Why does my battery stop charging?

[KHyde]

Has anyone else experienced a failure of their alternator (or some other electrical culprit) due to operation on the ground before takeoff in hot conditions? We are operating in Texas and it’s regularly 100F in the afternoons. When I start up everything is operating normally, but before I can make it to takeoff the battery stops charging. Our mechanic initially couldn’t duplicate the error, but after further discussion with him, we decided that the alternator was getting heat saturated and breaking down. Well we have a new alternator and I went to take the plane out for a test flight post maintenance and everything was great until about 7 minutes after starting. At that point the battery stopped charging. We have a 1989 J model. 
 

Any suggestions or ideas on possible culprits of the problem would be much appreciated. 
 

Thanks!

[ArtVandelay]

I’d do some troubleshooting:

.https://zeftronics.com/wp-content/uploads/2014/06/Z14PS.pdf

[EricJ]

What's the battery voltage when it stops charging?

 

[KHyde]

The voltage drops from about 27 when it’s charging to 24.3 and then slowly ticks down to around 24 while I try resetting CBs and what not to fix it. Sometimes it will charge for like a second after I flip the alternator switch back on but then it drops again. 

[EricJ]

Does it come back if you increase rpms?   If not, it sounds like the voltage regulator may be getting weak.

[PT20J]

Never heard of a heat soaked alternator. Might be a voltage regulator. Doubt it has anything to do with heat -- the regulator is in the cockpit -- but if you think so get out to the airport early when it's cooler and see if it makes a difference. What's the annunciator doing? If VOLTS is steady on, the OVP tripped.

[KHyde]

Increasing RPMs seems to have no change in the voltage or on the amperes gauge.  When I start up the gauge indicates a positive charge and the battery shows 27V. But then it just randomly drops off and the ampere gauge indicates in the negative and the battery shows in the 24V regime. The only thing that momentarily gets any sort of fluctuation in the ampere gauge is to cycle power on the alternator switch or reset the “Field/Alt 1” CB. 
 

I don’t recall seeing any lights on the indicator panel until I physically switched off the alternator switch while troubleshooting. The only big red warning was my engine monitor flashing red voltage numbers at me. The reason my hunch is heat related is because I have flown earlier in the mornings or on an afternoon that’s slightly cooler with no delays on the ground and have taken off with no issues throughout the flight. I’m just leery to take any family trips now, because it seems to be overall unreliable/unpredictable and I don’t want to get stuck off station or get into a self induced situation where I need the manual gear extension.  

[LANCECASPER]

35 minutes ago, KHyde said:

Increasing RPMs seems to have no change in the voltage or on the amperes gauge.  When I start up the gauge indicates a positive charge and the battery shows 27V. But then it just randomly drops off and the ampere gauge indicates in the negative and the battery shows in the 24V regime. The only thing that momentarily gets any sort of fluctuation in the ampere gauge is to cycle power on the alternator switch or reset the “Field/Alt 1” CB. 
 

I don’t recall seeing any lights on the indicator panel until I physically switched off the alternator switch while troubleshooting. The only big red warning was my engine monitor flashing red voltage numbers at me. The reason my hunch is heat related is because I have flown earlier in the mornings or on an afternoon that’s slightly cooler with no delays on the ground and have taken off with no issues throughout the flight. I’m just leery to take any family trips now, because it seems to be overall unreliable/unpredictable and I don’t want to get stuck off station or get into a self induced situation where I need the manual gear extension.  

Don Maxwell rebuilds voltage regulators. Call him and explain what you're seeing.

[GeeBee]

I had the same problem with my M20R a while back. Start out at 27.5 then gradually taper down to 24.3. Bad alternator. It was a Plane Power unit only a year old. I went back to the old Continental unit. The guys on Beechtalk say it is caused by out of round commutator rings which start the brushes bouncing which seems to be a recurring problem on the Plane Power units.

[RoundTwo]

1 hour ago, KHyde said:

I don’t recall seeing any lights on the indicator panel until I physically switched off the alternator switch while troubleshooting. The only big red warning was my engine monitor flashing red voltage numbers at me. The reason my hunch is heat related is because I have flown earlier in the mornings or on an afternoon that’s slightly cooler with no delays on the ground and have taken off with no issues throughout the flight. I’m just leery to take any family trips now, because it seems to be overall unreliable/unpredictable and I don’t want to get stuck off station or get into a self induced situation where I need the manual gear extension.  

As someone that went through something very similar last year on my 88 J, my experience may be useful. In my situation, there were no component failures and I spent a lot of time on my back, getting intimately familiar with the whole charging system, removing items and sending them off for testing, only to determine that everything was working as designed.

Can you download engine monitor data at high resolution to see what’s happening with your voltage? Once you understand what’s happening from start up, it will be easier to prevent wasting time like I did, chasing suspected failed components that were actually good.

[gabez]

I experienced the opposite low voltage on the ground at 1-1200 rpm and then it went back to working at higher RPM, it turned out it was the coupler 

[PT20J]

You changed the alternator to no effect, so it’s probably not the alternator.

27 V is low for a 24 V battery. The regulator should be set for something between 28 and 29 V.

When there is an over voltage, the over voltage protection circuit (OVP) in the regulator shuts down the alternator by removing the field excitation. There are two methods depending on the regulator. The OVP may just shut down the regulator field circuit internally, or it may “crowbar” - cause a short circuit - which trips the Field CB. Cycling power or resetting the breaker resets the OVP.

Since you said that it momentarily works when you cycle power, I would suspect something is wrong with the regulator, especially since the voltage also seems low. Something (maybe heat) causes the OVP to trip. Cycling power resets it, but it immediately trips again. 

[KHyde]

16 hours ago, PT20J said:

You changed the alternator to no effect, so it’s probably not the alternator.

27 V is low for a 24 V battery. The regulator should be set for something between 28 and 29 V.

When there is an over voltage, the over voltage protection circuit (OVP) in the regulator shuts down the alternator by removing the field excitation. There are two methods depending on the regulator. The OVP may just shut down the regulator field circuit internally, or it may “crowbar” - cause a short circuit - which trips the Field CB. Cycling power or resetting the breaker resets the OVP.

Since you said that it momentarily works when you cycle power, I would suspect something is wrong with the regulator, especially since the voltage also seems low. Something (maybe heat) causes the OVP to trip. Cycling power resets it, but it immediately trips again. 

Good to know that 27V is low. I had no idea. Sounds to me from all of the comments that it’s most likely the voltage regulator. Very helpful, thanks!

[KHyde]

On 7/4/2024 at 8:30 AM, RoundTwo said:

As someone that went through something very similar last year on my 88 J, my experience may be useful. In my situation, there were no component failures and I spent a lot of time on my back, getting intimately familiar with the whole charging system, removing items and sending them off for testing, only to determine that everything was working as designed.

Can you download engine monitor data at high resolution to see what’s happening with your voltage? Once you understand what’s happening from start up, it will be easier to prevent wasting time like I did, chasing suspected failed components that were actually good.

I do have the ability to download the engine data. I will have to take a look at what is happening with the voltage. What was the outcome in your situation? Were you able to get reliable operations back somehow?

[A64Pilot]

On 7/5/2024 at 9:40 AM, KHyde said:

Good to know that 27V is low. I had no idea. Sounds to me from all of the comments that it’s most likely the voltage regulator. Very helpful, thanks!

Don’t get too wrapped up on the numbers because a lot has to do with load and where it’s measured, but most often a 12V battery should be at 14V plus or minus .2V after being fully charged after starting.

A 24V battery is of course exactly twice the voltage, that’s why for example you hear that you have a 28V electrical system, but a 24V battery, because that’s actually correct.

Fully charged but engine off your battery should be approx 26V, 12V battery of course 13. At 24V resting your battery is on average about 50% charged.

[dkkim73]

On 7/3/2024 at 8:04 PM, PT20J said:

Never heard of a heat soaked alternator. Might be a voltage regulator. Doubt it has anything to do with heat -- the regulator is in the cockpit -- but if you think so get out to the airport early when it's cooler and see if it makes a difference. What's the annunciator doing? If VOLTS is steady on, the OVP tripped.

Regulator seems like a likely culprit for reasons you mention elsewhere. Maybe the apparent temperature dependence could be due to a thermal intermittent phenomenon? eg. a connection, which I would think could be in different locations. 

[PT20J]

The newer Mooneys use high quality connectors. Most connection issues are at the alternator because that’s where the most vibration occurs. I presume these were checked when the alternator was replaced. There have been some reports of loose connections at the alternator and master switches.