This can't be right.....can it ?

[walkman]

So I'm at the point of powering up my new electrical system and checking out systems and calibrating the EIS 4000. I'm seeing some measurements that I just don't believe. This is a z-12 architecture, 1 battery with 2 alternators. im using a b&c sd-8 as the stsndby alt.

This is all with engine off.

With a battery at 12.46v as measured with my digital multimeter I'm seeing about 11.4 volts at the main buss as measured on the EIS and confirmed with my multi-meter at several ground and buss locations.

If I switch off the master and turn on the e-buss feed (comes off the batt via a diode) I'm seeing very close to battery voltage.

This indicates there is a 1v drop through the contactor path.

I have two Hall effect sensors for ammeters. The first is on the battery positive cable between the battery and the contactor. The second is on the b lead for the standby alternator (after the regulator, between it and the buss).

With all breakers pulled and nothing on but the EIS, map sensor, and the EIS warning lamp (replaced with led) I'm seeing -5 for the amp1 and some tiny negative number (well less than 1 amp) on amp 2.

If I switch off the master and use the e-buss feed I'm seeing -4. That part sounds right, that the contactor take about 1amp to energize. I am just having a real hard time believing the EIS and map sensor are pulling 4 amps. Does that sound right?

When I turn on the strobe, landing, taxi, nav lights I see increases in discharge rate that seem sane.

So, I've got a 1v drop and a 4amp current drain that seems odd to me. Is it normal?

The only thing I haven't done yet is pull the fuse on the standby alternator b lead. Is it possible this is actually draining the battery and causing both issues?

 

[gasman]

If the alternators were not off line, the field will have a draw.

 

[walkman]

both off line

the engine was not running. Both field circuits off, both B leads are direct connections to the buss so obviously they were connected.

I didn't expect there would be a draw backwards through the B leads, but it would agree with what I was seeing on the second ammeter. I think it was showing -.4 or similar which would indicate a current flow backwards from the buss/batt to the standby alt.

 

[mburch]

Two thoughts...

1. I'm curious about your statement about "1v drop through the contactor path". That raised my eyebrows, since a properly functioning contactor should have negligible resistance. So I wonder if you can tell us exactly how you measured this. One thing that comes to mind is that if your measurement process was "measure battery voltage, turn on master switch, measure bus voltage", then you might not be getting the whole picture. The nature of lead-acid battery chemistry causes them to "float" up to a higher voltage when not under load, but drop to their normal operating voltage as soon as you draw any significant current. So if you made those two voltage measurements at two different times, you could be getting tricked into thinking your contactor is causing a big voltage drop when in fact everything is just fine. Something to check anyway.

2. Do you know for a fact that your Hall effect current sensors are accurate? The ones I'm familiar with have good linear response but can often have an output voltage bias offset of several mV, which can equate to an amp or two of measured current. I don't know what EFIS system you have, but the one I'm most familiar with provides a way to calibrate out any sensor error. Since you are only drawing a few amps here, it should be possible to temporarily hook up your handheld DMM as an in-line ammeter to double-check your current readings. Likewise, an EFIS with poorly-regulated transducer reference voltage output can also cause problems with Hall effect current sensor accuracy, since they are ratiometric to their supply voltage.

Hope that helps,
mcb

 

[Rupester]

Couple of thoughts ....

Do you know for a fact that your Hall effect current sensors are accurate? The ones I'm familiar with have good linear response but can often have an output voltage bias offset of several mV, which can equate to an amp or two of measured current.
mcb

This was exactly the case on my GRT system. After a call to the GRT Mother Ship, I had to adjust the current sensor offset at the EIS4000. Prior to that I was getting very odd amp draw indications.

When my two GRT EFISs are powered up, along with the master relay, I show a 4 amp draw. I suspect it's a little less than that because of the integer nature of the Aux point.

 

[walkman]

1. I'm curious about your statement about "1v drop through the contactor path". That raised my eyebrows, since a properly functioning contactor should have negligible resistance.

agreed, my concern also

So I wonder if you can tell us exactly how you measured this. ... if you made those two voltage measurements at two different times, you could be getting tricked into thinking your contactor is causing a big voltage drop when in fact everything is just fine.

As I started to write my response I thought back to my testing, I cannot say for sure now that I had the master on when I checked the voltage on the battery. I will verify that tonight. I think at this point there is a good chance I turned it off. I was on and off about a million times last night

2. Do you know for a fact that your Hall effect current sensors are accurate? ...I don't know what EFIS system you have, but the one I'm most familiar with provides a way to calibrate out any sensor error. ...

well, no. this was a "Setup and calibrate" session. I'm using a GRT EIS 4000 with two of their sensors. all new equipment. I entered the scaling factor and offsets provided in their documentation.

However, that being said, the changes in amperage when I add or remove devices such as batt contactor, strobes, landing light etc are very believable. That part seems correct.

I am driving two resistive fuel level senders, a fuel pressure sending, and the two hall effect sensors via the +4.5v excitation circuit from the EIS as well as powering the manifold pressure sensor. could that really be drawing 4 amps?

 

[walkman]

This was exactly the case on my GRT system. After a call to the GRT Mother Ship, I had to adjust the current sensor offset at the EIS4000. Prior to that I was getting very odd amp draw indications.

When my two GRT EFISs are powered up, along with the master relay, I show a 4 amp draw. I suspect it's a little less than that because of the integer nature of the Aux point.

At this point the EFIS itself is not installed. Only the EIS and associated sensors.

 

[walkman]

Confirmed

There is about a .7 volt drop when coming through the master relay circuit. I compared buss to ground with the EIS and sensors on as well as the master relay energized with the voltage across the batt terminals in the same condition.

I do not see the drop coming through the e buss feed. I have confirmed its not a grounding issue by checking various ground locations including the batt terminal.

This seems high to me.

Further, I am seeing high amperage readings on the EIS, but have not been able to confirm with an appropriate meter yet.

 

[skylor]

There is about a .7 volt drop when coming through the master relay circuit. I compared buss to ground with the EIS and sensors on as well as the master relay energized with the voltage across the batt terminals in the same condition.

I do not see the drop coming through the e buss feed. I have confirmed its not a grounding issue by checking various ground locations including the batt terminal.

This seems high to me.

Further, I am seeing high amperage readings on the EIS, but have not been able to confirm with an appropriate meter yet.

Are you sure that the EIS is measuring main bus voltage and not E-bus voltage fed from the main bus? 0.7V drop from the battery voltage coincides awfully well with voltage drop through a diode, such as the e-bus isolation diode. When you use the e-bus feed, you essentially bypass this diode, hence the lack of drop in that case.

Skylor

 

[Fred.Stucklen]

Bus Voltage

How are you measuring the buss voltage? Are you using an off-the-shelf volt meter, or using the EIS readout. Be advised that the EIS readout isn't always calibrated accurately....

There is about a .7 volt drop when coming through the master relay circuit. I compared buss to ground with the EIS and sensors on as well as the master relay energized with the voltage across the batt terminals in the same condition.

I do not see the drop coming through the e buss feed. I have confirmed its not a grounding issue by checking various ground locations including the batt terminal.

This seems high to me.

Further, I am seeing high amperage readings on the EIS, but have not been able to confirm with an appropriate meter yet.

 

[rocketbob]

Plain and simple, diode voltage drop.

 

[walkman]

I think I have a few issues with related symptoms that I was trying to treat as one issue.

I helped the voltage issue by finding a loose connection. Now voltage drop is around .4v and is only on the diode fed portion of the buss.

I am still seeing a significant amp draw, around 5 or 6 amps with just the EIS, sensors, and EFIS powered up. Primary alt won't come online, standby works fine. Batt volts drop quickly.

I tried to disconnect all the EIS sensors that require an excitation except for amp1 however for some reason that affected the amp1 sensor as well, it went to -99.

I don't know why I didn't think of this before. Tonight I will disconnect the eis and sensors from the aircraft buss and power them from a separate battery. If I see 0 on amp1 I will know its not a sensor issue.

 

[walkman]

4 problems in one

yep, turns out I had multiple problems with related symptoms masquerading as one issue. once I started thinking about the symptoms separately it made sense.

-erroneous ammeter readings showing a higher than actual load
-a loose nut on a connection between two sections of buss causing a higher voltage drop
-a weak battery that looses charge quickly, simulating a high load
-knocked a connection of the primary regulator preventing alt 1 coming online