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By marioair
#1556625
Our aircraft POH states that the "ammeter" is connected between the alternator and battery through a diode. The gauge reads 0 to +ve. I read this therefore as it is an alternator -----> battery load meter.

I need to dig out the wiring diagram but my assumption was that with engine running the alternator provides power to bus bar but NOT via the battery. Hence the only load that the meter should show is that's used to trickle charge the battery. However the POH states that with all electrics on the meter should read about 20A.

Two problems

1) it doesn't ever show 20A. Highest I've seen is aftee running battery alone on ground for long period or after multiple attempts to turn over the engine. Eg when battery is discharged. It soon settles back down to about 2amps once alternator is on. This leads me to believe the alternator does indeed power electrics through a route other than the load meter. Which contradicts the statement in the POH about seeing X amps with all electrics on
2) if the above assertions holds, what's the point of this gauge! If it only shows charge into battery then if the alternator fails in flight you'll see low volt warning so you know your running on battery and I wouldn't hang around to see how much life battery has in it. If battery fails on ground then engine won't start! So it's kind of a redundant gauge?
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By GolfHotel
#1556635
I'd expect it to show all the output from the alternator. So you can tell it's working. That's what mine does. Maybe some one has been rearranging things.
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By rikur_
FLYER Club Member  FLYER Club Member
#1556655
In my experience I've come across two main configurations of gauge:
1) If the minimum on the scale is zero, it measures the current being delivered by the alternator
2) If the scale has zero in the centre and read positive/negative, it measures the current be delivered to/by the battery

Does make me wonder if yours is wired for the latter, but has a gauge fitted designed for the former
By cockney steve
FLYER Club Member  FLYER Club Member
#1556676
Given your description, it's as the last 2 posters said . the alty feeds through the ammeter, as soon as the alty output exceeds battery -volts, it will start charging the battery, as well as servicing any electrical load. The diode is simply a "one-way valve" and prevents the battery from pushing current back through a low or non-producing alty.(sounds a bit odd, unless your alty also has an external regulator/rectifier set-up, as integral units have an inbuilt diode -pack thus, only the warning light needs switching. Automotive practice leavesthe main output permanently connected to the battery.

A load of 20 amps at 12 volts, is about 240 watts by my reckoning. Does sound like there's something wrong, somewhere. Unless you have Mag ignition, no Nav lights or electric instruments I'd take a wild guess that a radio on receive draws about 2 amps, so 24 watts @12V. After hitting a battery with a start, the alty should pump in a fairly high current for a few minutes....the battery voltage then rises to the regulated alty output, at which time the charge current reduces to a trickle.
This trickle will not normally fully recharge a partially discharged battery. a n external, plug-in mains charger will. It should be used to prolong battery life, especially where the battery is not used at least every week, at a minimum. otherwise, charge monthly.
Plenty of battery-care info on the net.
If you enjoy Armstrong starting, disregard all the above :lol:
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By marioair
#1556682
Yeah that's the issue. If the battery is discharged eg leaving it on, in the ground for a period or lots of attempts at starting, the meter shows a big load for a few minutes then dies down as expected. However in steady state having all electrics on or off makes no difference to the reading - it'll be between 0.5 - 2A.
It feels like there's one wire connecting alt to bat with a meter inline and the another from alt to all the electrics!
Have there been any AD/SBs from piper?

I could of course ask our engineer but I'm
Being lazy!
By cockney steve
FLYER Club Member  FLYER Club Member
#1556733
sounds, from that, you have, in essence, a split load. As you observe, the "output" side of the Alt/meter/ diode string, goes to the battery, however, it sounds like the aircraft electrics, (the load) are tapped the "wrong" end of the string, IE between Alternator and meter.
I would, if that's the case, expect the diode to be between the alternator output and the load....both items would be on the "input " side of the meter. I would also expect the meter to "peg" below the "0" on the meter, if the master and hence the load, is switched on prior to engine-start. In these conditions, load would feed from battery, backwards through meter and then through load and then return. It wouldn't flow back through the alty, because of the diode. When the engine's running, alty output appears to pass through the diode, forks off to load, and also passes through the meter to the battery. electric starter goes direct to battery (via solenoid or mechanically- operated switch) initial draw is well over 100 amps, normally, so you wouldn't want to meter that!
All is pure conjecture, of course, based on past experience.