Help me understand my Mooney's electrical system

[AlexLev]

I own an '68 M20G with a plane-power alternator conversion (instead of the generator).

For those technically minded that understand AC vs DC and how the electrical system on the Mooney works, mind helping me understand it?

I understand there's a 70A max load for the alternator and it's a 14v alternator. I have a Concorde 12v battery with 33 Ampere hours. The alternator generates AC and converts it back into DC via a rectifier (?) and the buses are fed off DC wires to the battery?

Apologies if this is totally wrong, but would love someone's help...also with the battery have 33 Ampere hours - does that mean if the battery dies, and the load I have draws approx 4A, it would take approx 8 (33/4) hours for the battery to die if it's fully charged? How many amp would radios/transponder/GPS draw?

Apologies if all these are novice questions, but want to learn/understand the plane and its electrical system better and I figure there's a few more technically minded folks here who could help me shed some light on understanding it.

[EricJ]

9 minutes ago, AlexLev said:

I own an '68 M20G with a plane-power alternator conversion (instead of the generator).

For those technically minded that understand AC vs DC and how the electrical system on the Mooney works, mind helping me understand it?

I understand there's a 70A max load for the alternator and it's a 14v alternator. I have a Concorde 12v battery with 33 Ampere hours. The alternator generates AC and converts it back into DC via a rectifier (?) and the buses are fed off DC wires to the battery?

Yes, there's nothing wrong in what you wrote above.

9 minutes ago, AlexLev said:

Apologies if this is totally wrong, but would love someone's help...also with the battery have 33 Ampere hours - does that mean if the battery dies, and the load I have draws approx 4A, it would take approx 8 (33/4) hours for the battery to die if it's fully charged? How many amp would radios/transponder/GPS draw?

Yes, a 33Ah battery can supply 1A for 33 hours or 33A for 1 hour.   You calculation is correct.

9 minutes ago, AlexLev said:

Apologies if all these are novice questions, but want to learn/understand the plane and its electrical system better and I figure there's a few more technically minded folks here who could help me shed some light on understanding it.

 

[Ragsf15e]

12 minutes ago, AlexLev said:

I own an '68 M20G with a plane-power alternator conversion (instead of the generator).

For those technically minded that understand AC vs DC and how the electrical system on the Mooney works, mind helping me understand it?

I understand there's a 70A max load for the alternator and it's a 14v alternator. I have a Concorde 12v battery with 33 Ampere hours. The alternator generates AC and converts it back into DC via a rectifier (?) and the buses are fed off DC wires to the battery?

Apologies if this is totally wrong, but would love someone's help...also with the battery have 33 Ampere hours - does that mean if the battery dies, and the load I have draws approx 4A, it would take approx 8 (33/4) hours for the battery to die if it's fully charged? How many amp would radios/transponder/GPS draw?

Apologies if all these are novice questions, but want to learn/understand the plane and its electrical system better and I figure there's a few more technically minded folks here who could help me shed some light on understanding it.

One thing to keep in mind about battery capacity… it will decrease over time.  They are supposed to be checked at annual and the standard is 80%, so you should hopefully get 80% of capacity as a minimum.  As far as draw? Depends a lot on your equipment, glass vs old stuff? Led vs old lights?  Radios draw a lot.  Theoretically, you should get at least a half hour (although I don’t have the reference for that so could be off).  You can shut off stuff (radios) to get more.  
Also, your amp meter is normally wired to show the charge/discharge flow to/from the battery, not the total power load on the system.  So you pretty much have to run everything with the engine off and look at the amp meter (drawing power from the battery) to see what your total load might be.  With the alternator running, you will only see the charge current to the battery which could be high with a discharged battery or low with a fully charged battery.  I usually see ~3amps in cruise.  Maybe 10amps right after engine start.

[Pinecone]

Some aircraft (my 252) are wired with a load meter.  This shows you the total electrical draw.

I see about 25% load on the working alternator, so about 15-18 amps total draw.  Or about 2 hours on a full charged, full capacity battery.  But I would count on 1/2 time on 80%% of rated, so more like 45 minutes to keep everything running.

[Fritz1]

yes to all, typically you will get about 1-2h of run time out of your battery, 80% capacity, depending on you ampere draw, I had a G model with an alternator conversion, lost power twice both times in VFR weather, voltage regulator died, alternator circuit breaker fell apart, carried a 12V tire inflator with 10Ah battery in cockpit and reverse fed through cigarette lighter socket, landed VFR within 45 min both times

[skykrawler]

The rectifier is built into the alternator.   The regulator adjusts for the load on the system by controlling the output of the alternator.  The load on the system is the battery (if it needs charged) and every electrical thing you have turned on.   When charged, the battery helps smooth the voltage changes from the changes in load.

The master relay alone requires about .5 amps.  When you transmit with the radio, the load increases by and 1 or 2 amps.  When you turn on pitot heat it probably pulls 10 or more amps until it starts to heat up and increases resistance.   All those panel lights draw 1..2 amps.

A 12v system should run about 13.5 volts on the bus.  If you don't have a voltage indication you should get one - even a cheap one that plugs into the aux plug is better than nothing.

[1980Mooney]

Don’t forget the effect of temperature. @AlexLev ‘s profile shows Buffalo, NY. Although Buffalo is a warm 53 deg F. today, the average low temperatures are usually closer to mid 20’s deg F in January. The lead acid battery loses about 20% of its capacity at that temperature. 
 

Also cold weather needs a lower charging rate in order to avoid battery damage .  Charging will warm the battery (and I don’t know where the battery in a G is located - if it’s like a C it wil get engine heat)  

 

[AlexLev]

3 hours ago, skykrawler said:

The rectifier is built into the alternator.   The regulator adjusts for the load on the system by controlling the output of the alternator.  The load on the system is the battery (if it needs charged) and every electrical thing you have turned on.   When charged, the battery helps smooth the voltage changes from the changes in load.

The master relay alone requires about .5 amps.  When you transmit with the radio, the load increases by and 1 or 2 amps.  When you turn on pitot heat it probably pulls 10 or more amps until it starts to heat up and increases resistance.   All those panel lights draw 1..2 amps.

A 12v system should run about 13.5 volts on the bus.  If you don't have a voltage indication you should get one - even a cheap one that plugs into the aux plug is better than nothing.

This is helpful. Does the alternator just charge the battery or does it supply power to everything electrical? Or does it split the load with the battery? How does that work?

[Pinecone]

Battery is about 12.8 - 13.1 volts.  Alternator output is 13.8 - 14 volts.  So the alternator supplies all the electrical power under normal load.

Only when you draw more than the alternator can supply (which may be none with a failure), then the battery supplies the power.

[N201MKTurbo]

15 hours ago, AlexLev said:

This is helpful. Does the alternator just charge the battery or does it supply power to everything electrical? Or does it split the load with the battery? How does that work?

The battery is there to start the engine and supply emergency power if the alternator fails.

The alternator is intended to supply all the power needs while the engine is running. The voltage regulator keeps the alternator voltage at a preset voltage no matter what the load on the alternator is. This is typically 13.6 to 14.1 volts depending on what battery chemistry you are using. 

When more current is required from the alternator, such as charging the battery after start, or when all the lights are turned on, the voltage regulator will supply more field current to the alternator to increase its output power so the bus voltage doesn’t drop. As the battery charges and the power requirements from the alternator decrease, the voltage regulator will decrease the field current so the bus voltage doesn’t get too high.

Lead acid batteries change their voltage according to their state of charge. This voltage is slightly different depending on the type of battery you have. Our planes will either have a flooded cell battery or an Advanced Gas Mat battery. They are more the same than different. The flooded cell battery has the electrolyte as a liquid between the plates and the AGM has the electrolyte as a gel in a fiberglass mat between the plates. The flooded cell battery will change the chemistry of the liquid electrolyte as it charges and discharges, the AGM changes the chemistry of the gas in the battery case as it charges and discharges. 

A fully charged battery will have an open circuit voltage of 12.6 volts. This is the voltage you will see with the engine off. If you apply a charging voltage, the battery voltage will rise a bit before charge current will start flowing into the battery. This is where the different types of batteries will show. There is a voltage depending on the state of charge where the battery will draw no current to charge it. This is referred to as the float voltage. The float voltage for a flooded cell battery is 13.6 volts and an AGM battery is 14.1 volts. Your voltage regulator should be set to these voltages depending on what type of battery you have. If you have a flooded cell battery set to a bus voltage of 14.1 it will over charge the battery and vent hydrogen and reduce its capacity over time. This is a slow process, so don’t lose any sleep over it, but your battery won’t last as long. If you have an AGM battery set to 13.6 volts it will not fully charge and you won’t have quite as much starting power, or emergency power as you could. It isn’t much of a difference so once again don’t lose any sleep over it. 

When a battery is not fully charged, such as right after starting the engine, it’s float voltage will decrease, so if it is connected to a bus with a higher voltage than its float voltage, current will flow into the battery to charge it. As it charges, its float voltage will rise and it will require less charging current until it is fully charged and its float voltage equals the bus voltage. 

[A64Pilot]

On 12/26/2023 at 2:46 PM, Fritz1 said:

yes to all, typically you will get about 1-2h of run time out of your battery, 80% capacity, depending on you ampere draw, I had a G model with an alternator conversion, lost power twice both times in VFR weather, voltage regulator died, alternator circuit breaker fell apart, carried a 12V tire inflator with 10Ah battery in cockpit and reverse fed through cigarette lighter socket, landed VFR within 45 min both times

That’s something I’ve never thought of, but I can’t fault the logic, I see no reason why connecting s decent sized 12V lithium battery thru the cigarette lighter wouldn’t increase the available amp hours, your limited to the rating of the cigarette lighters CB, but surely it’s enough.

I think you may really have something there

[A64Pilot]

On 12/26/2023 at 7:19 PM, AlexLev said:

This is helpful. Does the alternator just charge the battery or does it supply power to everything electrical? Or does it split the load with the battery? How does that work?

An alternator doesn’t supply pure DC, it’s pulsating DC. 

The battery being in the system is a really big electrical accumulator that functions to smooth the pulsating DC to pure DC, but other than that the alternator as long as it’s putting out enough power, powers the aircraft.

Sometimes like at idle it may not, then of course you run off of the battery.

The alternator in order to charge the battery has to output 14V, it has to be slightly higher than battery voltage to charge the battery, a fully charged battery is 13ish V.

The output voltage also determines how rapidly a battery is charged.

It confuses people but it takes a 14V system to keep a 12V battery happy.