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!