I'm bit and I can't get up
w1curtis said:
gmcjetpilot: "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."
w1curtis: OK George, I'll bite, why is the amount of current that the alternator is producing under normal conditions, in a light aircraft important? As long as it is producing what is demanded and charging the battery, "alternator is producitn xx Amps" is information that is not necessarily important. Most devices have power indications so you can usually tell by just looking at the device if it is using power.
Well because how hard the alternator is working is relevant to heat and health of the alternator. It's a double edge sword. We want small and light alternators, which are typically limited to 40 or 60 amps. Weight & size are directly proportional to max output. On the plus side modern electronics and lights (LED) may draw less juice, but on the other hand we keep adding and more electical load, heated seats, heated pitot, A/C, HID lights........ It's just nice to know the load. That's my story and I am sticking to it.
(Car makers run into this, even the smallest cars today have 130 amp alternators. You don't see amp-meters on cars any more. They just make sure the alternator is enough to handle all load and just a low volt idot light informs the driver. It works.)
In the OLD days, even before my time, generators typically had output of 25, 30 may be 35 amps. WOW! Big deal. Gens also needed RPM to make power; At idle they made almost nothing.
The BAT LEAD "Charge-Discharge" amp-meter (eg, needle center zero, + / - ) was real useful, because the generator could not keep up the load, often; It was the battery filling the gap (a lot). You saw a lot of Plus/Neg needle action on the old 50's cars and plane amp/load meters, depending on RPM and load. RV's with a modern alternator spinning like mad, a charge-discharge-amp-meter needle just sits a tad on the plus side and does not move. Not really exciting or useful in my opinion. If the LOW volt light comes on (which should be a mandatory idiot light on all planes like all cars have) you know the alternator is dead. Yes if that happens you will see discharge for the charge-discharge meter. I say so what? Low volts means shed load to essential items and land. There is no doubt the BAT charge-discharge amp-meter would be more useful after alternator failure; The LOAD meter will read ZERO, but that is useful info.
Now a days we have enough alternator capacity to provide 100% of the juice 100% of the time, and the battery is just going along for the ride 100% of the time, unless the ALT dies. If the alternator does die, USE the VOLT meter which tells you a lot as I said previously.
gmcjetpilot: "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."
w1curtis: A shorting device *should* become apparent with your circuit protection device, but even if you have no circuit protection device, the heavy discharge indication on the battery lead ammeter will also get your attention.
Sure you are right, detecting a short with a "LOAD" amp-meter is a stretch, you got me.
However I do have an example of an over-load shown by the LOAD/amp-meter; it does not apply to fixed gear RV's, but an electric gear extension/retraction motor drew more current because the gear was binding. The high draw was an indication of motor working harder. Gear adjustment and lubrication solved the problem! If you watch the amps during flap extension on the ground verses in the air you see difference in current draw.
A more practical example, inop item, where the LOAD/amp-meter tells you something like a NAV light or landing light is not working. In-flight they're hard to see some times. I know my NAV's draw 5 amps total, 2x2 amp-wing tips, 1 amp-tail. With a digital amp-meter, I can sell if a bulb is burned out by the LOAD meter draw. Big deal? No. As you said you can just look at the light and see it's not illuminate. Still, my two weak reasons are better than none at all.
gmcjetpilot: "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."
w1curtis: Well, we are all (most anyway) building airplanes here not retrofitting so I do not view this as a minus. The shunt's gotta go somewhere and if it is coming off the starter terminal lead or the alternator B lead, it's like six in on and half-dozen in the other.
I hear you but where do you put the battery shut? As you said off the starter contactor terminal to "THE BUSS" works. I say "THE" because many planes now have many buss's. Guys and gals out there in kit-plane building land are getting real sophisticated with their wiring, with multi HOT Buss's: Main, Aux and Hot all in parrellel. For example, with electronic ignition (Lightspeed), its recommended you go right off the battery, not thru the master contactor and main buss. When you have a second or third line off the battery in parallel to the main buss, one shunt is not enough. The hall-effect amp-sensor has the obvious advantage of not breaking the wire or carring the actual current, so it can go over the BIG BAT wire right off the battery and measure all current (you put the hall-effect on the negative battery cable/wire). Bottom line, the B-lead is easier to install in any application, in my humble opinion, shunt or hall-effect.
w1curtis: I guess my view on it in light aircraft is that both modes will give you information (different) when all things are operating normally, but only the battery lead mode will also give you information when things are not and the alternator has gone TU.
Well we disagree, that's cool. I do get it, I just think the alternator lead LOAD amp-meter is more useful overall in-flight. On the ground with the engine off or in the air alternator TU, the BAT charge-discharge meter is more useful, no doubt. My point the LOAD meter is useful 99% of the time, giving more info in-flight, with the alternator working, which is normal.
Bottom bottom line you don't even need an amp-meter. As
gasman said (don't want to know how you got that name
) the volt-meter is plenty to divine the electical systems health. Amp-meters regardless of what they measure are luxuries; they're not even required by FAR's for certified GA planes.
