Volt Gage Verses Amp Gage.

[PAUL161]

There has been, for a long time, confusion as which gage is the better instrument to use. The amp gage or the volt gage. The best answer I can give is to use both and I'll give you an example why. One of our trucks instrument panels had a volt meter verses an amp meter. Seems to be the norm these days as they are simple to read and do indicate the volts in the system along with voltage being put out by the charging system. But, there is a draw back, it doesn't indicate amps. Why is that important? This truck had a 200 amp alternator in it that was putting out 24 volts according to the volt meter, but for some reason the batteries were losing their charge. So much so that a driver just made it back to the yard after dark before losing all his lights. We know the alternator was good, as it was putting out 24 volts. We changed batteries, checked all the wiring and the lights for shorts, to no prevail. Finally our old mechanic said lets send out the alternator and have it tested. Well when we got the report from the alt. shop, they said the alternator was putting out 24 volts but only 3 amps. It takes amps to charge a battery. You probably see where I'm going with this. Had the truck had an amp meter, it would have indicated right away that the alt. was bad and saved us a lot of trouble and down time. Down time is money lost. Needless to say, ammeters were put in any vehicle that didn't have one. After that, New vehicles were ordered with them already installed.

 

[piman]

Hello Paul,

I have both on different cars. However, you say the truck was putting out 24 volts, that is insufficient to keep the battery charged. I don't know what your alternators are regulated to but my 12 volt car system runs at about 14 volts, without a higher voltage the battery won't charge.

Alec

 

[dklawson]

I see your point regarding the ammeter but there needs to be a cautionary note here... and your example shows exactly why traditional ammeters scare me.

Your truck alternator was rated for 200 Amps. Traditional ammeters pass all the current to/from the battery (excluding the power sent to the starter motor). Therefore, to be safe with a traditional ammeter you'd need a gauge that was rated for +/- 200 Amps minimum. In turn, you'd need cables running through the firewall that could handle this amount of current. Failure of a grommet or some other unforeseen accident could produce some rather serious damage.

Again... that was for traditional ammeters. There are shunt-type ammeters that would have worked in your truck example and been relatively safe. In a shunt-type ammeter, a large, low resistance resistor is placed in series with the current flow to/from the battery. As before, this doesn't include power to the starter motor. The shunt-type gauge has wires that connect to each end of the shunt and measure the voltage drop across it caused by the current. The gauge is really nothing more than a voltage gauge with a custom calibration/scale. The initial investment in a shunt-type gauge is higher but the gauge can be fused and very little current passes through the firewall.

Personally, I have neither gauge in my LBCs. When I have doubts about the charging system I pull out the test equipment. And I agree with Piman that the voltage out of an alternator needs to be slightly above the battery voltage if it is to charge the cells. I don't know what's correct for a 24V system but I agree that 14V (nominal) would be typical for our LBCs.

 

[Billm]

The resistor is placed in parallel with the gauge, not series.
Bill

 

[dklawson]

Sorry....

I did <u>not</u> say the resistor/shunt was placed in series with the gauge, I said the shunt was placed in series with the current to/from the battery and that the shunt-type ammeter was connected across the shunt. (Connected to each end of the shunt).

 

[Billm]

Yep!- sorry Doug, I misread it.
Bill

 

[Ed Kaler]

As I am not the Moderator Bill, this "topic", or in other words, your VERY good post should have been couple of threads below this one in <span style="color: #990000"><span style='font-size: 14pt'>"Amp Guage Questions"</span></span>

 

[PAUL161]

I see your point regarding the ammeter but there needs to be a cautionary note here... and your example shows exactly why traditional ammeters scare me.

Your truck alternator was rated for 200 Amps. Traditional ammeters pass all the current to/from the battery (excluding the power sent to the starter motor). Therefore, to be safe with a traditional ammeter you'd need a gauge that was rated for +/- 200 Amps minimum. In turn, you'd need cables running through the firewall that could handle this amount of current. Failure of a grommet or some other unforeseen accident could produce some rather serious damage.

Again... that was for traditional ammeters. There are shunt-type ammeters that would have worked in your truck example and been relatively safe. In a shunt-type ammeter, a large, low resistance resistor is placed in series with the current flow to/from the battery. As before, this doesn't include power to the starter motor. The shunt-type gauge has wires that connect to each end of the shunt and measure the voltage drop across it caused by the current. The gauge is really nothing more than a voltage gauge with a custom calibration/scale. The initial investment in a shunt-type gauge is higher but the gauge can be fused and very little current passes through the firewall.

Personally, I have neither gauge in my LBCs. When I have doubts about the charging system I pull out the test equipment. And I agree with Piman that the voltage out of an alternator needs to be slightly above the battery voltage if it is to charge the cells. I don't know what's correct for a 24V system but I agree that 14V (nominal) would be typical for our LBCs.

Doug, and to others, when I said 24 volts, I used that expression as a general statement, being a 24 volt system and yes the alternator does put out approx 28 volts. As far as how the ammeter was hooked up to the trucks that had none, that was the responsibility of the head mechanic. Whatever he did, he did it right, as everything works fine. As far as the new trucks, amp meters were installed at the factory and I haven't a clue how they were installed either. But, their there and they work.

 

[SwamperCa]

I just bought a Volt meter gauge VDO from summit racing for our 74.5 B. Almost everyone said not to put in an amp meter.

 

[Steve_S]

Almost everyone said not to put in an amp meter.

Who is everyone? Amp gauges are not dangerous, bad wiring is. There are thousands of T-Series and earlier MGs with 60+ year old amp gauges in them running around on today's roads. They haven't burned up yet and I don't expect they will. Countless other cars throughout history use them as well. Race cars, agricultural equipment, even non-automotive systems use them.

It's a simple device which cannot really fail in a way I can think of which would cause damage to the car. Heck, if a volt meter, which is considered by many to be safer, shorted out then it could potentially cause damage. An amp meter however cannot short because it's an inline device! There is no connection to ground. If it fails, it will either stop working or cut power to the systems that flow through it.

The perceived danger may come from the fact that so much amperage goes through the gauge. The only danger here is that if the wiring is allowed to get worn through and short to ground, it would be a magnificent show of sparks and smoke similar to touching the live wire on the starter to ground.

 

[SwamperCa]

That was the reasoning given for not putting one in. I guess it wouldn't be on a fused curcuit.

 

[dklawson]

Paul, thanks for the follow-up. Perhaps the mechanic installed a shunt-type ammeter in the truck you were referring to. I won't pretend to know what is installed by the factory in modern commercial vehicles with ammeters.

Steve's comments put a positive tone on my post above. The ammeter itself isn't dangerous, as both of us said (Steve said it more directly), the wiring is what can be dangerous.

Steve is also correct that a lot of cars ran ammeters for decades without issue. I don't take exception to that comment either. However, the cars mentioned from years gone by used generators/dynamos as I mentioned in my first post, most of them 22Amps or less. Once cars started putting in more powerful alternators the potential for damage if the wiring failed was more severe.

A volt meter's leads can be fused. They can use VERY tiny fuses rated under 200mA. Even without a fuse, a voltage gauge (or its wiring) that shorts to ground will result in a burned out voltage gauge wire, not a failure of the charging system.

You can indeed fuse an ammeter, however, the fuse has to be rated higher than the alternator and you need a buffer zone above that to handle inrush currents that happen when you switch on heavy loads. Perhaps you could write Dan Masters and get his recommendation for ammeter fuse ratings.

Install an ammeter if you wish but exercise caution as you select and run the wiring. Take steps to prevent any possibility of the ammeter wiring shorting to ground, or becoming chafed or cut by anything that is in contact with the chassis.

I encourage you to read the tech article at:
https://www.egauges.com/ATM_Tips.asp?TipPage=voltmeterammeter.htm
egauges.com is one of the resources I use when I purchase new gauges. In addition to the tech article they have a lot of inventory to select from including shunt-type ammeters.

 

[Sarastro]

I have both an ammeter and voltmeter, and I find the ammeter far more useful. What you want to know is whether the battery is charging or discharging, and how much. The ammeter tells you this directly. If something like the generator/alternator or regulator fails, it becomes obvious immediately from the charge current. The voltage is a much weaker and slower indicator of this.

The safety issue is a cogent one. Routing around a lot of unfused conductors carrying full battery output leaves me uncomfortable, to say the least. On the other hand, the battery output on my bugeye goes straight through the firewall to the wiring harness under the dash, comes back through the firewall to the fuse block, and somewhere splits off to the lights and such. All unfused. Eeek!

Another concern is simply the voltage drop and power dissipation in the shunt. The inexpensive meters used for these purposes will require a few tenths of a volt across the shunt, wherever it is located, and this comes right off the system voltage. At, say, 20 amps and 0.5V drop, that puppy is dissipating 10 watts, which can get pretty hot under some conditions.

I got around this in my ammeter by sensing the magnetic field generated by the wiring. Some electrical meters (e.g., clamp-on ammeters) work this way, so I'm surprised that automotive ammeters don't work this way. The description of my ammeter is here.

 

[dklawson]

Steve, that's very impressive! Very impressive indeed!

 

[Sarastro]

Thanks for the comment. This little bugeye has been a lot of fun, if only as an excuse to think up projects like this.

I enjoy reading your perceptive comments, too.

 

[Steve_S]

The bottom line is that any wiring failure can result in damage to other systems. Ammeters are connected inline with systems that use full power from the battery, therefore a failure or short in this line will be worse than just about any other. It would be the same as the live wire to the starter going bad.

Just like you ensure the starter wire is isolated and kept in good order, you should do the same for the line to and from the ammeter. I look at this as a moot issue since all wiring should be kept safe from chaffing, regardless of amperage or voltage.

As for the gauge itself, the ammeter in my MG TC is immensely useful. The voltage gauge in my BGT is far less so, but still nice to have.