Where does your bus voltage float during cruise flight? (14v aircraft)

[mcarterak]

Spin off from the other thread, where does your bus voltage float during cruise flight? I've been troubleshooting electrical gremlins on my 14v J, I managed to clean up some light corrosion, but can only get my bus to show 13.4 after start, and floats at 13.3 max during cruise. Am I chasing my tail? 

[N201MKTurbo]

Mine should be higher. I need to adjust it. I fried my Zeftronics and Put in a Plane Power and it is at 13.7. The Zeftronics doesn’t have any field current short circuit protection. The Plane Power does. Had to rewire the idiot light…

[mcarterak]

5 minutes ago, N201MKTurbo said:

I fried my Zeftronics and Put in a Plane Power and it is at 13.7.

Which model PP VR did you install? I don't see the J listed on the R1224 eligibility list. 

[PT20J]

From the Concorde battery manual…

[N201MKTurbo]

27 minutes ago, mcarterak said:

Which model PP VR did you install? I don't see the J listed on the R1224 eligibility list. 

The alternator is listed. The idiot light polarity is reversed. I fixed it.

[mcarterak]

17 minutes ago, PT20J said:

From the Concorde battery manual…

 

I've seen this before...is this supposed to be the maximum output? Or the float setting? If I pull every CB I can get the bus volts to show 14.0 but that's about it. I have a Zeftronics R15300 non-adjustable VR. 

[PT20J]

1 minute ago, mcarterak said:

I've seen this before...is this supposed to be the maximum output? Or the float setting? If I pull every CB I can get the bus volts to show 14.0 but that's about it. I have a Zeftronics R15300 non-adjustable VR. 

I don't know what you mean by float. If everything is working properly, the voltage regulator will maintain a constant bus voltage regardless of load. When the load current increases, the alternator output voltage wants to drop and the voltage regulator increases the field current to maintain a constant bus voltage. Conversely, when the load decreases, the alternator output voltage wants to rise and the voltage regulator decreases the field current in order to keep the bus voltage constant.

[Vance Harral]

If you're really trying to understand/fix bus voltage in the range of tenths of a volt, then one critical factor is how (where) you measure the bus voltage.  Ideally you'd connect the measurement device to the sense terminal of the voltage regulator to start, then sample a few other points: circuit breaker busses, power input of instruments, etc.  It's common to have a few tenths or hundreds of millivolts IR drop between the regulator and other locations.

Note that cigarette lighter plug-ins and other cheap, portable voltage measurement devices are especially susceptible to IR drop.  You don't want to crank up your regulator based on measuring a voltage that's electrically "far" from the output of the regulator itself.

[A64Pilot]

Ideally on a non temp compensated regulator and a lead acid battery voltage should be between 13.8 to 14.2, 14 plus or minus .2.

Same as older cars

Having said that it’s exceedingly common to read lower voltage as we aren’t reading alternator output voltage but voltage a long way down the road with many connections between where we are taking the reading and the source and it’s not uncommon to lose a little bit at each old connection.

To start chasing the problem I’d start by connecting a good multimeter directly to alternator output.

Ideal world we would charge batteries at 14.3V, then once charged float them at 13.3, those are middle of the road number as different batteries and different chemistries will require slightly different voltages, usually flooded requires higher voltage than AGM.

‘Some more advanced than what we run voltage regulators will follow a three stage charging profile, modern cars might, I think our Tesla does for example (it has a 12V lead acid battery for initial boot up prior to the big battery being connected)

 

[Ragsf15e]

10 hours ago, mcarterak said:

Spin off from the other thread, where does your bus voltage float during cruise flight? I've been troubleshooting electrical gremlins on my 14v J, I managed to clean up some light corrosion, but can only get my bus to show 13.4 after start, and floats at 13.3 max during cruise. Am I chasing my tail? 

I think you’ve got an issue to find.  You should see constant 14v +/- 0.1 or so.  The Zeftronics Troubleshooting guide is good, but I followed the voltage out of the alternator with my engine running to find my issue.

[Ragsf15e]

2 hours ago, A64Pilot said:

Ideally on a non temp compensated regulator and a lead acid battery voltage should be between 13.8 to 14.2, 14 plus or minus .2.

Same as older cars

Having said that it’s exceedingly common to read lower voltage as we aren’t reading alternator output voltage but voltage a long way down the road with many connections between where we are taking the reading and the source and it’s not uncommon to lose a little bit at each old connection.

To start chasing the problem I’d start by connecting a good multimeter directly to alternator output.

Ideal world we would charge batteries at 14.3V, then once charged float them at 13.3, those are middle of the road number as different batteries and different chemistries will require slightly different voltages, usually flooded requires higher voltage than AGM.

‘Some more advanced than what we run voltage regulators will follow a three stage charging profile, modern cars might, I think our Tesla does for example (it has a 12V lead acid battery for initial boot up prior to the big battery being connected)

 

Just to be clear for the OP though, the airplane will (should) keep the constant 14v you mentioned, not the lower float charge.  

[mcarterak]

1 hour ago, Ragsf15e said:

I think you’ve got an issue to find. 

Yeah, I think so too and I've gone around in circles without finding a smoking gun so far, but happy to have the confidence from others that what I'm seeing is low and to keep troubleshooting...

[Ragsf15e]

1 hour ago, mcarterak said:

Yeah, I think so too and I've gone around in circles without finding a smoking gun so far, but happy to have the confidence from others that what I'm seeing is low and to keep troubleshooting...

I went in circles for 2 years.  It’s a process.  Make sure you’re getting 14v out of your alternator, then at least you know the alternator and vr are ok.  Good luck!

[A64Pilot]

6 hours ago, Ragsf15e said:

Just to be clear for the OP though, the airplane will (should) keep the constant 14v you mentioned, not the lower float charge.  

Yes, I was saying that there is a such thing as float voltage, but our VR’s I believe are dumb, not even temperature compensated, but then they have been working just fine without all the bells and whistles for decades.

However if his voltage is really that low he’s likely killing his battery, a slow death from sulphation.

 

[N201MKTurbo]

15 hours ago, Vance Harral said:

If you're really trying to understand/fix bus voltage in the range of tenths of a volt, then one critical factor is how (where) you measure the bus voltage.  Ideally you'd connect the measurement device to the sense terminal of the voltage regulator to start, then sample a few other points: circuit breaker busses, power input of instruments, etc.  It's common to have a few tenths or hundreds of millivolts IR drop between the regulator and other locations.

Note that cigarette lighter plug-ins and other cheap, portable voltage measurement devices are especially susceptible to IR drop.  You don't want to crank up your regulator based on measuring a voltage that's electrically "far" from the output of the regulator itself.

So, the regulator will regulate to its sense terminals, but the important voltage is the voltage at the battery. Most electrical systems have a low impedance wire from the alternator, through the alternator breaker to the battery cable from the master switch. The connection from the alternator breaker to the battery buss is usually where the sense wire is connected. Of course the ground path back to the battery can screw everything up. The cigarette lighter plug is as good a place as any to measure the battery bus voltage, it is connected directly to the battery bus through its breaker or fuse and there would only be an IR drop if current was being drawn from the lighter plug.

[PT20J]

The electrical system is designed to supply a constant voltage to power the electrical equipment on the airplane. The battery is primarily for starting and secondarily for backup power if the alternator or generator fails. The electrical system charges the battery by applying a constant voltage higher than the battery open circuit voltage. There is no float mode.

Batteries self discharge. A  battery maintainer (or a charger with a maintenance) applies a float voltage to the battery which is just high enough to offset the self discharge and maintain the battery charged at full capacity.

 

[mcarterak]

1 hour ago, N201MKTurbo said:

So, the regulator will regulate to its sense terminals, but the important voltage is the voltage at the battery. Most electrical systems have a low impedance wire from the alternator, through the alternator breaker to the battery cable from the master switch. The connection from the alternator breaker to the battery buss is usually where the sense wire is connected. Of course the ground path back to the battery can screw everything up. The cigarette lighter plug is as good a place as any to measure the battery bus voltage, it is connected directly to the battery bus through its breaker or fuse and there would only be an IR drop if current was being drawn from the lighter plug.

Well, I feel pretty stupid. All of the wiring chasing I did, I decided to finally test voltage at the cigarette lighter plug with the engine running...14.18V, while the EIS showed 13.5V. 

So, seems like the charging system is working fine, but my voltage drop is somewhere either in the avionics bus or in front of the EIS...

[Ragsf15e]

39 minutes ago, mcarterak said:

Well, I feel pretty stupid. All of the wiring chasing I did, I decided to finally test voltage at the cigarette lighter plug with the engine running...14.18V, while the EIS showed 13.5V. 

So, seems like the charging system is working fine, but my voltage drop is somewhere either in the avionics bus or in front of the EIS...

Yes, I felt pretty stupid to when I realized mine was similar… 14v was coming through the firewall but the connection to the bus was bad.  Seems easy looking back.

Trace that 13v back to where it connects to 14v bus and you’ll find the issue.

[Vance Harral]

20 hours ago, N201MKTurbo said:

The cigarette lighter plug is as good a place as any to measure the battery bus voltage, it is connected directly to the battery bus through its breaker or fuse and there would only be an IR drop if current was being drawn from the lighter plug.

I speak from direct experience when I say the cigarette lighter plug may have significant IR drop.  There are two reasons for this.  The lesser is that some of the "cheap" USB chargers that display voltage actually draw non-trivial current.  The greater reason is that the wiring at the cigarette lighter, and the mechanism that connects it to the back of the port, was never designed for continuous use; and on older airplanes it has often been neglected and is in poor shape, and therefore resistive.  I have personally observed the measured voltage at that port be about 250mV lower than the voltage measured at the battery terminals.

I have no doubt many airplanes are in good shape and do not exhibit this behavior.  But it's worth noting.  You correctly state that the voltage at the battery terminals is the one that matters with respect to charging and battery health.  What I'm trying to convey - hopefully in a helpful manner - is that there is not a single value of "bus voltage" everywhere in the airplane.  Even with a theoretically perfect measurement device (infinite impedence), the voltage one will find at various points in the system (battery, regulator output, cigarette lighter, input to radios and instruments) varies by at least a few millivolts, and in some cases a few hundred millivolts.  This is understandably confusing to people whose V=IR educational punishment was light and/or long ago.

[PT20J]

The best way check the “voltage” is to use a DMM to check the voltage at several points. They should all be close. The regulator will adjust to maintain the set voltage at its sense point. This point probably varies depending on regulator and aircraft. In my 94 J with a Mooney regulator, it senses at the main bus at the CB panel. 

If the voltage at the battery is much different than the bus, the cause should be determined.