Ammeter Shunt Wiring

[Spaceman]

Howdy Folks,

I'm working on setting up the wiring on the front of my firewall so I need to decide where I'm going to put my ammeter shunt, which my EFIS will use to sense electrical load.

My intuitive first thought was that the most useful place to put it would be right after the battery positive terminal. That would sense any current going into (charging) or going out of (discharging) the battery. This would allow me to know how much power I was using during ground ops (alternator off), and also indicate if I was overloading my alternator (battery discharging in flight), or for that matter if the alternator quit entirely (full bus load shown as battery discharge current). I've illustrated this in diagram #1 below.

The problem with that is, the shunt instructions say not to run starter current through the shunt. If I wire it directly to the battery +, then it'll get all current that's ever used, including the starter. So I don't think I can do it that way. I think you could put a hall effect sensor there, but I have this type of shunt: https://www.steinair.com/product/g3x-shunt-100-amps/

The other common setup I've seen in my research is option #2, wiring it in series with the alternator output. That way, the shunt will measure all current from the alternator. The shortcoming I see with that is, it tells you nothing if the alternator is off. It would make it obvious if the alternator quit in flight because the current would go to zero, but you'd know if the alternator quit because your voltage would go from ~14v to ~12v. Plus, you wouldn't know how quickly you were discharging your battery, except for watching the voltage slowly decrease. So this seems slightly less useful to me.

Another option, #3, would be to wire the shunt in series with the power supply going to the main bus. This would tell you how much load you're pulling with all your systems, which could be good to know, but I don't think you'd really be able to discern whether that power was coming from the alternator or the battery.

Finally, I drew up option #4. I think this would meet the intent of option #1, while still keeping the shunt separate from the starter current. If I'm picturing this correctly, it would be the same as connecting it to the battery +. If the alternator output exactly matched the bus load, the shunt would show zero current. If the alternator was sending excess current to the battery (charging), it would show positive amps, and if the alternator wasn't keeping up then current would flow from the battery to the bus and the shunt would detect negative (discharging) amps. Assuming the polarity of the sensing wires are correct...

Can someone who's smarter on electrical stuff than me tell me if this sounds right? Now that I've typed it all up I think option #4 makes sense and will do what I want, but I wanted to make sure I'm not missing something obvious!

Thanks!

[lakespookie]

Option 4 looks good that's how I plan to do it shunt has access to everything except the dedicated power line to the starter.

Obviously I cant see the rest of that but I would presume you have some relay system to trigger the high power draw line to the starter triggered by your starting switch of choice.

[Spaceman]

Yeah the smaller wire to the starter solenoid comes from the start switch; the aerovee starter has its own solenoid so you don't need a separate contactor for that thing!

[Rynoth]

Which ammeter do you have? I got mine from MGL (http://www.mglavionics.com/Magnetic_current_monitor.pdf) and attached it directly to the battery lead, no worries about starter current and it's been very accurate once I got it dialed in.

Edit sorry I see you already linked your shunt. That's definitely a different type versus what I have.

[Rynoth]

#4 on your diagram looks good. The only thing you wouldn't be measuring is any "hot" battery appliances (turbo cooler, efis memory, etc.)

[sonex1374]

Chris,

In order to place the shunt, you first need to decide what you want to measure with it. You run through the various options, and the diagrams seem to match up properly with each of those. My thoughts on what to monitor come down to this: what decision will you need to make based on what the shunt tells you?

Do you care about alternator output, and do you need to know the exact number or just if it's working or not?
Do you care about the instantaneous draw of your panel and electrical devices?
Do you want to know whether current is flowing into or out of the battery, or just that it's not getting too low?

I'll submit that you really don't care to know the exact output value of the alternator, but rather that it is online and supplying current. Monitoring bus voltage will tell you this (when the alternator is working the bus voltage will be higher than battery-only voltage). Furthermore, the number of amps being consumed by the panel isn't really helpful, just whether the battery and alternator can supply the demand. Lastly, knowing current flowing into the battery isn't that helpful except to know that it's accepting a charge, and current flowing out of the battery could be useful to predict how long the battery can supply the power demands (assuming you know the current stored capacity inside). This is the most helpful of the three, but it's still just a number that needs more data to make sense of.

A more useful measure of the alternator output and battery capacity is voltage. The system voltage will tell you if the alternator is working because it will be higher than battery-only voltage. Voltage will tell you whether the battery+alternator are keeping up with electrical demands because the voltage will be stable (keeping up) or declining (not keeping up). And finally, voltage will help estimate the capacity of stored energy in the battery because every battery has a characteristic discharge curve that relates battery voltage to remaining capacity. You can (and should) create your own discharge curve by powering your plane with the expected loads and monitor the battery voltage over time as the battery depletes itself. When you plot this data you'll know that when the battery registers 12.1 volts you have XX minutes remaining, and when it reaches 11.7 volts you have YY minutes remaining, and finally when it reaches ?? volts the battery is pretty much depleted.

The shunt is included, so it seems like the thing to do is use it, and in that case I think you'll want to monitor power flowing out of the battery (minus the starter current, of course). But this information isn't terribly useful all on it's own, not like voltage is, so think thru what you'll do as a result of collecting this current flow information. You might find that you don't really need it at all.

Jeff

[Spaceman]

Ryan, I saw what you used on your blog and I looked into using that same MGL sensor for a few minutes, but it looked like making it work with the Garmin box was going to be a little convoluted. The analog thing I linked to is like the default G3X shunt so I decided I'll keep it simple and use what the recommend. I think it's geared towards way higher current than what we are working with but whatever!

Jeff, thanks for the insight! I'm with you in that it's probably not totally needed, but I have it so I guess I'll put it to use. Totally agree that bus voltage already tells you if the alternator is working or not so I don't think putting the shunt on the alternator lead is that useful. I think I will be curious to know how much power all my systems are drawing; maybe not so much in terms of absolute amps, but more in terms of like is the battery charging or discharging with the things turned on at that moment. If I can discern that it's discharging then I know I need to turn some stuff off before long!

[GordonTurner]

I would consider putting the shunt on the alternator load for your initial flight testing phase, give you a chance to see what your loads really are in flight conditions. Later it might prove more useful to change it to scenario 4.

[Spaceman]

That's a good point... For testing purposes, if I wanted to check the actual load of certain systems could I hook up the shunt like option 4 and then just turn the alternator off momentarily? Then note the battery discharge rate and turn it back on?

[builderflyer]

Chris,

The shunt is included, so it seems like the thing to do is use it, and in that case I think you'll want to monitor power flowing out of the battery (minus the starter current, of course). But this information isn't terribly useful all on it's own, not like voltage is, so think thru what you'll do as a result of collecting this current flow information. You might find that you don't really need it at all.

Jeff

Jeff has turned this conversation in the right direction. Other than for testing purposes, the most useless gage on the panel of our "Sonex" or any Cessna or Piper is an ammeter. For most of our inflight electrical concerns, a voltmeter tells us exactly what we need to know.

Art,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261