What could cause current to read discharge but voltage to remain steady?

[bkspero]

I could use some suggestions on what could be causing the subject issue with my airplane and what diagnostics we might do to narrow down the possibilities. I've already emailed my local shop, but know that they are swamped with other big ticket install work and short staffed, so anything I can do ahead of time could be the difference between a quick resolution and weeks on the ground.

Below are two graphs of charging amps and battery voltage for my Tiger. One is from a ca. 30 minute flight and is representative of what I expect (i.e., normal). Charging current (light blue trace) is high and positive immediately after starting (when the black, voltage trace jumps to ca. 14). Current then drops as the battery charge withdrawn during charging is replaced by the alternator. Eventually charging current stabilizes at about 1 amp and voltage is kept between about 13.9 and 14.1.

Startup for the abnormal profile is generally normal with high charging current initially falling as the battery is recharged. This time, however, shortly after the plane was airborne charging current went negative (discharged). I caught the anomaly at about the 5 minute mark and recycled the alternator side of the master switch. Current dropped to about -8 amps with the switch off, then back to positive (briefly) with the switch back on. Battery voltage appeared normal, though, and neither it nor current changed systematically with pitot heat or radios on/off (sorry, I did not record where I did those tests).

Since voltage was holding, I decided to continue the flight. Voltage remained within 13.9 -14.0 (maybe biased down 0.1 V vs. what I considered normal) despite about 10 minutes of a ca. 2 amp discharge reading. Near the end of the flight charging current increased from -2 to ca. -1, then to ca. +1 for a couple of minutes, then back to down to -1 as the battery voltage increased from 13.9 to 14. Voltage continued to rise and reached 14.1 despite showing a discharge current.

I later restarted the airplane hoping that power cycling the CGR-30P would eliminate the problem, but it did not. Current went negative in about 2 minutes, and after about 5 minutes it dropped to -15 amps for about 2 minutes, then back to -2 amps, then back to ca. -15 amps, then back to -2 amps, then stabilized around zero. Battery voltage was about 13.8 Volts during this time, but rose to about 14.1 Volts after about 2 minutes at zero charge current. I checked the battery voltage immediately after shutdown and it was 13.6 V. It was 12.8 V after resting for about 8 hrs.

For background, voltage and current readings are from an EI CGR-30P with its current shunt wired to show battery charge/discharge current. Battery is a 3 year old Concorde sealed AGM RG-25. Voltage regulator is a 19 year old solid state Zeftronics. I don't recall the brand or age of alternator.

What could cause this behavior. Failure of the CGR-30P voltage reading that is translated to discharge current is an obvious potential culprit. But this is a relatively new (ca. 3 years old) solid state device which appears to be working normally otherwise. Any suggestions on how to test whether it is the Volt or Amps reading(s) that is (are) in error? Could battery failure somehow explain this behavior? Or loose/dirty connections? Or worn alternator brushes? I don't see how, but also don't claim particular expertise. Are there readings I can take which will help narrow down the cause?

Thanks in advance for your assistance.

 

[Let'sgoflying!]

So, the airplane is working normally?

Just wonder if its an instrument anomaly; usually people start these topics with functional symptoms like ‘dead battery’ etc

 

[AKBill]

Neat graphs. 5:30am here will have to let the brain wake up before I make further comments..
Edit: Got to run to hangar to start my preheat

 

[AKBill]

So, the airplane is working normally?

Just wonder if its an instrument anomaly; usually people start these topics with functional symptoms like ‘dead battery’ etc

Don't think it's an instrument anomaly, the second graph shows the drop expected when the field was disconnected. That said the first step I would take is verify what the instrument is showing. I could be wrong but I don't think it's the battery. Battery's don't normally fail fast, unless they are frozen... I guess I would start at the alternator. Check output and field current. Loose connections and corrosion can cause strange voltage readings. Just my 2 cents, good luck. Hope it's not an expensive fix.

 

[1RTK1]

Had a similar intermittent problem at one time (didn’t have any monitor) it turned out to be a bad master switch. Be sure and diagnose the problem before throwing money at it.

 

[Dan Thomas]

Or loose/dirty connections? Or worn alternator brushes?

Those are what I would go after first.The way the output was jumping around is typical of either worn/contaminated brushes, or some loose/corroded connection, likely between the regulator and field.

I would pull the connector off the regulator and put an ohmmeter between ground and the F wire in the connector to see what the field resistance is. It should be between 3 and 5 ohms, and turning the alternator shouldn't change it much. If you're going to turn the prop to rotate the alternator, disconnect the sparkplugs first. Look for a loose field wire connection at the alternator and at that terminal at the regulator. Frayed wires are common at the alternator end, caused by vibration flexing the wires if they're not properly supported.

 

[bkspero]

I heard back from the shop. As I feared, they don't know when they will be able to work on the plane, but they are ok with me starting the diagnosis process under their direction. They want me to look at the current shunt first. Check the connections and the fuses/fuse holders in the sense wires. I should also be able to compare the voltage drop across the shunt with the current display. The drop will be low but measurable if I can get the current draw high enough without starting the engine (shunt resistance is only 0.001 ohms...and I don't want to be concentrating inside the cabin with the prop spinning on the ground). I will also be able to measure the bus voltage and compare it to the readout. Those should at least confirm or eliminate the CGR-30P as at least a part of the problem.

 

[bkspero]

The problem was poor connections at the sense leads to the current shunt resistor. First I checked the bus voltage using a multimeter and it matched the bus voltage reported by the CGR-30P. So my voltage readings were likely reliable.

Next, I removed the top 37 pin connector from the box that collects all the signals (the amp leads are on pins 32 and 33 of that connector) and measured the resistance between the respective connector pin and the shunt terminal block to which it was connected. One was 10 ohms where it should be near zero. I removed and cleaned the connections at the shunt for both sense wires and also treated their connector pins with Stabilant. Resistances dropped to near zero. Did a ground check and it looked normal. Buttoned things up, did the paperwork, and did a flight check for 30 minutes. Things continued to look normal. I think that was it.

BTW, I decided not to dig into the charging system based on the data from the last "abnormal" flight and the fact that the voltage readings on the CGR were good. During that last abnormal flight there were several periods during the last half of the flight when the discharge current went way low (10-12 amps). If that were real it would mean that my alternator was supplying no current, as the base electrical load on my system without the pitot heat or lights is about 8-10 amps (what I see if I turn off the alternator field in flight). But if the alternator was not supplying current, then all the electrical power would have been coming from the battery and the bus voltage should have been battery voltage at 10 amps discharge rate. That's down near 12 volts. But the bus voltage was steady near 14 volts during those periods of 10-12 amp discharge. So the alternator must have been supplying current and it was the CGR-30P amp reading that was inaccurate.

 

[bluerooster]

Voltage seems to be in the ball park. Current OTOH has decreased. So, that leads to high resistance somewhere, (corrosion?) A resistance in series lowers current, voltage remains constant. So, the fact that it shows a discharge, indicates to me that the problem lies somewhere between the alternator, and the battery.

 

[bkspero]

Rooster, the problem was with the current reading due to high resistance which developed at the connections in the amp measurement sense wires that run between the calibrated current shunt and the box that aggregates the CGR-30P signals. The charging system, battery, and things in between all seem to be working fine once those sense wire connections were cleaned.