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Voltage Readings

Postby akskyhawk » Wed Oct 22, 2014 4:45 am

I have discovered that the voltage reading displayed by EFIS can have as much as .5 volt errror!
I found on my system with engine not running, that I had a battery voltage reading of 13.2 volts and the EFIS is showing 12.7.
GRT confirmed there can be a difference normally of .2 to .5 volts because of filters and other things in the EFIS.
So don't go and change your voltage regulator to increase voltage to the norm 14.2 when engine and alternator running. You might ruin your battery!
The fix according to GRT is to run a voltage input from your battery bus to a Anolog input thru a 240 ohm resistor to get true voltage reading.
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Re: Voltage Readings

Postby psychose » Mon Oct 27, 2014 4:06 pm

Does this voltage discrepancy still apply when the EIS is displaying voltage information and feeding it to the EFIS? In this case, does the EFIS display voltage data as received from the EIS or from its own circuitry?

Is the amperage reading using the CS-01/CS-02 current sensors via the EIS subject to a similar error?

Thanks,

Normand RV8 Finishing
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Re: Voltage Readings

Postby GRT_Jeff » Fri Oct 31, 2014 7:25 am

Each unit is measuring the voltage at its input. The EIS computes its own voltage and sends the digital value over to the EFIS. It can also have a difference, but the difference should be smaller than the EFIS simply because the EIS uses less power.

The EFIS can display the voltage from all 3 of its power inputs and the EIS power input. The data boxes and bar graphs have separate selections for these inputs. If you choose to display EIS Volts in a particular location, the EFIS will always show EIS Volts there.

Sensors are not affected unless there's a problem with the ground. For example, you could connect a high current device to the same ground wire as the EIS or a sensor and throw off the sensor readings if the ground wire isn't heavy enough.
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Re: Voltage Readings

Postby psychose » Fri Oct 31, 2014 1:10 pm

Thanks Jeff, that clarifies a lot.

Normand RV8 Finishing
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Re: Voltage Readings

Postby psychose » Wed Mar 15, 2017 7:38 pm

Hello,

Have been thinking about this difference between actual battery voltage and voltage as measured by EIS/EFIS. Given how critical it is to maintain proper system voltage, would it be possible to add a bias function in either EIS or EFIS software, permitting us to adjust displayed voltage to make it equal to actual voltage?

Thanks,

Normand RV8 C-GNGN
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Re: Voltage Readings

Postby Bobturner » Wed Mar 15, 2017 8:31 pm

Normand,
I think you misunderstood Jeff. Between the battery and the EFIS are various contacts, connections, etc, which introduce unintended voltage drops; and fuses and/or CBs which, by design, introduce voltage drops. But how large those drops are depend on how much current is being drawn, so you would need different 'adjustment factors' for different situations, including whether the screen brightness was high or low - not very practical.
Skyhawk,
What kind of battery do you have that puts out 13.2 volts?
All,
Best approach is as described above - run a separate, fused wire from the battery to an analog input. Because of the relatively high input impedance this will draw very little current so there will be very little voltage loss across the fuse. I did this to a mid-priced digital voltmeter, and it and the EFIS voltage were within 0.1 or 0.2 volts with the engine running (I wanted to verify the charging voltage). About as good as you could expect with this setup.
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Re: Voltage Readings

Postby Rob Henderson » Wed Mar 15, 2017 9:13 pm

My hxr reads .5v low on all 3 inputs as measured by my fluke meter at the same points.
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Re: Voltage Readings

Postby psychose » Thu Mar 16, 2017 4:15 pm

Hello Bob and Rob,

I will measure my voltage readings with engine running this weekend and report back. From my readings of Aeroelectric Bob and Mike Busch, battery life is extremely sensitive to charging voltage, thus the need to monitor accurately. I would thus argue that even a 0.1 difference may merit adding a bias function to the software.

Normand RV8 C-GNGN
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Re: Voltage Readings

Postby Sportsman » Wed Apr 12, 2017 8:14 pm

This is a topic which has bugged me for quite awhile now... The GRT equipment reports the voltage on its three power input pins, but in fact it's reporting that voltage as measured somewhere deep within the box. There is consistently a lower reading on all three inputs when compared to actual DVM readings on those same wires on the connector (yes, I measure it live with the EFIS powered up - I'm a professional, don't try this at home!). From what I can deduce, GRT has a diode in series with the power input and the unit is sensing voltage somewhere downstream of this diode. I suspect the power inputs are simply isolated by diodes in order to allow the box to be powered from any of the three inputs.

As has been suggested earlier, the input voltage reported is NOT anything like aircraft bus voltage. If I want to know what my battery voltage is on y Essential bus battery (located exactly one switch, one CB and three feet of wire from the battery itself) I have to take the reading reported by GRT and add 0.7 volts.

I agree wholeheartedly that it would be an excellent idea to add a voltage bias calibration factor to the software. This would be far easier than running three separate wires to three separate anolog inputs through three 240 ohm resistors.

What about it, GRT? We know it's just a simple A-D convertor - let us add a user-configurable bias to your factory-calibrated slope/offset on the A-D converter. If unable to do it this way, then please adjust your factory-calibrated offset to dial out the diode drop that occurs downstream of the input connector pins. Let's get these voltage numbers reading something close to reality.
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Re: Voltage Readings

Postby GRT_Jeff » Thu Apr 13, 2017 4:45 am

It's not the diode drop. It's the voltage drop through the power and ground wires from the current and wire resistance. If you measure the voltage by shoving tiny probe leads into the power and ground pins on the back of the connector (or otherwise get as near to the display unit connector as possible) you should see the same voltage reported on the screen. You can see that the voltage on the screen is closer to your bus voltage if you decrease the dimmer level. A simple bias correction won't fix this because the current can change significantly, and it would change based on which of the 3 inputs is supplying the current. The single power input case could be helped by adding multiple corrections at several dimmer levels, which changes the current the most.
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