What Causes GPU Charging Current To Rise?

[Fly Boomer]

Airplane has not been run for a couple of weeks.  Went to hangar over the weekend and, while doing some other things, I plug in the GPU.  Usually, the current jumps way up as soon as I push the NATO AN2551 plug in, but then starts dropping precipitously until it gets down below 1 amp.  This time, it jumped up to about 20 amps, followed by a fairly quick drop to about 18, followed by a slow climb back up to a little over 20 amps, which is all my GPU can put out.  Batteries are only a few months old -- pair of RG-35AXC batteries in series.  Any ideas?

[EricJ]

It's supplying a load somewhere.   Is the battery the only thing in the circuit?    

[201Mooniac]

I would have disconnected the GPU and turned on the master to see if for some reason the batteries had been completely discharged, like leaving the interior lights on.  Otherwise as Eric says, there is a load somewhere

[1980Mooney]

1 hour ago, Fly Boomer said:

Airplane has not been run for a couple of weeks.  Went to hangar over the weekend and, while doing some other things, I plug in the GPU.  Usually, the current jumps way up as soon as I push the NATO AN2551 plug in, but then starts dropping precipitously until it gets down below 1 amp.  This time, it jumped up to about 20 amps, followed by a fairly quick drop to about 18, followed by a slow climb back up to a little over 20 amps, which is all my GPU can put out.  Batteries are only a few months old -- pair of RG-35AXC batteries in series.  Any ideas?

If they have been drained dead, this may save you about $1 K

 

[Fly Boomer]

26 minutes ago, 1980Mooney said:

If they have been drained dead, this may save you about $1 K

Thanks for the video.  I checked to see if any switches had been left on (including cabin and baggage light), but everything was off.  What I didn't think to do is check the OCV before I plugged the GPU in.  I'll check that voltage tomorrow.

[Fly Boomer]

1 hour ago, EricJ said:

It's supplying a load somewhere.   Is the battery the only thing in the circuit?    

Unless something has shorted, the battery is the load.  It has been typical for the current draw to drop to something under an amp when I unplug and go home.  Before I put in the new batteries, a Concorde engineer said "To accommodate the variations in the voltage vs temperature chart, just pick something in the middle like 28.2 or 28.3 volts."  I run the GPU as close to 28.2 as I can get.  The CMM says between 60dF and 113dF 28.0 is an okay setting for the voltage regulator, so 28.2 is a tiny bit hot but probably okay.

[Fly Boomer]

1 hour ago, 201Mooniac said:

I would have disconnected the GPU and turned on the master to see if for some reason the batteries had been completely discharged, like leaving the interior lights on.  Otherwise as Eric says, there is a load somewhere

Nothing was left on, but I didn't turn on the master.  I can probably capture something pretty close to the OCV with a cigar lighter voltmeter I have.  I'll try that tomorrow.

[EricJ]

How long did it stay at 20A?    You could disconnect everything and check the open circuit voltage of the batteries just to make sure nothing has gone wonky, like a dead cell or something.

 

[MikeOH]

Yeah, I was thinking dead cell...a little weird if the both batteries are new; maybe 'infant mortality'?

Conclusive test would be to lift the ground cable off the battery and rig the GPU straight to the battery...then you'll know if it's the batteries causing the 20A load.

[Fly Boomer]

41 minutes ago, EricJ said:

How long did it stay at 20A?    You could disconnect everything and check the open circuit voltage of the batteries just to make sure nothing has gone wonky, like a dead cell or something.

 

I watched it for about 10 minutes, and my GPU topped out just over 20 amps (the max).  Eventually, the voltage started coming down below where it had set it (28.2).  I don't understand how my GPU works, but I can set either the voltage or current, and the other will float.  Anyway, when the voltage started coming down, I unplugged it.

[N201MKTurbo]

If the battery was a bit low, and you did a constant voltage charge, it will start charging at the supplied voltage, as it charges, it heats up the battery and its internal resistance lowers because the chemical reactions go faster when it is warm. When it hits the current limit on your charger, the charger goes into constant current mode and it will charge at that current until the charge state of the battery increases to where it goes back to constant voltage mode and the charge current will drop until the float voltage of the battery  rises to the voltage of your charger where the charge current will go to near zero. 

[MikeOH]

What voltage did it indicate before you shut it down?

[Fly Boomer]

10 minutes ago, MikeOH said:

Yeah, I was thinking dead cell...a little weird if the both batteries are new; maybe 'infant mortality'?

Conclusive test would be to lift the ground cable off the battery and rig the GPU straight to the battery...then you'll know if it's the batteries causing the 20A load.

It's a good idea, but I would probably have to pull the battery box forward in the tail in order to get the lid off and unhook a cable.  Some things need to be removed in the avionics bay, and then a plywood ramp installed to slide the battery box forward.  There is a smallish inspection cover on the side of the fuselage, but only big enough for one hand at a time.  This is a Rocket, and the batteries are WAY back in the tail.  Each battery weighs 32 pounds, and the battery box with lid weighs about 10 pounds, so that's about 75 pounds to manhandle.  If I can't figure out anything else, this will be the way.

[MikeOH]

Ugh!  Yeah, that sounds like quite a 'project'; my 'F' has one battery right inside the access cover and is super easy to disconnect.

[MikeOH]

As @EricJ said, checking the open circuit voltage should tell you if a cell is shorted/dead.

[Fly Boomer]

2 minutes ago, N201MKTurbo said:

If the battery was a bit low, and you did a constant voltage charge, it will start charging at the supplied voltage, as it charges, it heats up the battery and its internal resistance lowers because the chemical reactions go faster when it is warm. When it hits the current limit on your charger, the charger goes into constant current mode and it will charge at that current until the charge state of the battery increases to where it goes back to constant voltage mode and the charge current will drop until the float voltage of the battery  rises to the voltage of your charger where the charge current will go to near zero. 

So, if I understand this correctly, what I was seeing might be the expected behavior, and unlikely to damage the batteries?

[N201MKTurbo]

3 minutes ago, Fly Boomer said:

So, if I understand this correctly, what I was seeing might be the expected behavior, and unlikely to damage the batteries?

Yes, it sounded like perfectly normal behavior for a run down battery.

[Fly Boomer]

5 minutes ago, MikeOH said:

What voltage did it indicate before you shut it down?

I didn't let it run very long after the voltage started down -- it may have come down to twenty-seven point something.  I don't remember exactly.  I had never seen this behavior from my GPU, so I put a stop to it.  Until this episode, I am accustomed to seeing the voltage stay where I put it while the amps start at 10 or so, and come down quickly.

[Fly Boomer]

6 minutes ago, MikeOH said:

As @EricJ said, checking the open circuit voltage should tell you if a cell is shorted/dead.

That will be my next step.

[MikeOH]

2 minutes ago, N201MKTurbo said:

Yes, it sounded like perfectly normal behavior for a run down battery.

What sounded odd to me is that @Fly Boomer indicated that the voltage DROPPED

[Fly Boomer]

3 minutes ago, N201MKTurbo said:

Yes, it sounded like perfectly normal behavior for a run down battery.

Well, if the OCV is in the "down but not dead" range, I'll hook it back up, and sit with it for a bit to see what happens.

[N201MKTurbo]

1 minute ago, MikeOH said:

What sounded odd to me is that @Fly Boomer indicated that the voltage DROPPED

When the charger hits its current limit, the only way it has to reduce the current is to lower its voltage. That happens when it goes into constant current mode. 

[MikeOH]

Just now, N201MKTurbo said:

When the charger hits its current limit, the only way it has to reduce the current is to lower its voltage. That happens when it goes into constant current mode. 

Correct, but he had NOT seen this behavior before.  However, if one cell is shorted then I would expect to see a drop to around 27V.

[N201MKTurbo]

If you think about it, you charged at 20 amps for 5 minutes. That is 1.6 Amp/Hours of charge. If it is a 35 Amp/Hour battery, It was only down about 4.5% from full charge. 

[N201MKTurbo]

6 minutes ago, MikeOH said:

Correct, but he had NOT seen this behavior before.  However, if one cell is shorted then I would expect to see a drop to around 27V.

If one cell is shorted, it will overcharge the rest of the cells and outgas some hydrogen. It will probably eventually get to a charge state where the current will go down to near zero. It is not good for the battery. The only way to check for a dead cell is to charge the battery, then unhook it from the charger and monitor what voltage it stabilizes at.

A 28 volt battery should stabilize at 25.2V, with a dead cell it should settle to 23.1V. These voltages are temperature and charge dependant. The service manual has charts in it that give you these values.

These are the open circuit voltages of the batteries. Anything above this is coming from the charging system.