Battery capacity checks

[MikeOH]

@Vance Harral

FWIW, my understanding is that the alternator field is the rotating part which, if the field wire becomes disconnected, might have residual magnetism if parts are ferrous.  The stator is three coils acting as the armature.  Thing is, I believe each of those stator coils is connected to a diode which makes your observation even more strange!  No 'parasitic load' should exist if all three of those diodes are functional.

I wonder if the magneto return currents introduce enough of a ground offset voltage to explain the discrepancy??

[PT20J]

2 hours ago, Vance Harral said:

On an anecdotal side-note about battery capacity...

This weekend, one of my airplane partners had an alternator failure in our airplane - exactly the contingent situation being discussed here.  This occurred in IMC, though fortunately he was only a few minutes from home, with a healthy battery and a dual G5 setup that has it's own backups.  Anyway... we've done capacity checks on our battery using the "turn on the stuff in the airplane in the hangar" method, and being a bit of an electrical engineering nerd, I even know what the voltage vs. remaining capacity curve looks like for our Concorde RG-35A battery, driving an "emergency" electrical load.  The interesting thing is that when the alternator failed - which my partner was alerted to immediately via the bus voltage annunciator - he observed an indicated battery voltage of 11.7V.  That's abnormally low for a healthy battery, even taking into account the IR drop between the battery terminals and the voltage gauge.  It's much lower than we ever saw on that gauge in our capacity test.  Once he got on the ground and shut down, the indicated voltage immediately went back to 12.3.  Voltage at the actual battery terminals was slightly higher, of course; but the point is that battery voltage with the engine running was lower than with it stopped.

The cause of the failure turned out to be simple: just a broken field wire at the alternator.  But his observations suggest that a spinning alternator with a broken field wire actually puts an additional electrical load on the battery, beyond the "turn on all the switches" load one might use in a practical capacity test.  I can see how that might be the case, because the alternator output wire to which the battery is connected, would at that point also be connected to an un-energized rotating electro-magnetic system.  I confess I haven't dug out my electromechanical machines textbooks to verify that idea, though.  Anyone know if this is actually the case?

If the hypothesis is correct, then in the event of electrical failure, it might be a good idea to pull the main alternator breaker (not the field breaker) to disconnect that parasitic load.  That assumes you can pull the breaker, though.  The main alternator breaker in our airplane is of a type that cannot be manually pulled, for whatever reason.

Anecdotally, it seems like the most common cause of alternator failure is a broken field wire at the crimp to the ring terminal where it attaches to the alternator. I have added strain relief to mine and also inspect it every time the cowling is off.

The differences in voltage engine on and off are interesting and worth exploring to better understand. At first look, it doesn't make a lot of sense that the alternator could be the cause. First the field is the rotor and so with no voltage applied, the rotor doesn't do much except spin around. Unlike generators, alternator fields have very little residual magnetism when unpowered. The stator output is connected to the ALT breaker through diodes so current should not flow in reverse. You could verify this if you could pull the ALT breaker, but yours isn't the push on-pull off type (This is probably because the designer never thought there would be a reason to pull it. The breaker has a higher rating than the alternator output and it's purpose is to protect the wire to the alternator from excessive current from the battery should there be a short circuit).

I might fly around long enough so that the battery is fully charged and then land and note the voltage, and then pull the field breaker and not the voltage, and then shut down the engine without turning anything off and note the voltage again to see if it is reproduceable. 

[Vance Harral]

5 minutes ago, PT20J said:

Anecdotally, it seems like the most common cause of alternator failure is a broken field wire at the crimp to the ring terminal where it attaches to the alternator.

This is exactly what happened during my partner's flight.  Trivial fix, but the strain relief idea is a good one that we didn't think of.  I'll bring this up with our mechanic at the next inspection.

 

6 minutes ago, PT20J said:

I might fly around long enough so that the battery is fully charged and then land and note the voltage, and then pull the field breaker and not the voltage, and then shut down the engine without turning anything off and note the voltage again to see if it is reproduceable. 

Excellent idea, we'll give this a try and report back.

[Pinecone]

Some power supplies allow you to set the max current and the max voltage.  So you could set the current to the C/10 rate and the voltage to the 34 (0r 17) volts max for the conditioning charge and leave it for 16 hours.

 

[Fly Boomer]

12 hours ago, PT20J said:

most common cause of alternator failure is a broken field wire at the crimp to the ring terminal where it attaches to the alternator. I have added strain relief to mine and also inspect it every time the cowling is off.

What sort of strain relief did you employ?

[PT20J]

2 minutes ago, Fly Boomer said:

What sort of strain relief did you employ?

Heat shrink and I zip tied the field wire securely to the large output wire. 

[Fly Boomer]

1 minute ago, PT20J said:

Heat shrink and I zip tied the field wire securely to the large output wire. 

Makes sense.  My wires seem kind of floppy.  I will start with half of your solution, and zip tie them together.  Thanks.

[dkkim73]

Capacity testing sheet above from Concorde says to discharge ideally at C1, which is 13.6 (Ah->A) for the Concorde RG24-15, so power = 24V * 13.6 = 326W. 

Most of the less expensive testers don't go that high. This one (https://www.amazon.com/KP184-Electronic-Precision-Capacity-Resistance/dp/B0CD7LM5R3/) does but is pricier that the one above. @EricJ mentioned on that thread that a "correction factor" could be applied if not done at C1. Does anyone know the proper method for that? (is it just "farther down" the capacity curve, which I would think is nonlinear, or something more nuanced). 

Also, if one needs 34V for a conditioning charge for a 24V nominal battery, than that would also push to a higher-voltage charger than mentioned above. 

DK

[BlueSky247]

So, serious question. If one were to buy a new concorde from aircraft spruce, would they then need to do that deep charge, or what?

[Jerry 5TJ]

20 minutes ago, BlueSky247 said:

So, serious question. If one were to buy a new concorde from aircraft spruce, would they then need to do that deep charge, or what?

They should charge it per the capacity test procedure and then do the C1 capacity test to provide a baseline for the new battery before installation.   

[PT20J]

32 minutes ago, BlueSky247 said:

So, serious question. If one were to buy a new concorde from aircraft spruce, would they then need to do that deep charge, or what?

It’s all in the manual:

[EricJ]

1 hour ago, dkkim73 said:

Capacity testing sheet above from Concorde says to discharge ideally at C1, which is 13.6 (Ah->A) for the Concorde RG24-15, so power = 24V * 13.6 = 326W. 

Most of the less expensive testers don't go that high. This one (https://www.amazon.com/KP184-Electronic-Precision-Capacity-Resistance/dp/B0CD7LM5R3/) does but is pricier that the one above. @EricJ mentioned on that thread that a "correction factor" could be applied if not done at C1. Does anyone know the proper method for that? (is it just "farther down" the capacity curve, which I would think is nonlinear, or something more nuanced). 

Also, if one needs 34V for a conditioning charge for a 24V nominal battery, than that would also push to a higher-voltage charger than mentioned above. 

DK

Here's a screenshot of a MathCAD sheet I did to play with it.    This example is for the typical Concorde RG35-AXC, discharging at half the rated current (33A), or 16.5A, which yields and expected discharge time of 2.219 hours.    So compare the discharge voltage to that time to determine % capacity instead of one hour.  The basic relation is shown as the "actual discharge time to capacity".   The first tester I had was limited to less than 33A, (I think it was 20A or something), so I used 1/2 current and used the adjustment factor.  There's a link to a reference document as well (not a live link, though...sorry, cut-and-paste from MathCAD is sketchy).

[MikeOH]

1 hour ago, dkkim73 said:

Capacity testing sheet above from Concorde says to discharge ideally at C1, which is 13.6 (Ah->A) for the Concorde RG24-15, so power = 24V * 13.6 = 326W. 

Most of the less expensive testers don't go that high. This one (https://www.amazon.com/KP184-Electronic-Precision-Capacity-Resistance/dp/B0CD7LM5R3/) does but is pricier that the one above. @EricJ mentioned on that thread that a "correction factor" could be applied if not done at C1. Does anyone know the proper method for that? (is it just "farther down" the capacity curve, which I would think is nonlinear, or something more nuanced). 

Also, if one needs 34V for a conditioning charge for a 24V nominal battery, than that would also push to a higher-voltage charger than mentioned above. 

DK

While I suppose you can get a fair approximation using Peukert's correction factor I went ahead and purchased the Kunkin KP184 that you referenced above so that I could perform capacity testing IAW the Concorde MM. It was like $165 with tax and shipping...yeah, pricier than $39.95 but in aviation terms the cost is lost in the rounding (and I'm a total CB!) and I can perform the specified testing.  Plus it only takes under 1 hour instead of 2.219 hours

[Fly Boomer]

1 hour ago, BlueSky247 said:

So, serious question. If one were to buy a new concorde from aircraft spruce, would they then need to do that deep charge, or what?

I think the CMM says to check the voltage.  If it's high enough, you don't do anything.  If it's below some threshold, it means the battery has been sitting idle for a bit, and there is a specified procedure to bring it back up.  Here is some of the pertinent stuff:

C. The open circuit voltage (OCV) of a fully charged battery is approximately 26.0 volts (13.0 volts for a12-volt battery). As the battery state of charge drops due to self-discharge, its OCV also declines.
D. Batteries should be boost charged per Section 6 when the OCV declines to 25.0 volts (12.5 volts for a 12-volt battery).
E. Batteries with an OCV below 25.0 volts (12.5 volts for a 12-volt battery) must be capacity tested per Section 7 before being placed in service.
F. Batteries in storage for longer than 24 months must be capacity tested per Section 7 before being placed in service.

"Boost Charge" is defined elsewhere in the document.

[Fly Boomer]

10 minutes ago, MikeOH said:

While I suppose you can get a fair approximation using Peukert's correction factor I went ahead and purchased the Kunkin KP184 that you referenced above so that I could perform capacity testing IAW the Concorde MM. It was like $165 with tax and shipping...yeah, pricier than $39.95 but in aviation terms the cost is lost in the rounding (and I'm a total CB!) and I can perform the specified testing.  Plus it only takes under 1 hour instead of 2.219 hours

Does that device automatically bring the battery back up to full charge after the capacity test?

[MikeOH]

6 minutes ago, Fly Boomer said:

Does that device automatically bring the battery back up to full charge after the capacity test?

No. It's just an electronic load.  I use my battery minder to recharge.  It is capable of 8 Amps which exceeds the 0.2C charge rate specified in the Concorde MM.

[dkkim73]

Thanks! Very helpful responses. 

Two practical questions (for the FIKI Acclaim):

- is it more practical to pull the batteries for testing? (Eg at home)

- are there any labile "memories" in the plane that make it bad to pull both? 

[dkkim73]

Thanks, all, for the great info. I did buy a Kunkin KP-184 electronic load (tester). It's actually quite nifty and did the job well with just a bit of programming. Really just configuring the battery test function, setting the endpoint voltage to drain to (EDV = 20V in this case, for an RG24-15) and the discharge voltage to C1 (=13.6A in this case)... Then hit "On" and it drains and calculates the total A*h (charge, in amp-hours or the unit you specify). The manual is, well, a bit inscrutible  but not too terrible to figure out. 

The Concorde "Component Maintenance Manual" is nice and clear and covers a lot of the basics of battery testing, also. 

Kunkin KP-184 via Amazon: https://www.amazon.com/gp/product/B0CD7LM5R3/

Also bought the Matrix MPS-3210 32V 10A supply ( https://www.amazon.com/gp/product/B0BBZK7H4Y ) recommended in an earlier thread. 

That supply technically does not go to 34V as recommended by the Concorde CMM, but seems adequate for the conditioning charge (brought up from 92% to 105%) and easily does the standard "constant potential" charge. 

Great recommendations, both, thank you! 

[EricJ]

I had an interesting development lately.   I've been capacity testing my Concorde RG-35AXC battery every year or so, and it's been consistently hitting 95-100% capacity testing both during a capacity test and verified during a recharge cycle with my Topdon charger, which also measures capacity during the recharge.   

But the battery performance has still been dropping during that time, as measured by slower cranks, and deeper voltage sags during starter operation.    I did some engine work a couple weeks ago, and when I went to start it after reassembly it did a quarter turn and stopped.   Erk.  On a second try it turned enough to start, and I figured I'd try a local flight and see what it did.   It started for that as well, but then during a hot start at the fuel pump after returning it was definitely starting to turn a lot slower just before the engine started.

So I got a new replacement, which is now back to spinning the prop like crazy with the skytec, and thought I'd check the old battery to see how it's doing.

Open circuit voltage on the old battery was still great, 12.8V when I took it out of the airplane, which showed as 12.9V with a different multimeter before I started the capacity test after bringing it home.   I got a Kunkin tester to replace my old semi-homebrew unit which could ony test at half current, so this test went more quickly.   The result was 27.13Ah capacity, or 27.13/33 = 82% capacity.    When the Topdon recharged it, it reported a 28.1Ah capacity, which is still only 85% capacity, which is Concorde's recommended threshold for removal from service.   OCV after the recharge was 13.3V.  Since it was already having trouble I didn't think it was worth attempting a rehab cycle, especially since I already had a replacement.   If replacements weren't available a rehab attempt would have definitely been an option.

The battery is nearly six years old, and has lived in Phoenix its entire life and we're not yet through a brutally hot summer with long strings of contiguous record-breaking high temps, so it may have just been enough to push it over the edge.    The more remarkable thing to me is how quickly it went from testing 95+% capacity to notably tired and questionably servicable.   The last capacity test was in March and tested at 34.2Ah, which was confirmed during the recharge by the Topdon at 35Ah (both a little over 100%).   The summer was either hard on it or it was just done, but the change from testing 100% to out-of-service in about six months suggests that other tests, like the voltage sag during start or just observation of starting behavior, may be a more reliable indication of a failing battery.    The capacity test seemed to not provide adequate warning under these particular circumstances.

Anyway, just thought I'd pass that along.    A successful capacity test may not be a reliable indicator of the ability of the battery to make it another year.

[Fly Boomer]

11 hours ago, EricJ said:

A successful capacity test may not be a reliable indicator of the ability of the battery to make it another year.

Does your newer equipment allow a capacity check as documented in the Concorde CMM?

[EricJ]

13 minutes ago, Fly Boomer said:

Does your newer equipment allow a capacity check as documented in the Concorde CMM?

Yes, the load capacity test is pretty straightforward.   The constant-current discharge rate and ending voltage are programmable.  

 

[dkkim73]

13 hours ago, EricJ said:

Anyway, just thought I'd pass that along.    A successful capacity test may not be a reliable indicator of the ability of the battery to make it another year.

@EricJ thank you for the detailed observations. That is food for thought. 

My batteries are only a couple years old and recently tested well. I've been kicking around biting the bullet and getting the approved BatteryMinder connectors and a Minder. As a EE, do you have a sense of how much this helps? (if the plane flies about once every week or two)

[MikeOH]

@dkkim73

I fly once or twice a week and have never used my battery minder (just use it to recharge after a capacity test).  My Concorde RG35-AXC tested over 90% last July and is 5 years old.  Personally, as an EE, I don't think there's a benefit for my usage pattern and, while low risk, I don't like the idea of the battery minder plugged in and running in my hangar when I'm not there.

[EricJ]

4 hours ago, dkkim73 said:

@EricJ thank you for the detailed observations. That is food for thought. 

My batteries are only a couple years old and recently tested well. I've been kicking around biting the bullet and getting the approved BatteryMinder connectors and a Minder. As a EE, do you have a sense of how much this helps? (if the plane flies about once every week or two)

The Concorde reps come to our local IA seminars and give talks every year, and they have some very good reasons why a minder is a good idea, and they recommend using one.   This is why I use one, plus it's always nice knowing the battery is essentially topped off whenever you go to use it.   It mostly has to do with the way AGM batteries are constructed and behave, and their batteries in particular.   One of the main benefits is the desulfating cycles in the minder that keep the plates clean, which is a primary reason for battery failure.   The rehab process in the CMM is intended specifially to attempt to desulfate the plates, so if they're kept clean by a minder in the first place, the expectation is that the battery life is extended by the minder.

They do only recommend the minder listed specifically for their batteries, since apparently the desulfating and maintenance cycles are best for how their batteris are constructed.

https://www.aircraftspruce.com/catalog/elpages/batteryminder_11-19239.php

For pertinent details I'd recommend attending an IA seminar where the Concorde reps give a session.    IA seminars are usually free and open to pilots and owners as well, usually with Wings credit.    Concorde actually sponsors on-line webinar IA renewal seminars, and there's one in January:

https://www.concordebattery.com/training/ia-renewal-2025.html

Our local IA seminar is usually blessed with sessions by both Concorde and Gill, and I often attend both.   

[MikeOH]

@EricJ

Maybe I need to re-evaluate the risk that my battery minder will burn down my hangar