammeter, it weighed 5 kg and was the size of a 5ltr Oil can (and was made in 1950).

Anyway - here's my suggestion pry open an Ammeter from any old voltage stabiliser at home (they all had Ammeters) - you'll easily find a shunt resistance in their.

On voltmeters being delicate - I guess you are talking about moving coil voltmeters. All new ones are solid state. But even talking about the moving coil voltmeter - what it the difference between a moving coil voltmeter and a moving coil ammeter?

[tsk1979]

There is also the "jugaad type test" which most electricians employ. They start the car, and disconnect battery. If voltage is too low, engine dies. It does not catch slightly low voltages(eg 12.5).
surprisingly, I have not seen auto electricians carry voltmeters. They work from experience, but do not have any idea about the why!

[Sutripta]

Originally Posted by Aroy

All other electricals, including ignition are supplied through a separate conductor which in most of the cars is routed through the ignition switch.
No, not really. You should be able to park and lock your car with the parking lights on.

Coming back to the ammeter, in older cars the ammeter was wired in series after the ignition switch. It would work only if the ignition was on.
See above.

Originally Posted by vina

The books did talk about ammeter being connected in series and voltmeter in parallel, but then there were sections about how to measure voltage with ammeter and current with voltmeter


Bus, relays, glow plugs ! How is this at a high school level ?
I agree. Less than high school. Our roadside mechs handle it without a problem.

I don't know specifics of car eletricals so here's a question - is the battery directly connected to the "power bus" or is there a switch/relay mechanism in between?
For all loads except self starter, through some protective device like a fusible link.

also is there only one "power bus" or are there two or even more (perhaps different vehicles have it done differently)? From what I know car chassis is used as ground for all return currents (by the way, how does it work when chassis goes carbon/fibreglass)
The body/ chassis is used for the return path.
Self starter best thought of separately. (My view only)
Some highly specialised applications have separate power sources/ busses. eg separate batteries for winching or other prolonged high drain applications. Or auxillary windings in the alternator for highly specialised applications.

If I were designing things I would define different "domains" - at least one for mission critical stuf - attached to the battery and another for non critical stuff (like ICE) directly connected to alternator with optional switch to battery, and perhaps more.
Any views on ISRO satellite designs?

Second paragraph:
Cars did NOT eveolve differently. This has nothing to do with cars.
Wow!
But let me ask you a different question (if you want to stick to the "ammeter is series" theology) - if you want to measure the o/p current of a local substation's transformer, do you seriously believe you will put the "ammeter" is series?
See later.

As people wrote on other threads - at start the electricals take currents as high as 300A - the small thin wire of the coil in the ammeter will be glowing red in no time if anything like that passed through it. In fact it will be glowing red if the nominal 50A passed through it.
As mentioned several times, self starter handled separately.

Thoughts on voltage/ current measurement.
At the heart, we have a measuring device. Popularly called the movement. It will show FSD for a certain voltage across its terminals, which will cause a certain amount of current to flow, and consume a certain amount of power. This is the basic measuring device. This can be anything from moving iron to powered (digital) systems. Principle does not change. To measure higher ranges of voltage, we drop the excess voltage across a series resistance. To measure higher currents, we divert the excess through a shunt. Whether the series resistance/ shunt is built into the movement housing, or external and visible does not change the principle. Though I've never seen a automotive ammeter with an external shunt!

High value AC is measured using CTs/ PTs.
But any idea how you measure currents in say an aluminium smelter?

Originally Posted by vina

While the part you wrote about starter cables being separate is almost certainly true - the last time I saw an ammeter that was really a series ammeter, it weighed 5 kg and was the size of a 5ltr Oil can (and was made in 1950).

Anyway - here's my suggestion pry open an Ammeter from any old voltage stabiliser at home (they all had Ammeters) - you'll easily find a shunt resistance in their.
In fact it will be instructive to take apart an automotive ammeter!

On voltmeters being delicate - I guess you are talking about moving coil voltmeters. All new ones are solid state. But even talking about the moving coil voltmeter - what it the difference between a moving coil voltmeter and a moving coil ammeter?
Answered previously.

Regards
Sutripta

[Abes]

Are they really required? I mean there are lights for indicating any malfunction, do we need to be over-worried about things that we need to keep a constant watch on ammeter & voltmeter? How often does the modern cars give problems in these areas? No offence meant to anyone here!

Long time ago, when I had my first Maruti 800, the service foreman told me to watch out these while starting the car: the battery light should be lit, when turning the key to crank, else there is trouble with rectifier or alternator! The battery light should go off after the engine is started, else there is a charging problem! In 14 yrs of owning the M800, only 2 times I had problem with the rectifier/alternator!

When the cars go for regular service, dont they check for these kind of problems, as there are proper equipments for checking these now-a-days?

At least here they do! Last time my car went for service, the electrician at the agency told me that the battery is weak. He gave me a print out from an equipment (attached here) & told me to get the battery replaced outside, as it is expensive in the agency! I ignored the advice, as the indicator on the battery was green, & the battery died on me in about a months time! So the indicator on the Acdelco battery is not reliable!