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Amp Gauges vs Volt Meters...Separating Fact From Fiction


If you own an antique vehicle built before 1965 chances are it has an amp meter in the dash. One thing you need to understand is what an amp gauge is actually telling you. An amp gauge measures the actual amount of electrical current flowing into or out of the battery. The amp gauge needle will move to the (+) when the generator output is recharging the battery. The amp gauge needle will move to the (-) when current is flowing out of the battery, such as when the generator output cannot keep up with the electrical load. 

An amp meter does not (as many car owners assume) tell you if the generator charging system is working properly. While indirectly you may "assume" if current is flowing into the battery that the generator is charging and producing an output. Just keep in mind that an amp meter does not directly measure electrical output from the generator.

In other words an amp meter measures the volume of current being used from the battery, much like a water meter measures the amount of water passing a given point. Amp meters will also show you the amount of current coming into the battery to recharge the battery. Amp meters were standard on all types of vehicles beginning in the early 1900's up thru the early 1970's.  

Before alternators were invented generators provided the electrical current and most of them did not have an output at idle and low rpms. As a result current flowed into and out of the battery on a regular basis and the amp meter simply measured the amount of current and which direction is was going. 

As a vehicle owner back in the day you knew that if the amp meter showed a discharge at highway speeds or even above fast idle, that the charging system needed attention because current was flowing out of the battery when the generator should be replacing it.

To give you an idea how this worked in the old needed to drive you antique vehicle ten miles at highway speeds in order for the generator to have time enough to replace the current used from the battery for one start of the engine. That explains why you always see the amp gauge two thirds the way over to charge most all of the time but the battery never seems to get fully charged. The generator, especially with a lot of town driving or short trips couldn't keep the battery fully charged. It was common even up thru the 1950's to connect a car battery to a battery charger overnight at least once a week, especially if the vehicle was only driven a short distance too and from work.

Volt meters were introduced in the mid 1960's soon after alternators became common. A volt meter simply measures the voltage or electrical pressure behind the current. It does not measure the volume of current (amps) being used. As charging systems became more reliable and were able to provide a constant output at idle and low rpms, the constant flow of current into and out of the battery was all but eliminated. That is why batteries last much longer today than they did in the generator days.

For the first time a battery, became a storage battery. It stored the current used for starting the engine, then the alternator took over and provided the current to run the electrical accessories and could replace the current in the battery in a matter of minutes. As a result in was no longer necessary to measure the volume of current flowing into or out of the battery. Only a small amount of current was used from the battery for starting which was quickly replaced by the charging system.

So a volt meter simply measures the pressure or "voltage" present in the electrical system. The assumption is that if the voltage is between 13.8 and 14.2 (a 12 volt electrical system) that the battery is fully charged and the electrical system is working properly. Alternators are built using solid state components which means less mechanical parts to wear and a more accurate charging system with better and more accurate control of the voltage output. So a volt meter makes sense for modern applications.

But for the antique vehicles we drive and build, I still use and recommend using an amp gauge especially with a generator charging system. I want to know how much current is being used. With mechanical gauges and non solid state electrical accessories, the electrical load in an antique vehicle can vary greatly.

So... if your antique vehicle came with an amp gauge, leave it in there. It will work fine even with an alternator. Because an alternator charges at idle and low engine rpms you will see about 10 amps charging when you first start your vehicle then the needle will fall back to about a needle's width above zero in less than 30 seconds (most applications). That tells you the alternator has already recharged the battery. An amp gauge will get less of a workout with an alternator, than it did with the original generator charging system.

When you increase the electrical load such as turning on the headlights or heater blower motor, the alternator will automatically increase the output to cover the increased electrical load. The additional electrical current needed to run the headlights will come from the alternator, not the physical battery. As a result, there will be no change in the amount of current going into, or coming out of the battery, so the needle on the amp gauge in the dash will stay slightly above zero just as it was before the headlights were turned on.

Is An Amp Gauge Safe...?

Absolutely they are safe. The were the standard measure of automotive electrical current in all types of vehicles for the first 60 plus years the automobile existed! While all of the output of the generator or alternator passes thru the amp gauge on the way to recharge the battery or to deliver battery current to the electrical system, the same amount of current is also traveling around thru the rest of the wiring harness. Bare wires and missing insulation are dangerous in any wiring harness.

Are They Safe To Use With An Alternator...?

Of course. Most original antique vehicle electrical systems used in the neighborhood of 40 amps or less, even if you turned on everything at the same time.  You can add an electric fuel pump or electric radiator cooling fan and you will seldom exceed that 40 amps, because you will not have everything turned on at the same time. Even if you do... the alternator will pickup the increased electrical load so the amount of electrical current passing thru the amp gauge will not change.

Common Sense Applies...

It stands to reason that if your plan is to add a thousand watt stereo and a 150 amp alternator to run it, in your 40 Ford Coupe, that the stock amp gauge will not be large enough, especially if the battery can't keep up while you are cruising around rattling all of the windows in the neighborhood. If this is your plan you will have to upgrade the stock wiring harness, switches and a few other things as well, to accept the increased electrical load.  Just don't drive thru my neighborhood when you are done!

A 65 amp alternator is more than big enough for the average antique vehicle application. You will have plenty of current to run all of the original electrical accessories with enough reserve to power things like electric fuel pumps and electric cooling fans. (you will also not have to upgrade the original wiring harness and switches if they are in good condition) At 65 amps you have nearly twice what the stock generator put out on a good day and the alternator has the advantage of an electrical output at idle and low engine rpms, something the generator could not deliver.

Don't get suckered in by the magazine mechanics who tell you you need a 100 amp alternator for your antique vehicle. You don't have near enough accessories nor could you add enough accessories to require a 100 amp alternator. Besides a 100 amp alternator will require a significant upgrade to the wiring harness (20 percent larger diameter wire for example) to handle the extra current. You will also have to upgrade all of the switches and related hardware for the increased current load. You would be the accident looking for a place to happen.

Your goal should be to make your charging system as SIMPLE and as RELIABLE as possible. You should work on your antique vehicle only when you want to... not because you have too. Go drive it and enjoy it...that is the reason you have it in the first place.

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Since 1987, Fifth Avenue owner, Randy Rundle, has been making antique, classic and special interest vehicles more reliable and fun to drive.