Getting low voltage annunciator light if below 1100 RPM (1988 M20J)

[IslandPilot]

Voltage drops below 28V when go below 1100 RPM.  Time to change out the alternator?

[ArtVandelay]

I would check for the cheapest and easiest to fix problem first; is the belt loose?

[Ragsf15e]

20 minutes ago, IslandPilot said:

Voltage drops below 28V when go below 1100 RPM.  Time to change out the alternator?

 

I wouldn’t throw parts at an electrical problem.  A little bit of troubleshooting with a digital voltmeter will narrow it down.  Could be lots of things.  Possibly want to start by checking actual voltage at the cig lighter to see if it’s really low at 1000 rpm and not low above that.

[PT20J]

That's not unreasonable. My M20J drops out somewhere around there. Unless it has suddenly changed, I wouldn't worry about it.

[ta2too]

I was told once you get below 1200 rpm's the LV light will flash, and it will drop below 28v.

Mine does the same thing.

[Ragsf15e]

4 minutes ago, PT20J said:

That's not unreasonable. My M20J drops out somewhere around there. Unless it has suddenly changed, I wouldn't worry about it.

I didn’t realize the 28v systems did that.  My 12v system seems to be at 14v down to 1000rpm or less.  After start I don’t notice it drop off at all.  I wonder if it’s just a difference between 14v and 28v?  I do have a newish alternator and zef vr.

[ttflyer]

Completely normal on our 1988 J.  As long as the light goes out when I push the power up for runup/takeoff, I don't give it a second thought...

[A64Pilot]

I think it may be depending on electrical load too, like if you have the old type landing light on.

I don’t think my 14V system does it, but I do get a low vac light right about there.

[231LV]

My 231 runs on the 14v system with all the standard electronics including an Aspen 1000 glass panel...I find that after I fly (no low volt annunciator light at anytime unless I drop below 980 rpm) my battery minder says the battery is discharged by more than half...it's a new battery so I can only surmise that the alternator can't put a charge into the battery as fast as the electronics are pulling the volts out...too bad Mooney never got an STC for a higher load alternator for the K models (and any others)

[A64Pilot]

28 minutes ago, 231LV said:

My 231 runs on the 14v system with all the standard electronics including an Aspen 1000 glass panel...I find that after I fly (no low volt annunciator light at anytime unless I drop below 980 rpm) my battery minder says the battery is discharged by more than half...it's a new battery so I can only surmise that the alternator can't put a charge into the battery as fast as the electronics are pulling the volts out...too bad Mooney never got an STC for a higher load alternator for the K models (and any others)

If your maintaining 14V in flight, it’s charging as in not depleting the battery, if your significantly lower than that you have a problem that needs fixing, it’s unlikely you have exceeded alternator capacity because most can’t put out 100% continuously or they burn up, they don’t have a 100% duty cycle, or ones I’m familiar with don’t.

‘If your battery minder soon shows full charge that’s likely normal as they work that way, meaning they are looking for absorption voltage which is usually low 14’s but your battery is less than that so initially it comes on at absorption voltage for several minutes (amps are dropping but you don’t see that as you have no amp gauge on the charger) then once amps drop to set point charger drops to float voltage and shows fully charged. 

If your battery is truly depleted to 50% it will take likely until the next day for a minder to fully charge it as their output is very low, often just 1.5 amps or so.

You can verify this theory simply by unplugging the battery minder then after a min of two plug it back in and see what it displays, if it shows fully charged in less than an hour battery wasn’t depleted by much if at all. If a charger is 100% efficient and nothing is, but a 1.5 amp minder can only put 1.5 amp hours back in one hour.

I think modern glass is pretty efficient as in low amp draw, nothing at all compared to pitot heat or landing light and of course you can fly forever with those on.

[Ragsf15e]

49 minutes ago, 231LV said:

My 231 runs on the 14v system with all the standard electronics including an Aspen 1000 glass panel...I find that after I fly (no low volt annunciator light at anytime unless I drop below 980 rpm) my battery minder says the battery is discharged by more than half...it's a new battery so I can only surmise that the alternator can't put a charge into the battery as fast as the electronics are pulling the volts out...too bad Mooney never got an STC for a higher load alternator for the K models (and any others)

That’s pretty normal for the battery minder display.  It’s kind of misleading as its not an indicator of battery health.

[231LV]

Good points...yes, the voltage is steady at 13.7v during flights and I'm glad to understand how the minders work so I am not terribly worried...didn't mean to hijack the thread...apologies

[toto]

12 hours ago, IslandPilot said:

Voltage drops below 28V when go below 1100 RPM.  Time to change out the alternator?

 

Totally normal for my 89J 

[Pinecone]

17 hours ago, 231LV said:

.I find that after I fly (no low volt annunciator light at anytime unless I drop below 980 rpm) my battery minder says the battery is discharged by more than half...it's a new battery so I can only surmise that the alternator can't put a charge into the battery as fast as the electronics are pulling the volts out...too bad Mooney never got an STC for a higher load alternator for the K models (and any others)

That is NOT what your Battery Minder its telling you.  The light next to the icon showing 1/2 battery level is just telling you what mode it is charging.

Page 10 - https://www.batteryminders.com/content/BATTERY MINDR CEC2 GUIDE_ (003).pdf

It will show that for a short while even if the battery is fully charged.  It does into Constant Current (Bulk) mode and monitors the battery response.  Then it may switch to Constant Voltage (Absorbtion) mode in a short period of time.  Only the Red and the Full sort of indicate the actual state of charge.

[Fly Boomer]

15 hours ago, 231LV said:

Good points...yes, the voltage is steady at 13.7v during flights and I'm glad to understand how the minders work so I am not terribly worried...didn't mean to hijack the thread...apologies

I would say if the voltage is 13.7 during flights, there is a problem.  Concorde says voltage regulator should be set for about 14.0 or 14.1 (the ideal charge voltage changes depending on temperature, but 14 or a little over is good for most temperatures).  Even if you don't have a Concorde, any AGM battery (and perhaps any lead-acid battery) will be the same.  If your charging system is only making 13.7, you are likely discharging during flight.

[ArtVandelay]

I would say if the voltage is 13.7 during flights, there is a problem.  Concorde says voltage regulator should be set for about 14.0 or 14.1 (the ideal charge voltage changes depending on temperature, but 14 or a little over is good for most temperatures).  Even if you don't have a Concorde, any AGM battery (and perhaps any lead-acid battery) will be the same.  If your charging system is only making 13.7, you are likely discharging during flight.

13.7 is charging, if you don’t believe me, pull your field CB and then look at the voltage. However, it’s charging at a low rate.
A good thing to do is check the voltage drop from your battery terminal to the alternator, it shouldn’t be more than 0.1V. A bad connection or relay can cause significant voltage drop which means your alternator may drive the voltage to 14.x but the battery could only see 13.x.

[Fly Boomer]

33 minutes ago, ArtVandelay said:

13.7 is charging, if you don’t believe me, pull your field CB and then look at the voltage. However, it’s charging at a low rate.
A good thing to do is check the voltage drop from your battery terminal to the alternator, it shouldn’t be more than 0.1V. A bad connection or relay can cause significant voltage drop which means your alternator may drive the voltage to 14.x but the battery could only see 13.x.

Not when the temperature is below 113.  This is from the Concorde manual.  Could be different for other batteries, but I doubt it.  Ignore the yellow highlight.

 

[ArtVandelay]

Not when the temperature is below 113.  This is from the Concorde manual.  Could be different for other batteries, but I doubt it.  Ignore the yellow highlight.

 

AFAIK, our 1970s VR technology doesn’t do temperature sensing so that level is going to be the same when it’s 13.

[Fly Boomer]

1 minute ago, ArtVandelay said:

AFAIK, our 1970s VR technology doesn’t do temperature sensing so that level is going to be the same when it’s 13.

Exactly.  For almost any temperature we are likely to encounter (global warming not withstanding), 13.7 volts is too low (according to Concorde).  For the very low temperature you mention (13), 14.7 is more like it.

[A64Pilot]

On 8/9/2023 at 10:34 AM, Fly Boomer said:

Not when the temperature is below 113.  This is from the Concorde manual.  Could be different for other batteries, but I doubt it.  Ignore the yellow highlight.

 

I respectfully disagree.

13.7 is an operating alternator, it’s well above open circuit voltage of the battery. I think that was his point.

But I agree it’s below optimum set point, however before I did anything I’d verify actual voltage at the battery, depending on where the reading is taken on our older airplanes there is some wiring resistance due to age, my panel voltage reads 13’s, but my Garmin 696 indicates 14.1V. If I went by the ship installed voltmeter I’d be chasing a non existent problem.

Actually and call Concorde to verify but 13.7 will charge a battery, not as fast as they would like but it will charge. Float voltage is around 13.3 V and even that will charge, but it takes days to do so, literally. Any voltage above open circuit will charge, how high within reason determines charging speed along with available current of course.

Depending on where you look because vehicles including airplanes voltage are not set for this or that battery, but very often voltage should be 14 plus or minus .2

Are any of our voltage regulators adjustable?

[natdm]

@231LV my battery was reading 11.5v at 1000RPM. I had been told by everyone that it was normal. 

Went to a mechanic in Tucson who was fixing avionics and he noticed that in a picture I had sent him and said it was not normal and could be fixed. He noticed the battery ground on the airframe was just "absolutely terrible". Now, I make about 13.7 at 1000RPM.

[ArtVandelay]

Are any of our voltage regulators adjustable?

The common Zeftronics 15300 is, open it up there’s a pot you can adjust with a screwdriver.

[A64Pilot]

I hate to sound like I’m preaching, but before you spend $$$ chasing low voltage, be sure you measure the voltage at least at the main bus. Often the cigarette lighter is connected pretty directly to the main bus, but in truth it can have issues too like dirty contacts.

Actually for ANY reading out of spec, fuel oil pressure whatever first step is verify the gauge, often the engines fine, it’s the old wiring / gauge that’s bad.

Best to run a couple of jumpers directly to the battery and use a good multimeter, but honestly one of those volt meters that plug directly into the cigarette lighter aren’t bad for just diagnosis, test it in your car first. Don’t adjust your VR to a cheap often unreliable volt meter though, to adjust read voltage directly with a known good multimeter.

Unfortunately and I’m going to pick on the Chinese, but many products sold they just lie about accuracy, output whatever. With tools treat yourself to good ones, resist the temptation to save money on tools. I’ve always had good luck with Fluke multimeters, they may be Chinese for all I know but they seem to be good quality, or I hope they still are

[jlunseth]

The answer is probably going to vary with the model. I have a good engine monitor (930) and all the lights have long since been converted to LEDs. Even so, at anything less than 1100 I will most likely get a Low Volts annunciation. This is a known issue in the 231. It has a direct drive alternator and the alternator simply does not make much current at low RPMs. For awhile, Mooney was supplying 100 amp alternators (the standard one is 70 or 80 as I recall), but that did not change the issue with low volts at low RPMs. 231 owners have a choice of riding the brakes on and off during taxi, or watching the low volts lights flash. 

That’s the 231 and its direct drive alternator. Other models have completely different alternators and drive systems. Since yours is 28V it is obviously not a 231, but how good the charging is a low RPMs is still going to vary from one model to the next, and what kind of load you have on your system.

[201Mooniac]

I was watching this thread as I don't recall seeing a low voltage light on my 1980 M20J when running low idle.  I checked this morning and I had 14.0v at 800 rpm so a low voltage at low rpm isn't always expected.