Piper Ammeter

[Code90]

Exposing my ignorance, but on a left zero ammeter on a piper lance, what exactly am I reading from the gauge?

My understanding was that it showed the amp drain on the system. The more lights and electronics I turn on would give a higher number. How does that give me any information about the alternator? Am I understanding it incorrectly? Is it really giving me the charging state of the battery AFTER subtracting the pull of the electronic devices I am using?

 

[Norman]

Left zero is a load meter. It shows how much the alternator is having to put out to meet system needs. It doesn't tell you a whether the battery is charging or not.

 

[Skip Miller]

Left zero is a load meter. It shows how much the alternator is having to put out to meet system needs. It doesn't tell you a whether the battery is charging or not.

Correct. If you have a voltmeter (by itself or as part of some avionics) you can determine if it is charging or not. This may not be an instant determination... a slow charging system will take some time to raise a drained battery's voltage to normal.

-Skip

 

[Graueradler]

A voltmeter showing system voltage should read close to 14V (normal alternator output voltage as determined by the voltage regulator) with the engine running and 12V (battery voltage) with the engine not running. The ammeter, as stated above, shows alternator output current but doesn't tell you where it is going. If the battery is good, 14V will be charging it.

Of course, if you have a 24V system, the voltages double.

 

[RussR]

How does that give me any information about the alternator?

It's not telling you anything useful about whether or not the battery is charging. What it IS telling you is how much current is flowing OUT of the alternator, which by extension tells you if it's working. If you ever see the load go all the way to zero (and doesn't come up some if you turn more things on), it means your alternator is out. You can prove this to yourself by turning the alternator switch off briefly in flight - you will see the loadmeter drop to zero.

 

[BigBadLou]

However, the Piper-style amp-meter is very useful in checking (somehow supposedly accurately) operation of devices that you cannot normally see. While taxiing and checking all IFR equipment, I also flip on the ldg light, look for increase in alt load, then do the same for pitot heat.

Sometimes I wish I had a bus voltmeter. Though the GPS gives me a "Low bus voltage" msg so that is handy to notice an alt failure in flight early.

 

[Clark1961]

However, the Piper-style amp-meter is very useful in checking (somehow supposedly accurately) operation of devices that you cannot normally see. While taxiing and checking all IFR equipment, I also flip on the ldg light, look for increase in alt load, then do the same for pitot heat.

Sometimes I wish I had a bus voltmeter. Though the GPS gives me a "Low bus voltage" msg so that is handy to notice an alt failure in flight early.

Got a cigar lighter/power port in the panel? get a digital voltmeter from your local autoparts store that plugs into the port...

 

[Caramon13]

On my Piper the ammeter always reads right around 20, slightly higher sometimes. I do have a voltmeter and for the longest time I thought I had an alternator problem or something else as it would always read 12.4 V.

Then, I started the engine, shut down all the electrical equipment except the voltmeter and it was upwards of 13.3 V (ish).

It also depends on where they connected the voltmeter as to what you are actually seeing on it. You will never and should never see a 14V number on your voltmeter with everything running, shouldn't even be close, but it should be above 12V which is what the battery is responsible for.

If you do see close to 14 V I think you're probably overcharging it or need to look at your voltage regulator.

 

[Graueradler]

If you do see close to 14 V I think you're probably overcharging it or need to look at your voltage regulator.

My observations on the 71 Cherokee 140 that I had for years do not match yours. I saw 14V as long as the engine was running and never had a problem with battery life.

The VR is a pretty high gain device and cranks up the field current to get the output voltage it is set to and can do it as long as the load is within the capacity of the alternator. I had a digital voltmeter plugged into the "cigarette lighter".

 

[Jim_R]

In my Cherokee with the same ammeter as the OP, I check for battery charging immediately after engine start by observing the needle gradually drop down a bit and then stabilize (as the slightly-depleted battery regains full charge). After that, there's no way to know positively that the battery is charging, but the laws of physics make it pretty likely.

If the ammeter suddenly drops to 0, then you know your alternator has gone offline. If you have a bus voltage monitor elsewhere in the plane, then you'd also see a voltage drop from alternator voltage (~14.0V) to battery voltage (~12-13V at full capacity, and dropping as the battery depletes).

 

[Caramon13]

My observations on the 71 Cherokee 140 that I had for years do not match yours. I saw 14V as long as the engine was running and never had a problem with battery life.

The VR is a pretty high gain device and cranks up the field current to get the output voltage it is set to and can do it as long as the load is within the capacity of the alternator. I had a digital voltmeter plugged into the "cigarette lighter".

Not a single plane I've flown shows 14V (or 28V where applicable) with everything on, none. Maybe if all you had on was the clock then yeah I'll buy a reading of 13.5 at the most because the clock that measures the volts is going to be taking some of that current.

If you're seeing 14V on your voltmeter all the time, that is most certainly NOT a good thing.

 

[weirdjim]

Not a single plane I've flown shows 14V (or 28V where applicable) with everything on, none. Maybe if all you had on was the clock then yeah I'll buy a reading of 13.5 at the most because the clock that measures the volts is going to be taking some of that current.

If you're seeing 14V on your voltmeter all the time, that is most certainly NOT a good thing.

Just a few things ...

• I guess I've seen my share of airplanes, some from the left front seat and some getting wrenches all greasy.
• Normal voltage in normal flight (without landing lights or pitot heat) is somewhere between 13.6 and 14.0 with 13.8 being the most common setting. 13.6 is a little light for full charge and 14.0 is a little high and may boil out some electrolyte.
• Perhaps you might want to tell us how a clock measures volts? And how the milliampere or two drawn by a voltmeter (NOT a clock) is going to affect battery/alternator currents measured in dozens of amps?

Jim

 

[Jim_R]

Not a single plane I've flown shows 14V (or 28V where applicable) with everything on, none. Maybe if all you had on was the clock then yeah I'll buy a reading of 13.5 at the most because the clock that measures the volts is going to be taking some of that current.

If you're seeing 14V on your voltmeter all the time, that is most certainly NOT a good thing.

Speak for yourself. At cruise RPMs, the alternator on my Cherokee is able to maintain voltage (as sensed by the engine monitor) between 13.9 and 14.0 regardless of load. Has done so for years now.

 

[Caramon13]

Just a few things ...

• I guess I've seen my share of airplanes, some from the left front seat and some getting wrenches all greasy.
• Normal voltage in normal flight (without landing lights or pitot heat) is somewhere between 13.6 and 14.0 with 13.8 being the most common setting. 13.6 is a little light for full charge and 14.0 is a little high and may boil out some electrolyte.
• Perhaps you might want to tell us how a clock measures volts? And how the milliampere or two drawn by a voltmeter (NOT a clock) is going to affect battery/alternator currents measured in dozens of amps?

Jim

Jim, the clock is an M803 Davtron. The "clock" feature is just one part of it, but since that's its primary function I call it that. It also does OAT and measures volts.

@Jim_R: Interesting. I've already had the system checked, but might be worth doing again. Annual is coming up in a couple months.