Shunt location when using VP-X?

[jeffa]

I'm trying to figure out where and how many ammeter shunts need to be installed using the VP-X, a single battery, and a single alternator.

According to the current install manual and pilot's guide, it's possible to get three current indications....one for the current draw through the VP-X (provided by the VP-X), alternator output, and battery charge (or discharge) current.  Are two shunts needed to provide both alternator output and battery charge?  If so, where does the 2nd shunt plug into the EMS?  If only one shunt can be installed, where does the Skyview get the other current value....is it calculated?  If I put the ammeter shunt in position 'B' (for alternator output), is value 'A' (battery charge) calculated by subtracting the VP-X load from the alternator output?  If I put the shunt in position 'A', is alternator output calculated by adding battery charge current and VP-X load?

I apologize if this is covered in the manual....just looking for clarification.

 

[jeffa]

Or am I wrong altogether, and I only get to have alternator output or battery charge, but not both.  The pictures of the VP-X page show all three amp values.....is it just showing those three at the same time to illustrate "possible" indications?

 

[sandaville]

You do not need shunts with the VP-X system. The current monitoring is all done within the VP-X box. The current values displayed are all derived by the VP-X box itself and sent to the Skyview via RS232 serial link.

See http://www.verticalpower.com/VPX.html for more information on what else you don't need if using a VP-X box. scroll down until you get to the Cost Analysis section.

 

[jeffa]

Only partially correct I think.....

Although my beautiful VP-X box is incredibly smart, I'm not sure that is correct. Yes, it has the ability to know how much each individual circuit is drawing, and add them together for a total system draw.....but to my knowledge, the VP-X box has no means of knowing where the power it is receiving is coming from, either the battery or alternator, or both. It has the ability to know how much the alternator FIELD is drawing, but not the alternator OUTPUT.

To get all three current indications shown on page 13-4 of the installation manual, one or more shunts would have to be installed (I think). The amp indication under the VP-X box will always be there (I'm sure), but the alternator output, and/or battery amps with arrow need a shunt (I think). Interestingly, figure 45, page 5-8 of the pilot's guide does not show the amps of the alternator or battery. Trying to figure out if both the alternator output and battery charge indication can be displayed simultaneously?

 

[preid]

I have the VP200, it does exactly what you are saying,I would be surprised if the VP-X would be any less. There is even a line in my installation manual that states no Shunts are needed.
I have a red wire coming from the alternator that connects to the VP to display the OUTPUT
Call Marc at VP, he'll confirm if you need the shunt.

 

[dynonsupport]

You have two options:

1) Install no shunts, and you get only the VP-X current reading, as seen below.

2) Install ONE shunt and you get both the battery and alternator current. Why only one shunt? Kirchhoff's current rule says that the sum of current in and out of a node must be zero. Thus, if we know the VP-X is reading 10 amps, and the battery is putting out 2, the alternator must be doing the other 8. So yes, one of the readings is calculated.

This is a unique feature to the VP-X implementation on the Dynon SkyView- It really has nothing to do with the VP-X except that it uses the VP-X as one of the 2 current readings, and no other EFIS does this.

The issue with the VP-X is that it only tells you what the current draw of all your equipment is- you have no idea if the alternator or the battery is sourcing it. You'll find the ammeter in very few planes reads the avionics load- it almost always reads the battery load/charge or the alternator load, but with a VP-X alone you don't know this.

Theoretically, the battery voltage gives you a good indication if the alternator has failed, but we thought that some people might want to install the shunt and have additional information. It's not required, but it is nice to have.

 

[dynonsupport]

With one shunt installed, you always see all three currents:

 

[jeffa]

Thank you!  You guys are great!  

I would prefer to see all three indications, so now I just have to figure out where I'm going to install the shunt....

Thanks again.

 

[jbeaver]

That's not completely correct though. If you have components connected directly to the battery or battery bus (e.g. electronic ignition), their current isn't accounted for. This can cause a false display for the power flowing in/out of the battery or out of the alternator depending on where the shunt is located.

 

[mmarien]

When the shunt is between the alternator and the rest of the system (Position B in the install manual) I suspect it will always show the total current including the current flow to (or from) the battery. The 2.5A in the above picture is to the battery. So if a device is connected directly to the battery the alternator will reflect the current draw from it in the total and show it as current to the battery.

I suspect that if the alternator isn't producing enough current, some current will flow from the battery. In that case you wouldn't know how much the device connected to the battery is drawing. But that isn't going to be the case as long as the alternator is (working and) designed to supply all the power and some to recharge the battery.

In position C the current would equal the VP-X draw but you wouldn't know if the current is coming from the battery or the alternator so it doesn't supply any new information. Position A would just show the current to (and from) the battery and any devices connected directly to the battery. I'm going to put a shunt in position B as shown in the manual. It supplies the most information.

I guess that begs the question. Does the Skyview formula described above work if the shunt is in position A or C?

 

[dynonsupport]

As the install manual says:

If you have a Vertical Power VP-X system in your aircraft, use either position A or position B. Position C is not useful in a VP-X installation because the VP-X measures aircraft loads directly.

This is Kirchoff's Current Law. If you know 2 out of three legs, you can calculate the third.

True, if you hook a current load right to the battery and not to the VP-X, this doesn't work. Don't know why you would invest in a VP-X and bypass it though. If you do this, you should tell SkyView that no shunt is installed so it doesn't calculate your non-VP-X currents.

 

[mmarien]

I agree. I struggled with backup systems for a couple of months now but decided that the best method would be to run everything from the VP-X. However, my ELT switch is wired directly to the battery as is the clock in some systems so I don't think there is an alternative for some devices. These currents are probably not measurable.

I'm a bit confused with Kirchoff's Current Law though. If the battery draws 2.5A to recharge, wouldn't connecting a 5A draw directly to the battery make the total recharge draw from the battery 7.5A? Shunt position B: Alternator - VP-X = battery. The current from the alternator should reflect all the current draw no matter where from.