What Causes GPU Charging Current To Rise?

[MikeOH]

9 minutes ago, N201MKTurbo said:

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.

All true.

My point was that a shorted cell is consistent with his observation of the CHANGE in GPU behavior.  As you pointed out, in order to test for OCV you'd need to charge the batteries which will be BAD for the battery if, in fact, a cell is shorted.

It sounds like it is difficult to get to his batteries but it would be useful to measure each battery's OCV at whatever their present charge state is.  A roughly 1V mismatch would be highly suggestive of a shorted cell in the battery with the lower voltage.

[N201MKTurbo]

2 minutes ago, MikeOH said:

All true.

My point was that a shorted cell is consistent with his observation of the CHANGE in GPU behavior.  As you pointed out, in order to test for OCV you'd need to charge the batteries which will be BAD for the battery if, in fact, a cell is shorted.

It sounds like it is difficult to get to his batteries but it would be useful to measure each battery's OCV at whatever their present charge state is.  A roughly 1V mismatch would be highly suggestive of a shorted cell in the battery with the lower voltage.

No harm in charging a battery with a shorted cell, it is already toast, you have nothing to lose.

[N201MKTurbo]

Shorted cells usually come from a seriously deteriorated battery, where parts of the lead plates falls to the bottom of the cells. These lead chunks short the cell. I'm not sure this can happen with an AGM cell. I have never seen it, but I haven't seen everything either.

[MikeOH]

2 hours ago, N201MKTurbo said:

No harm in charging a battery with a shorted cell, it is already toast, you have nothing to lose.

I believe he has TWO batteries in series, so the concern is damage to the OTHER, presumed good battery.

[MikeOH]

2 hours ago, N201MKTurbo said:

Shorted cells usually come from a seriously deteriorated battery, where parts of the lead plates falls to the bottom of the cells. These lead chunks short the cell. I'm not sure this can happen with an AGM cell. I have never seen it, but I haven't seen everything either.

Yuup.  This may turn out to be something entirely unrelated to the batteries.

[1980Mooney]

29 minutes ago, MikeOH said:

I believe he has TWO batteries in series, so the concern is damage to the OTHER, presumed good battery.

No. It has 2 batteries in Parallel. This is a Rocket Engineering conversion. They did the same thing with the Rocket and Missile conversion. There are 2 batteries in the tail - In addition to more cranking power, they also act as big Charlie weights to offset the extra 200+ lbs added forward of the firewall. 
 

Early J’s and K’s had two (2) 12 V Conchorde’s and later ones had two (2) 24V Concorde’s. 
 

 But you are right that the other battery could become overcharged and damaged due to excessive charging of the possibly dead cell undervoltage battery. 

Edited September 5 by 1980Mooney

[Fly Boomer]

44 minutes ago, 1980Mooney said:

No. It has 2 batteries in Parallel. This is a Rocket Engineering conversion. They did the same thing with the Rocket and Missile conversion

They are in series.  My airplane started life as a 252.

[Fly Boomer]

3 hours ago, N201MKTurbo said:

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

I think my cigar lighter volt meter will give the best reading I can get.  The cigar lighter is hot all the time, so I don’t need to turn on the master — just plug it in.

[Fly Boomer]

3 hours ago, MikeOH said:

sounds like it is difficult to get to his batteries but it would be useful to measure each battery's OCV at whatever their present charge state is

I can get pretty close to the OCV with the cigar lighter voltmeter.  Not the individual batteries, but the pair in series, which is functionally the same as a single 24 volt battery.

[MikeOH]

@Fly Boomer

The problem is that you don't know the state of charge; at 50% OCV is 24.10V, while 100% is 26.00V.  So, if one cell is dead you could still read 25V.

[1980Mooney]

54 minutes ago, Fly Boomer said:

They are in series.  My airplane started life as a 252.

The 252 was the first 28 volt model. 

From Mooney Chronology  

https://www.mooneyevents.com/chrono.htm

1986 M20K  252 

25-1000 to 25-1066 

Continental TSI0-360-MB(1) (210 HP). Replaces 231 with intercooled engine, Garrett variable wastegate. First model with curved side window. Also, enclosed landing gear; 28V electrical. Four place.

“The next new model, the M20K 252, appeared in early 1986 with a top speed of 252 miles per hour (406 km/h). It replaced the 231 and achieved its higher speed with the same 210 horsepower (160 kW) engine. It featured a new 28-volt electrical system to power additional equipment and to improve cold-weather starting.”

[Fly Boomer]

On 9/4/2024 at 8:05 PM, MikeOH said:

@Fly Boomer

The problem is that you don't know the state of charge; at 50% OCV is 24.10V, while 100% is 26.00V.  So, if one cell is dead you could still read 25V.

The batteries have started the engine 12 or 15 times since new.  No other heavy load or abuse since new.  Doesn't rule out a dead cell, but I'm hoping they are just run down from a few starts without time on the alternator to fully recover.  I'll try the GPU again, and see what happens.  News at eleven.

[Fly Boomer]

On 9/4/2024 at 8:35 PM, 1980Mooney said:

The 252 was the first 28 volt model. 

Most Rockets were modified 231s, but a few were 252s.  The drawings show the batteries in parallel for the 231 (14v), and in series for the 252 (28v).

[MikeOH]

6 minutes ago, Fly Boomer said:

The batteries have started the engine 12 or 15 times since new.  No other heavy load or abuse since new.  Doesn't rule out a dead cell, but I'm hoping they are just run down from a few starts without time on the alternator to fully recover.  I'll try the GPU again, and see what happens.  News at eleven.

I agree that it seems unlikely to be the batteries as they are new.

My point with state of charge was only to point out that measuring the total OCV (both batteries) won't be indicative of a shorted cell UNLESS you know the percent of charge.  You need to be able to measure them separately.

[Fly Boomer]

9 minutes ago, MikeOH said:

I agree that it seems unlikely to be the batteries as they are new.

My point with state of charge was only to point out that measuring the total OCV (both batteries) won't be indicative of a shorted cell UNLESS you know the percent of charge.  You need to be able to measure them separately.

Point well taken.  I don't currently have a means to separate them but, if the OCV cannot be raised to a near-100% SOC, they will have to come out again.

[EricJ]

35 minutes ago, Fly Boomer said:

Point well taken.  I don't currently have a means to separate them but, if the OCV cannot be raised to a near-100% SOC, they will have to come out again.

You can also measure the OCV of each battery independently.   If there is a bad cell in one, it may show up that way.   A dead or weak cell is just one thing that could be wonky, it may not be likely, but it should be fairly easy to do some basic checks (like overall OCV, OCV of each, etc.), and recheck the charge current characteristic and get some basic ideas about the battery health.

[1980Mooney]

48 minutes ago, Fly Boomer said:

Most Rockets were modified 231s, but a few were 252s.  The drawings show the batteries in parallel for the 231 (14v), and in series for the 252 (28v).

I went to the Concorde site to look at approved applications.  Amazingly Rocket Engineering never got the 24 volt Concorde batteries approved for either the later model M20J's or M20K 252's. (all with 28 volt systems).  All older Missiles (M20J) and Rockets (M20K) with 14 volt systems have 2 batteries in parallel.  The smaller number of later Missiles and Rockets with 28 volt systems have two (2) 12 volt batteries in series.  @MikeOH was correct in your application/model.  I am not sure which (parallel vs series) is more easily damaged by overcharging if one cell on one battery is bad.  I have always changed out both batteries together.  They typically last 6-7 years.  Never had any charging problem.

[1980Mooney]

As you previously mentioned, that battery box is about 75 lbs and has to be pulled down a wooden ramp (battery board) that you temporarily install.  In order to get to it and reach the box, you have to wedge your body into the tailcone a bit.  You will wind up leaning on something in the tailcone putting additional weight back there.  

[1980Mooney]

On 9/4/2024 at 2:46 PM, Fly Boomer said:

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.

Before you pull the battery box, are you sure that there is not a current drain somewhere?  There are two (2) hot wires that go from the battery to the front of the plane when the Master is off if I am not mistaken - 1) the hot wire going to the cabin and baggage lights (and a clock on some models) and 2) the wire going to the Master Switch which energizes the master power solenoid/relay when flipped on.

If I were you, I would disconnect  the 24 v. positive line coming from the battery box right at the master power solenoid/relay (which is in the tail cone next to where the original battery before the conversion was located.)  Use an ammeter to see if there is any current flow that would be slowly draining the battery.  You can also check the voltage before the Master is on.

If you have a drain, then the batteries are just run down.  You will need to find where the current leak is.

If it is a bad cell in a battery, then they should be within the Concorde 6 month Full Replacement Warranty period.

[Fly Boomer]

12 hours ago, EricJ said:

You can also measure the OCV of each battery independently.   If there is a bad cell in one, it may show up that way.   A dead or weak cell is just one thing that could be wonky, it may not be likely, but it should be fairly easy to do some basic checks (like overall OCV, OCV of each, etc.), and recheck the charge current characteristic and get some basic ideas about the battery health.

If I end up needing to measure them separately, the battery box with batteries must be pulled out of the tail.  It's about halfway back on a custom shelf, and the box with batteries weighs about 75 pounds, so I'll have to get some help if it comes to that.

[Fly Boomer]

12 hours ago, 1980Mooney said:

I am not sure which (parallel vs series) is more easily damaged by overcharging if one cell on one battery is bad. 

I spoke to a Concorde engineer when I was still trying to resurrect the old batteries.  He said there is no difference between one 24-volt battery with 12 cells in series and two 12-volt batteries wired in series.  I suppose there might be some resistance in the cable that connects the two batteries, but aside from that, no difference.  It made sense when he explained it.  A battery of cells is multiple cells wired in series, and packaged in a sealed box, so two batteries in series is just more cells in series.

[A64Pilot]

On 9/4/2024 at 5:00 PM, 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. 

Constant current often isn’t a “mode”, it’s simply what happens, but marketing likes to call it modes making people think their charger is more sophisticated and is actually changing modes. However even if current is adjustable, current should be maintained and voltage should increase not decrease.

Most often, just like in this example it’s just the limit the charger can provide, often with good chargers charging is three stage, first constant current because you hit the limit the charger can provide. During this phase voltage is below set point and voltage slowly climbs until it hits set point, then the charger maintains voltage and amperage drops. If you have a three stage charger ideally once you hit .5% acceptance rate of the batteries rating in Amp hours the charger drops to float, but those are expensive chargers, most chargers simply drop to float based on a timer or just continue at the higher voltage because they don’t have a float mode.

Many maintainers actually drop to float based on output current.

However what he has isn’t a charger specifically, it’s a power supply, if the voltage is set on the power supply then that voltage should be maintained, unless of course it’s power limited. then it hits its max power output and if left connected over time the battery voltage will climb until it hits set point, at that time current will slowly decrease, but will never reach zero, but the voltage shouldn’t drop unless the load is increased.

However even if current limited a power supply connected to a partially charged lead acid battery, the voltage shouldn’t drop during charging it should slowly climb. Unless somethings wrong as in bad connections or something the only way the voltage will drop is if there is a reduction in amperage or increase in load, turn on your landing light and of course voltage will drop for instance.

My theory is either there is a high resistence connection somewhere or the power supply started reducing amperage due to it hitting a thermal limit as a way to protect itself from overheating.

[Fly Boomer]

9 hours ago, 1980Mooney said:

There are two (2) hot wires that go from the battery to the front of the plane when the Master is off if I am not mistaken - 1) the hot wire going to the cabin and baggage lights (and a clock on some models) and 2) the wire going to the Master Switch which energizes the master power solenoid/relay when flipped on.

The current check is a good idea.  Last time I was in the hangar, I double checked that the master was off and that both the cabin lights were off.  As you point out, those are hot all the time, and can kill a battery.  What I forgot to remember is the clock, which probably puts a small constant drain on the battery.  Also, I believe there is a small drain for the fuel computer so it can remember some things between flights.  That said, an ammeter is the only way to determine what the drain is.

[Fly Boomer]

5 minutes ago, A64Pilot said:

My theory is either there is a high resistence connection somewhere or the power supply started reducing amperage due to it hitting a thermal limit as a way to protect itself from overheating.

I don't think it was on long enough to reach any thermal limit, and the internal fan cycled on and off which tells me the machine is protecting itself.  What happened is the voltage was set at 28.2.  Usually the current inflow starts high and comes down rapidly.  The current did go high (higher than I had seen it before), and the current did come down, but eventually the current started going back up and, when the power supply hit the maximum current it could supply at 28.2 volts (a little over 20 amps), it switched from constant voltage to constant current and started reducing the voltage.

[A64Pilot]

Most multimeters can measure small amounts of current.

If you disconnect a battery cable when yiu touch it back to the battery there should be no or at most a tiny spark, if there is a significant spark then of course you have s drain.

With the cable disconnected the multimeter an be placed inline to the cable and battery and will measure any current flow, everyone I’ve owned is fuse protected if the draw exceeds the current capacity of the multimeter, which isn’t large