I can't seem to understand how the electrical system works:

What does the ammeter display?

What is said on my POH is that the ammeter displays the alternator output. My question is what output is that? In amps?

Does the battery power the DC bus?

It seems the battery takes AC from the alternator, convert it to direct current, and then feed the entire electrical bus system while still being charged. Is that correct?


2 Answers 2


I think you misunderstand how the alternator, battery, and electrical system are connected. An alternator puts out DC voltage into the electrical bus. It is used to provide electrical power to aircraft systems, as well as to recharge the battery from cranking and pre-start light and radio usage.

You can see this in the wiring diagram in the POH: Archer III Electrical System

You can see the alternator about 2/3 of the way up the diagram, just to the left of center. The 'B' lead coming out of it goes directly into the ammeter and to the DC power bus (the big bar along the top).

The path back to the battery is a little more complicated, but there where it says "Battery Feeder", you can see it goes through a 70A fuse, down to where it connects to the battery contactor. So when the alternator switch is on and the battery switch is on (closing the battery contactor), the alternator can charge the battery. [In small Cessnas this is a split master switch, but I'm not sure what it looks like in an Archer.]

To answer your question specifically about what the ammeter shows, this will give you the amount of current the alternator is providing in amps. After startup you should see slightly more than usual as the battery recharges. You will probably see the number go up while you are transmitting on the radio. If you see this number get really large for no reason, there may be a fault in the electrical system somewhere. If you see this number go to ZERO, it's possible that the alternator has stopped working completely and your lights and radios are now running on battery for a limited amount of time!

I hope that is helpful. Note that nothing here should suggest a specific course of action -- this is just general electrical system information. Consult the emergency procedures or an experienced flight instructor for specific information about your airplane.

  • 1
    $\begingroup$ Technically the alternator does put out an AC voltage, it just uses a rectifier and regulator to convert it over to DC. All of this is internal to the alternator though. $\endgroup$
    – Ron Beyer
    Commented Feb 25, 2017 at 2:47
  • $\begingroup$ So, uh, it puts out DC voltage then. $\endgroup$
    – Daniel
    Commented Feb 25, 2017 at 2:49
  • $\begingroup$ When you get through the rectifier and regulator, yes, I know it sounds a little strange but it is important to know how the alternator works internally, the "alternator" part of it is exactly that, it produces an alternating current. $\endgroup$
    – Ron Beyer
    Commented Feb 25, 2017 at 2:51
  • $\begingroup$ Yes, all of us who design electrical systems know that, but I feel like this is extraneous and confusing in this context. On this schematic, the "alternator" puts out DC voltage. $\endgroup$
    – Daniel
    Commented Feb 25, 2017 at 3:03
  • 1
    $\begingroup$ Oh I see, so the alternator gives power to the electrical bus system as well as charging the battery correct? If I turned off the battery master switch off, the electrical system would still be working right? (If the engine is running, of course). $\endgroup$
    – nyorkr23
    Commented Feb 25, 2017 at 3:04

Daniel gave you an excellent, detailed description of how the Piper ammeter (load meter) works.
Now I'm going to dumb it down to "What the heck does this mean in the cockpit?" and make all the other EEs on the site cringe.

Let's start with the assumption that the ammeters in small aircraft are going to read current, in amperes DC. (All electrical explanations aside the alternator is, for our purposes, a DC power source: We're only ever going to see its output after rectification).

In Piper aircraft the ammeter functions as a load meter: It shows how much current (in amps) the aircraft is demanding from the alternator for all purposes: running instruments/avionics, lights, and charging the battery.
Piper Ammeter - Load Meter Arrangement

Practically that means that the ammeter shows the total load the alternator is supplying (electrical accessories and battery charging) at any given time.

  • It will read zero any time the alternator is not providing power - running off the battery on the ground or with the alternator switch off.
  • It will initially show a high current load when the alternator is brought on-line, and this will reduce once the battery charge has been topped off from starting.
  • It should always show a load comparable to the running electronic equipment. For example when you turn the pitot heat on the load should go up by the amount of current that draws (around 10A) and stay there until you turn it off.

The only practical difference in the Warrior III and Archer III from previous generations of Pipers is that the meter is digital now, so instead of a needle with zero on the far left and the maximum alternator output on the far right you have a nice LCD display.

By contrast Cessna ammeters are arranged as a charge/discharge meter: They show how much current is flowing To or From the battery, and have their zero point in the center. Positive numbers represent charging the battery, and negative numbers represent discharging it.
Cessna Ammeter - Charge/Discharge arrangement

Practically that means once the battery has been recharged after starting the plane the Cessna ammeter will show zero, or close to it, under normal circumstances.

  • It will show a discharge (negative numbers) when the alternator is not providing power - running off battery on the ground or with the alternator switch off.
  • It will initially show a charge (positive numbers) when the alternator is brought on-line and will drop to zero once the battery is topped off from starting.
  • It will show zero (no charge or discharge) when everything is operating normally in flight.
  • It will "twitch" when you add or remove load from the bus as the battery initially supplies the current demand until the alternator/regulator compensates for the new load. Using the same pitot heat example as for the Piper the Cessna load meter will briefly show a discharge when you turn pitot heat on, and may briefly show a charge when you turn it off, but will return to zero once the bus is stable with the new load.

Both systems have advantages and disadvantages, but most importantly the troubleshooting procedures are very different (in a Piper zero is bad and means your alternator is offline, in a Cessna zero is good and means everything is working) so it's important to know which system your aircraft has and how to interpret what it's telling you.

  • $\begingroup$ Well, can't the answer be as simple as: "The ammeter shows whether the battery is charging or being discharged." (it shows current (Ampere)). If the engine and alternator provide more power (current) than the equipment uses, the displayed value will be positive, and negative otherwise. $\endgroup$
    – U. Windl
    Commented Aug 13, 2022 at 0:41

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