Why do we still need ammeters?

... cars did away with them years ago ...

I'm working in car electronics sector and I know that your assumption is wrong:

Modern (combustion motor) cars use a so called "battery management device", a computer which measures the voltage and the current of the battery; in some cases it even measures multiple voltages inside the battery and multiple temperatures of the battery.

The truth is that the display has been removed from the cars because the current is monitored by the computer. If there are problems with your car battery, you will typically see some warning message in the car's dashboard.

Why is this not done in airplanes?

My suspicion:

The safety regulations for avionics are much stricter than for car electronics (*). Therefore, building electronics for airplanes is much more difficult and more expensive than electronics for airplanes.

So if it can be avoided to use some electronics in a sports airplane, it is avoided.

... voltmeters seem mostly enough.

Especially if the battery is old (and bad), a voltmeter does not give you any information:

A voltage of 12.5 volts may indicate a full battery being discharged or an empty battery being charged.

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(*) Just one example: In passenger airoplanes the infotainment devices at the passenger seats are already classified as "safety critical" (DO-178B level "D").

Voltmeters tell you when your battery is flat.

Ammeters tell you when your battery will soon be flat.

Your battery is being charged by an alternator, only an ammeter will tell you if the alternator is working. If you don't have an ammeter then you might wait several hours before realising you've got a problem

Yes a voltmeter indicates state of charge with some provisos. It shows issues like insufficient charge and excessive load by implication. Simply put, an analogue meter needs offset and expanded scale to show much detail, and responds to charge anomalies more slowly. It needs to be accurate. An ammeter in the battery line indicates charging or discharging more assertively, and shows the important states clearly. Not charging, discharging or well charged (assuming the charge voltage is correct for the battery). With the engine/charger off it could show load current.

An explanation of the electrical side is to consider there are three connection points, the charger, the battery and the load. These could all be wired to meet at a common point on the active side, so they are all in parallel. Additional chargers like solar panels and portable generators, and batteries would each connect separately to that common point. In some cases blocking diodes may be needed. A single ammeter can be connected in series with the battery, the charger or the load, indicating:

1. Battery charge or discharge, with the direction of current in or out of the battery. The current shown does not indicate the charger or the load current directly, but the battery current. This is the traditional usage, because it indicates a charger or a load issue. It does not need much accuracy to be useful, so long as it reads zero properly.
2. Charger current, which shows the sum of the battery and the load current. It shows if the charger current is sufficient. Note the charger is ideally a current source with a voltage limited to the maximum allowed battery voltage. This means nominally constant current once the battery voltage is exceeded by some small margin, until the maximum voltage is reached, so the need for this meter is not great. In fact the voltmeter indicates what a charger is doing. The battery holds the charger down to the battery voltage.
3. Load current, which is just the load itself, part of which could be from the charger or the battery. It shows the load current is approaching limits. Normally the load would be within the charger limits for a vehicle, aircraft or boat, with the battery supply brief surges in current that exceed the charger capability.

The starter is usually wired separately so the current does not pass through the ammeter. The other side of the supply may be ground return (using a metal frame as in cars) or use go and return wires, with a single ground point. All the load, charger, battery, starter and connections would be insulated from ground. The regulations or codes of practice for particular systems will spell out how to do it, reducing the need to be a design expert.

It is also possible to use current shunts, and a switched ammeter (really a voltmeter calibrated in amps). This can help reduce the amount of heavy current wiring to solve layout problems. This would give a more complete diagnosis of where a problem lies, along with a system voltmeter. No one would bother with a car, but for a boat? The battery current ammeter shows current in charge or discharge direction, and could be simplified to a few lights, charge current, discharge current, very little current. In the past the ammeter did not need to be accurate, and acted more like three lights. They were cheap and rugged. A useful analogue voltmeter was expensive and delicate. An accurate, expanded and offset scale type would be unusual at least before the days of zener diodes. Though I do not doubt it was possible with an appropriate magnetic circuit it would have been unattractive in a car, boat or aircraft. Nowadays a digital voltmeter overcomes this easily. Personally, I would prefer the center zero charge ammeter or electronic equivalent, with decent digital battery voltage meter. Visible in all lighting conditions of course. There was little choice in the past, a battery ammeter with center zero was the choice. Nowadays a voltmeter can be used. Both methods need interpretation, and probably it depends on who is using it.

The reason ammeters were dropped in cars is they were (and are as used in airplanes) wired to the battery and only show the current flow into or out of the battery. The battery's nominal voltage is 12, but the system operating voltage is about 14, and the voltage regulator varies the alternator field to keep roughly 14 volts in the system so that the battery is always getting some small amount of charging current. The current flowing from the alternator will be whatever is needed to sustain 14 volts under the demand of electrical services plus charging, or keeping the battery charged.

With the alternator running and a heavy electrical load on the main bus, the alternator's output may be, say 10 amps, with most of that going to the lights and heaters and radios etc, with a small surplus going to the battery if it's fully charged, whatever it needs to stay charged. The ammeter is showing what is going to the battery and it should normally be 0 amps or a slight positive surplus once the battery is recharged following a start (because the 12 volt battery is connected to wires with 14 volts in them, so there will be some trickle flow through the battery even when fully charged).

In cars it was decided that a voltmeter was sufficient because if the alternator is running it will show roughly 14 v in the system, and if the alternator is failed it will show somewhere around 12, at some point declining further from there until it's down below 10 volts or so and everything stops working. So the voltmeter is sufficient to tell you the alternator is running and sustaining system voltage, or not.

Having the ammeter there as well provides a more complete picture because it's showing current to and from the battery as well as system voltage. For example, when you start, the engine is running with the alternator is putting out current to charge the battery that was depleted somewhat by the start. The voltmeter will just show 14 volts, but the ammeter will show the heavy charging current going to the battery, which can be good information to have if you want to be aware of the battery condition. You will normally see the temporary heavy charging current for perhaps a minute or two until the battery is brought up to charge. Charging current the goes much longer than this could be a sign of problems in the battery, like high internal resistance.

Similarly, when you turn on heavy load services with the alternator running, the ammeter reveals the temporary current drop at the battery resulting from the voltage drop from the service taking "juice" off the bus, and there is a slight delay as the voltage regulator reacts and raises the alternator field to put out more current. The ammeter will show a short discharge spike, then recover. That's the voltage regulator in action, and it can be good information to know. The voltmeter will show a voltage drop/recovery as well but it's more subtle.

Bottom line is, once you learn to interpret them, having a voltmeter and ammeter together provides a more complete picture of what your electrical system is doing and its overall health. And pilots are expected to be more concerned with that sort of thing than drivers, so there you go.

With cars, where you drive it until it won't, not such a big deal unless you're a hardcore enthusiast, so why not save a couple bucks by deleting one of the meters. And overall, if you have to give up one of those meters, you will generally prefer to keep the voltmeter because it's information is more important of the two in the general scheme of things.

Some history from the automotive side, and a bit of information from the electrical engineering side.

To measure amps, you need to have the meter in series with the load. Earlier systems had the amp meter directly in line, so all the power of the vehicle ran through the ammeter. This could be dangerous if something went wrong, as you had 30+ amps running through the gauge. Later years, they used a shunt. A wire between the battery and a junction. The ammeter measured across the shunt and scaled the reading based on resistance. This is safer, as you can measure 60 amps, with less than 5 amps running through the gauge.

Measuring voltage is MUCH easier. Voltage is wired in parallel. Just wire one side to the key on power, and the other side to ground. On a car where the gauges are controlled by the ECU, the ECU is already getting a voltage signal from the battery that it can measure. As some have said, at this point, it is usually a light rather than a gauge.

The change from ammeters to voltmeters came around the time they switch from a generator and voltage regulator to an alternator with an internal regulator. I don't know if this is just a coincidence. The alternators tend to be more reliable, and provide more power at low RPM (idle).

So in terms of automotive, I think the main reason for the change is simplicity and cost.

The more unpleasant a failure of some system is, the more tools you get to prevent, diagnose and mitigate the failure.

An ordinary passenger car can stop in a middle of nowhere because the alternator failed and the battery depleted. Quite unpleasant, but rarely fatal - and modern cars tend to be reliable enough anyway.

An offroad rig with winches, extra lights, radios, compressors, etc... usually has not only an amperemeter, but a full-blown battery controller with voltmeter, amperemeter, ampere-hour metter (integrating amperemeter) and so on. If you are out of electricity, you are in a quite worse position than in a passenger car. And you have quite a deal of extra failure modes.

Same for marines. They adopted these controllers even before the offroad cars.

In aircraft, "no electricity" situation is even worse, but you have the extra limitation of not overloading the pilot with extra information that they rarely need. That's why volt + ampere is okay-ish.

Because it displays how much current, and thus how much power, your electrical system is consuming. This provides a quick reference for diagnosing problems. Negative values can indicate a malfunctioning alternator and large power draws can indicate a short circuit somewhere in the system.

Congrats on earning your wings!

Because ammeters measure the flow of current (positive or negative) they can immediately show when the alternator is no longer providing power and the battery is discharging instead. In a voltmeter, like most modern automobiles have, you would have to wait to see the charge level on the battery drop, and by that point, several minutes (or longer) may have passed. In an aircraft, where it may be important to quickly shutdown electrical devices and conserve battery power, every moment counts and the ability to see flow shift from positive to negative is very helpful.