AMP Reading appears to low

[The RV8 Pilot]

New flying RV-8 with an all SkyView system and the shunt in position "B". When I turn on the ALT field after engine start I see AMP's in the 14 to 16 range, The battery voltage reading goes to 14.7. The Amps changes a little higher and a little lower depending on devices being used like seat heat and so on. It appears that the alternator, Plane Power 60 amp, is functioning fine as I have been doing some longer 2 hour flights to break in the engine and no issues. Any ideas why the AMP's are reading low?

Thanks,
Michael

 

[lancair360]

I don't draw much more than that on my Lancair. Have you added up the systems running and done the math?

 

[RV8JD]

I also have an RV-8 with a Plane Power 60 amp alternator. It's equipped with an all Dynon system including one 10" SkyView Touch, radio, intercom, Transponder, ADS-B IN, autopilot; and an Aux power plug powering an iPad, Aera 660, GDL 50 and an iPod. I normally fly with the strobes and Wig-Wag lights on. That all pulls 13 amps.

The 14.7v sounds a little high. I normally see 14.4v to 14.5v.

As a side note, I normally start with the Alt Field switch on.

 

[The RV8 Pilot]

Thanks guys for the replies, for some reason in my head I thought I should be seeing more output. My setup is a 10" touch, 7", 2 COM's, transponder, ARINC, ADS-B, intercom, AP head, Garmin GTN-625xi, seat heat and USB-ports for iPhone. I also run AeroLED's land/taxi, strobes and NAV lights. I will keep and eye on it and play around with all the items powered up to see the draw. I will also recheck the battery volts to see if it is staying at 14.7 or fluctuation.

 

[lancair360]

Output will only be what is needed. Sure it's a 60A alternator but you're only using 14A or so. Perfectly normal.

 

[DBRV10]

Just a thought, and it is a personal preference thing, but every installation I do, I explain the information gained from each shunt "position". In every case when the discussion has been had, POS A is selected. Especially with a VPX installed.

The reason is under normal conditions you are interested in what is going in or out of the battery. The system load is taken up by the alternator and if you want individual loads and have a VPX installed you can see it all there.

When you are on the ground or if you have an alternator failure, you are then interested in load shedding and watching the -Amps. This applies even with a backup alternator as they usually cannot cope with a full system load alone.

In my opinion.....there is only one option worth using from a practical stand point, position A.

 

[The RV8 Pilot]

Just a thought, and it is a personal preference thing, but every installation I do, I explain the information gained from each shunt "position". In every case when the discussion has been had, POS A is selected. Especially with a VPX installed.

The reason is under normal conditions you are interested in what is going in or out of the battery. The system load is taken up by the alternator and if you want individual loads and have a VPX installed you can see it all there.

When you are on the ground or if you have an alternator failure, you are then interested in load shedding and watching the -Amps. This applies even with a backup alternator as they usually cannot cope with a full system load alone.

In my opinion.....there is only one option worth using from a practical stand point, position A.

I thought one of the benefits of the VP-X is that you can see all the current draws? I can pull the VP-X screen up in the air or on the ground and see what each device is drawing and then decide to shed as needed.

 

[Marc_J._Zeitlin]

I thought one of the benefits of the VP-X is that you can see all the current draws? I can pull the VP-X screen up in the air or on the ground and see what each device is drawing and then decide to shed as needed.

DBRV10 spoke to this issue, but I'll discuss all three possible shunt locations, and why (IMO) "A" is the best choice.

"C" tells you what the aircraft load is - that's essentially what the VP-X does, if you have one, and the VP-X also gives you individual current draws. If you don't have a VP-X, you don't get the individual draws, but by turning things off and watching the total draw, you can determine what individual draws are. 99.9% of the time, you don't care what the individual draw is - even in an emergency, if you've done this test once, you know what the big draws are and which to turn off (lights, mostly). This position ("C") is essentially useless in normal use if you designed your electrical system correctly so that you're never pulling more than 80% of your alternator output on a constant basis. "C" would only be useful if you didn't "design" your electrical system, and just threw a bunch of electrical components at the airplane without considering the ability of the alternator to drive them. In which case, I'd argue that the right answer is to redesign your system, not put the shunt in a less than optimal location.

"B" tells you the alternator output - this is the total of the aircraft load added to the battery charging current. It's not particularly useful, again, if one has designed one's electrical system correctly so that in normal use with a charged battery, the constant load (not intermittent stuff, like gear retract, flaps, etc.) is below 80% of the alternator output capacity. You don't need to watch the alternator output current to know if your alternator is healthy and working correctly - the voltmeter will tell you that. If the voltage is 14.2V - 14.5V, or whatever you set your regulator to, then your alternator is working. If it's ~13V or below, or 15V or above, then it's not.

"A" is the most useful because it tells you the health of your battery. Since the voltmeter tells you the health of your alternator, and you've designed your system correctly so that it doesn't overload the alternator output, you only need an additional indicator for battery health. "A" provides that. Right after starting, there will be a high positive current INTO the battery as it recharges. After a few seconds to a few minutes, depending upon battery size and type, the current into or out of the battery will be about zero, when the battery is again fully charged. If the current INTO the battery stays high, so that it's always charging, you've got a sick battery. If the current OUT of the battery is anything more than an amp or three, you've probably also got a low voltage indication and your alternator or regulator are on the fritz. If you see -1 to 1 Amp on a regular basis, you have a healthy battery.

I'm with DBRV10 - "A" is the most useful position for a current measuring shunt, and what I recommend to all my customers when I'm installing a system in their plane.

 

[DBRV10]

I thought one of the benefits of the VP-X is that you can see all the current draws? I can pull the VP-X screen up in the air or on the ground and see what each device is drawing and then decide to shed as needed.

Yes you can pull up the VPX screen, and yes the VPX will display all the attributes, but nobody flies along all day with the VPX screen up. So the widget you want displayed on your EMS page is the shunt current that is the most meaningful. That is POS A. Simple as that.

POS B or C is simply pointless with or without a VPX. In fact if you do not have a VPX, POS A is even more critical. And for the reasons Marc has described above.

 

[mmarien]

Your battery is for starting the airplane. After that who cares. The battery doesn't as it's job is done. It could be dead and the plane will still fly. The avionics will still work and the lights will stay on as long as the alternator is working. If the battery is dead or I have no battery I can still start the plane by hand propping. No battery - no loss.

I do care what the alternator load is. It's the device that is supplying the power to my equipment. If it quits then there is a finite time until my lights (avionics) go out. Since we are talking personal preferences, I personally like to see what the load is on the alternator. If it quits then I know my lights (avionics) are eventually going to quit. I can calculate how long I can fly by the negative draw on the battery. I also can determine how much farther I can go when I turn devices off as I can see the change in the draw. No memorizing or guessing involved.

It's kind of like looking at your fuel gauge. If there is fuel you know that your engine run (position A). If you know the fuel flow (draw) you know how long your engine will run (position B and C).

I vote for B. or C.

 

[Marc_J._Zeitlin]

Your battery is for starting the airplane. After that who cares. The battery doesn't as it's job is done. It could be dead and the plane will still fly.

Sorry I'm 4 weeks late on this reply.

Actually, your battery also stabilizes the output of the alternator. Depending upon what kind of alternator you have and what kind of regulator you have, a dead (completely dead) battery can cause alternator instability, which could damage equipment or cause an overvolt crowbar shutoff.

Without a battery, you have no backup for alternator issues, and if you have dual EI's (or EFI), you have no redundancy, even if the alternator DOES keep working correctly.

I do care what the alternator load is. It's the device that is supplying the power to my equipment. If it quits then there is a finite time until my lights (avionics) go out. Since we are talking personal preferences, I personally like to see what the load is on the alternator. If it quits then I know my lights (avionics) are eventually going to quit. I can calculate how long I can fly by the negative draw on the battery. I also can determine how much farther I can go when I turn devices off as I can see the change in the draw. No memorizing or guessing involved.

So with Position A, you still get ALL that information that you state you have to guess at, and you get it directly from what the battery is doing in the case of an alternator failure. If you've designed your electrical system correctly, you cannot overload the alternator, so you don't care what it's doing, and if you have a voltmeter, you'll immediately know if the alternator has gone south - you don't need an ammeter to tell you that.

As far as shedding load goes to extend the life of the battery in the case of an alternator failure, there's no more accurate way of doing that than by looking at the current flowing OUT of the battery, with position A for the shunt - it will tell you exactly what happens as you shed load. No guessing required, no memorization required.

It's kind of like looking at your fuel gauge. If there is fuel you know that your engine run (position A). If you know the fuel flow (draw) you know how long your engine will run (position B and C).

You've got it backwards. With position A, you know the fuel flow from the thing that can run out of fuel if in use. If you use B or C, you know the fuel flow from the thing that creates its own fuel and can never run out, so there's zero additional information from those shunt positions. Position A gives you far more and far more useful information than B and C.