Ammeter: Battery Lead vs. Alternator Lead

w1curtis · #1 09-24-2007, 12:23 PM

There are two common ways Ammeters are wired in light aircraft; Battery Lead mode and Alternator Lead mode. Those that are use to flying Cessna's are familiar with battery lead mode. Those that fly Pipers are usually familiar with the alternator lead mode.

Some folks designing their RV electrical systems are copying designs and may not be familiar with the pros and cons with each mode. Certainly I have my preference and that has been reinforced by an in flight alternator (single) failure at night.

I'd like to hear other opinions on the merits of each.

Battery lead: Ammeter (shunt) is wired in the path from battery to buss-excluding starter. Normal indications of the ammeter is how much current is being taken by the battery in charge, or used from the battery in discharge. When the alternator fails, the indication on the ammeter is how much power (amps) is being consumed from the battery by the attached devices.

Alternator lead: Ammeter (shunt) is wired in the path from Alternator to buss. Normal indication of the ammeter is how much current is being produced by the alternator. When the alternator fails, ammeter will read zero.

My preference is for the battery lead mode since it allow you precise power management in the event of an alternator failure. You can power down devices to a specific Amp load and then monitor the voltage as a measure of battery reserve. Also it provides a good indication of battery health. When I have not flown for a while, monitoring the Ammeter after start-up, my ammeter reading may be around 5 amps, but after some tim, this slowly reduces to around 1 amp which indicates the battery is about fully charged.

asav8tor · #2 09-24-2007, 12:55 PM

How about 2 shunts, a switch and both functions with one meter.

Piloto.Mendes · #3 09-24-2007, 02:56 PM

Quote:

Originally Posted by asav8tor

How about 2 shunts, a switch and both functions with one meter.

I like this ideia.

But I don't know what type os switch I should buy, as it needs to be wired to four terminals, swapping two at a time.

Any idea?

gmcjetpilot · #4 09-24-2007, 07:20 PM

I'll take the Con to the debate. My vote is for alternator lead, or LOAD meter. How hard is your alternator working? What electrical energy you are using in-flight? The only way to know is with a Alt lead Amp-meter. If there's an inop electrical item you know it when turning on or off has no affect on load. If an item is shorting or drawing too much current you know it. That could be a sign of failure. Also right after start before you turn on the electrical items you can see the battery charge and watch the amp-meter go down as the battery recovers after the start. Also one last thing to recommend the Alt Lead amp-meter is installation is easier, right off the B-lead. With the Bat lead amp-meter, the location of the shunt can be a pain. That pain is mitigated if you use a Hall-effect amp-meter, since no break in the wire is needed.

With the Bat Lead amp-meter, once the engine is running its just basically telling you how much charge your battery is taking, since all power is from the alternator. On the plus side as stated above, if you loose the alternator the Bat amp-meter will NOW give you the total load, other wise it just sits there at 1 amp or what ever the battery takes charging.

I figure I'll have an alternator most of the time, may be always if lucky. Most of the time total electrical load is on the Alt lead. I'm not going to worry about the alternator failure scenario, simply because I'm going to shed load and land. You should have a handle on the drain from your essential items anyway.

Either way its preference, mine is on the alternator. It gives more info most of the time (when the engine & alternator are running). On the ground (engine off) or in air (dead alternator) the Bat lead is more useful. There is NO right or wrong and I do get jealous when guys with the Bat Lead Amp-meter turn their elec sys on on the ground and get a wiggle out of their Amp-meter while mine reads zero (until the alternator comes alive).

PS: Most forget about the VOLT meter. The volt meter will tell you the amount of energy left in the battery. +12.6 volts is fully charged, less than 12.2 volts 50% left and 11.7 volts 10%. If the Alt quits the Volt meter is what I'd look at as much or more than amps.

David-aviator · #5 09-24-2007, 07:39 PM

I have the amp meter hooked up to the alternator because I like to know what is going on with the electric system load wise. Also, if the alternator quits, a warning light comes on, the meter goes to zero and the message has been sent - find an airport and land ASAP. The battery is on board to get you to that airport.

I don't buy into managing battery load. Sure, it is good technique to reduce the load, but there's no way of knowing how long it (they) will last. Theoritically, one can do the math on it, but the condition of the battery is unknown and to fly past one airport for another because the battery should last is folly....."but I should have had 5 amp hours remaining" won't cut it at the hearing.

This of course assumes an IFR flight or with the electric Subaru. VFR and 2 mags, press on, subject Captain's discretion.

L.Adamson · #6 09-24-2007, 09:35 PM

I'm battery mode, with a seperate voltage meter & low voltage light.

Of course this subject is highly debated throughout various aircraft forums. Just a lot of personal opinions with no clear winners!

L.Adamson

asav8tor · #7 09-24-2007, 09:45 PM

Quote:

Originally Posted by Piloto.Mendes

I like this ideia.

But I don't know what type os switch I should buy, as it needs to be wired to four terminals, swapping two at a time.

Any idea?

DPDT, is what you will find. It has 6 tabs/screws/solder tabs; the middle 2 go to the meter.

w1curtis · #8 09-24-2007, 09:50 PM

Quote:

Originally Posted by asav8tor

How about 2 shunts, a switch and both functions with one meter.

Yup, this is a great idea! I'll have to find a place for the switch now and hope the EFIS doesn't go berserk when you switch modes.

David-aviator · #9 09-24-2007, 09:54 PM

Quote:

Originally Posted by L.Adamson

I'm battery mode, with a seperate voltage meter & low voltage light.

Of course this subject is highly debated throughout various aircraft forums. Just a lot of personal opinions with no clear winners!

L.Adamson

Heavy metal machines use alternator (generator) load systems, it is the only way to know what's going on. No load, something is not working; heavy load, something could be getting ready to make smoke.
On light airplanes, it usually doesn't matter. Same with a car.
That's the difference.

asav8tor · #10 09-24-2007, 10:46 PM

Quote:

Originally Posted by w1curtis

Yup, this is a great idea! I'll have to find a place for the switch now and hope the EFIS doesn't go berserk when you switch modes.

It won't. The shunt doing all the heavy work. You are just measuring a voltage differential off each shunt. In fact without the shunt the ammeter is just a very sensitive volt meter.