Alternator

[Col_T]

I have an imperfect understanding of 12v electrical generation, so some questions to try and improve that a  little.

 

Alternator output - this is typically rated in Amps e.g. our boat has two alternators, a 70A for the domestics and lower rated one for the engine start battery. I know that the battery bank limits the amount of charge, in Amps, that it will accept during charging but what determines the maximum output of the alternator - is this defined by the design, or 'imposed' by internal circuitry? Just a high-level answer will be fine; I wouldn't understand anything detailed!

 

Regulator - I think this is that part of the alternator that controls the actual voltage output, preventing the alternator from exceeding a 'designed in' maximum voltage, but does it also sense when the battery has decided enough is enough, and if so, how?

 

Alternator output (2) - our boat spends most of it's time connected to a land-line with the battery charger keeping the batteries topped-up, so what does the alternator 'do' when we go cruising and pretty much the first thing the alternator sees are fully charged domestics? Yes, I know it stops generating electricity but, if I understand it correctly, the actual generation is the product of a mechanical operation - copper windings spinning is a magnetic field produce the electricity, so where does the electricity 'go' if the batteries are fully charged? Or are is the magnetic field electrically generated?

 

All answers gratefully received, especially those that don't get too technical !!

[MoominPapa]

Maximum output is determined by design. The manufacturers don't use more iron and copper than is required to produce the specified output.

 

The regulator measures the output voltage of the alternator, which is practically the same as the battery voltage. The characteristics of the battery mean that when it is discharged, it takes a very large charge current to reach the regulator cut-off voltage, but when it's full, a small current is sufficient.

 

The magnetic field is electrically generated, and controlled by the regulator. When the batteries are full, the regulator cuts back the field strength and the output drops. The alternator also becomes easier for the engine to spin, so conservation of energy is maintained.

 

MP.

 

[Alan de Enfield]

6 minutes ago, Col_T said:

Alternator output (2) - our boat spends most of it's time connected to a land-line with the battery charger keeping the batteries topped-up, so what does the alternator 'do' when we go cruising and pretty much the first thing the alternator sees are fully charged domestics? Yes, I know it stops generating electricity but, if I understand it correctly, the actual generation is the product of a mechanical operation - copper windings spinning is a magnetic field produce the electricity, so where does the electricity 'go' if the batteries are fully charged? Or are is the magnetic field electrically generated?

 

Imagine the electrical circuit in a house - electricity comes into the fuse box and is distributed around the main circuit until it gets to the 3-pin sockets. The electricity doesn't 'go anywhere' until a circuit is made.

The same with an alternator - no electricity until a circuit is made.

[Boater Sam]

2 minutes ago, MoominPapa said:

Maximum output is determined by design. The manufacturers don't use more iron and copper than is required to produce the specified output.

 

The regulator measures the output voltage of the alternator, which is practically the same as the battery voltage. The characteristics of the battery mean that when it is discharged, it takes a very large charge current to reach the regulator cut-off voltage, but when it's full, a small current is sufficient.

 

The magnetic field is electrically generated, and controlled by the regulator. When the batteries are full, the regulator cuts back the field strength and the output drops. The alternator also becomes easier for the engine to spin, so conservation of energy is maintained.

 

MP.

 

A good succinct answer, and not too technical, even I understood that. Green is the colour.

[Tony Brooks]

1. The output is actually rated in Watts BUT for mere mortals it tends to be stated in amps and what voltage I have no idea, I suspect the regulated voltage. From this flows further answers. The wattage is defined by the design and in theory it can never be exceeded. If something were to try to make it then the voltage would drop to maintain the wattage.

 

2. It is the batteries that control how much current they want so the regulator has no direct control over the current supplied to the batteries. All the regulator on the vast majority of alternators that we use simply limits the voltage to a safe battery charging level once the output current has fallen low enough to allow the output voltage to reach the regulated voltage. Before that point the regulator simply sits there doing nothing.

 

3. Its the batteries that decide when they "have had enough", not the regulator.

 

4. With fully charged batteries the output current will be very low so the POTENTIAL voltage would be high but the regulator limits the output voltage to whatever it is set to so regulated voltage o say 14.4 volts. Fully charged battery voltage 13.2 volts or maybe more for a while. The difference in voltage 1.2 volts is what pushes the current into the battery and  that will only allow a few amps to flow. With discharged batteries the voltage difference will be greater so more current flows. Again, at any given voltage its the batteries that control the current they will accept.

 

 

[WotEver]

1 hour ago, MoominPapa said:

When the batteries are full, the regulator cuts back the field strength and the output drops.

Actually, no. The regulator will be doing nothing other than limiting the voltage to the regulated voltage (14.4V or whatever). It is the batteries that no longer demand much current flow. 

1 hour ago, Alan de Enfield said:

Imagine the electrical circuit in a house - electricity comes into the fuse box and is distributed around the main circuit until it gets to the 3-pin sockets. The electricity doesn't 'go anywhere' until a circuit is made.

Clearly described

 

[Tony Brooks]

2 hours ago, MoominPapa said:

Maximum output is determined by design. The manufacturers don't use more iron and copper than is required to produce the specified output.

 

The regulator measures the output voltage of the alternator, which is practically the same as the battery voltage. The characteristics of the battery mean that when it is discharged, it takes a very large charge current to reach the regulator cut-off voltage, but when it's full, a small current is sufficient.

 

The magnetic field is electrically generated, and controlled by the regulator. When the batteries are full, the regulator cuts back the field strength and the output drops. The alternator also becomes easier for the engine to spin, so conservation of energy is maintained.

 

MP.

 

 

14 minutes ago, WotEver said:

Actually, no. The regulator will be doing nothing other than limiting the voltage to the regulated voltage (14.4V or whatever). It is the batteries that no longer demand much current flow. 

Clearly described

 

 

I feel the above may not be clear for many.

 

The regulator does its thing when the charging VOLTAGE reaches a set level, 14.4 in Wot Ever's example. Before and after that point it is the batteries that control the current flow at whatever voltage the alternator with or without the regulator working is producing.

[Alan de Enfield]

2 hours ago, Col_T said:

All answers gratefully received, especially those that don't get too technical !!

 

2 hours ago, Col_T said:

Just a high-level answer will be fine; I wouldn't understand anything detailed!

 

How about "because they do"

 

Trust me !!

[bizzard]

Ok, I won't give a scientific explanation.

[WotEver]

41 minutes ago, Tony Brooks said:

I feel the above may not be clear for many.

 

The regulator does its thing when the charging VOLTAGE reaches a set level, 14.4 in Wot Ever's example. Before and after that point it is the batteries that control the current flow at whatever voltage the alternator with or without the regulator working is producing.

My post was with regard to when the batteries are full. At that point the regulator is holding the voltage at its regulation level. The post I was quoting stated that it also reduced the output to the batteries at this time. I was pointing out that it doesn’t, it is the batteries that are reducing their demand to virtually nil. 

[Tony Brooks]

Just now, WotEver said:

My post was with regard to when the batteries are full. At that point the regulator is holding the voltage at its regulation level. The post I was quoting stated that it also reduced the output to the batteries at this time. I was pointing out that it doesn’t, it is the batteries that are reducing their demand to virtually nil. 

I 100% agree and despite your excellent battery charging primer elsewhere on the forum we still have to keep on emphasising that point - its the batteries that control the charging current, not the alternator.

[Col_T]

Very many thanks for a fine body of answers, and I think I even understand most them! However . . . .

 

Am I right that the alternator never completely stops outputting, even when the battery has decided it's had enough, it just drops the output to a float charge voltage.

56 minutes ago, Alan de Enfield said:

How about "because they do"

Sometimes that just ain't enough and a soul wants to know why they do.

 

58 minutes ago, Alan de Enfield said:

Trust me !!

I didn't think you were a doctor?!!!

[WotEver]

1 hour ago, Col_T said:

Am I right that the alternator never completely stops outputting, even when the battery has decided it's had enough, it just drops the output to a float charge voltage.

Yes and no. Firstly, taking your last part, the alternator does not drop the output voltage to a float voltage. Explaining this was the purpose of my correction in posts #6 and #10 The alternator will continue to output its regulated voltage until you stop the engine. 

 

So yes, the alternator continues generating a voltage even though the batteries are demanding virtually nothing, but no to the dropped float voltage. 

 

Remember Alan’s point about the 13A sockets in a house. Just because they’re there doesn’t mean anything is using the available power, and just because they can supply 13A doesn’t mean that a device plugged into them will draw 13A. 

[Chewbacka]

8 hours ago, Tony Brooks said:

I 100% agree and despite your excellent battery charging primer elsewhere on the forum we still have to keep on emphasising that point - its the batteries that control the charging current, not the alternator.

Pedant alert -  actually it’s both.  The alternator aims to always have an output of whatever it’s regulator is set to, say 14.4 volts.  However if the effective resistance of the battery is low (low state of charge) then the amount of power the battery can adsorb is greater than the amount of power that the alternator can generate so the alternator regulator will protect itself from overload by limiting the output power by reducing the voltage.  As the battery charges up the effective resistance increases thereby reducing the amount of power it can adsorb for a given charging voltage, so the alternator regulator will increase the output voltage to maintain rated power until it reaches its design maximum- typically 14.4v.  At which point the battery will take progressively less power and the alternator will be outputting a lot less than it’s max capability.  As power is a bit fiddly to measure, we tend to monitor current which gives a good indication as to the soc.

 

added - the alternator rated output will vary with rotational speed, temperature etc, so don’t expect (as an example) 70A from a 70A alternator in anything other than ideal conditions.

Edited February 24, 2019 by Chewbacka

[WotEver]

50 minutes ago, Chewbacka said:

Pedant alert -  actually it’s both.  The alternator aims to always have an output of whatever it’s regulator is set to, say 14.4 volts.  However if the effective resistance of the battery is low (low state of charge) then the amount of power the battery can adsorb is greater than the amount of power that the alternator can generate so the alternator regulator will protect itself from overload by limiting the output power by reducing the voltage.  As the battery charges up the effective resistance increases thereby reducing the amount of power it can adsorb for a given charging voltage, so the alternator regulator will increase the output voltage to maintain rated power until it reaches its design maximum- typically 14.4v.  At which point the battery will take progressively less power and the alternator will be outputting a lot less than it’s max capability.  As power is a bit fiddly to measure, we tend to monitor current which gives a good indication as to the soc

No, it’s not both. The current is dictated by the battery and only the battery. The regulator does not limit the current in any way; it only limits the voltage in order that the total power demanded by the battery doesn’t exceed the output power of the alternator. To say “it’s both” confuses the issue for someone who has asked for a non-technical reply. 

[Chewbacka]

37 minutes ago, WotEver said:

No, it’s not both. The current is dictated by the battery and only the battery. The regulator does not limit the current in any way; it only limits the voltage in order that the total power demanded by the battery doesn’t exceed the output power of the alternator. To say “it’s both” confuses the issue for someone who has asked for a non-technical reply. 

The regulator controls the power output of the alternator.  Power is the product of volts multiplied by amps.  To control the output power the regulator will adjust it's output.  Obviously as the voltage rises so does the current, and likewise the current reduces if the voltage reduces, so the power output goes up or down.   So is the current limited by controlling the voltage or is the voltage  controlled by adjusting the current??  So I would say that the regulator controls the power by controlled the voltage, but you could also argue that it controls the power by controlling the current.  Either way it’s the regulator that controls (regulates) the power generated and transferred to the battery.  However the regulator is designed not to apply excessive power during charging a ‘flat’ battery and not to overcharge a charged battery by allowing the voltage to rise to an unsafe level.

 

 So whilst a non tech person wants a simple answer, to say the battery decides is not really true, the regulator decides what to do considering the battery state of charge.

[WotEver]

12 hours ago, Chewbacka said:

So is the current limited by controlling the voltage or is the voltage  controlled by adjusting the current?? 

Neither. 

 

The power output is controlled by lowering the voltage to constrain the output power to within the alternator’s design limitations (with a flat battery). Once the power demanded by the batteries is within the alternator’s capabilities the regulator does nothing other than prevent the voltage rising above the set value. It’s still not adjusting the current; that is set by the battery. 

12 hours ago, Chewbacka said:

So whilst a non tech person wants a simple answer, to say the battery decides is not really true, the regulator decides what to do considering the battery state of charge.

But it remains incorrect to suggest that the regulator adjusts the current because it doesn’t. It limits the maximum voltage. 

Edited February 25, 2019 by WotEver

[WotEver]

 

The regulator is not reducing the voltage at all. The alternator is running flat out and its voltage reduces simply because it can’t supply any more with the current being demanded by the batteries. 

 

 

Edited February 25, 2019 by WotEver
Too much scotch late at night!

[dmr]

1 minute ago, WotEver said:

 

The regulator is not reducing the voltage in order to reduce the current, it’s reducing the voltage to keep the power within the design limitations of the alternator while the batteries are demanding a high current. 

No that's not right, the regulator limits the maximum voltage to protect the batteries from an overvoltage, it is not protecting the alternator, I reckon you've been on the beer!

 

During bulk charge the alternator is running flat out, current (or power) is limited by fundamental design, the regulator is not limiting things, its driving the rotor as hard as it can.  Its only when battery demand is less than alternator capacity that the regulator starts to work.

 

There are maybe a few cases where cunning regulator design plays a part in preventing the alternator getting too hot,but that's another story.

 

..............Dave

[Chewbacka]

25 minutes ago, WotEver said:

Neither. 

 

The power output is controlled by lowering the voltage to constrain the output power to within the alternator’s design limitations (with a flat battery). Once the power demanded by the batteries is within the alternator’s capabilities the regulator does nothing other than prevent the voltage rising above the set value. It’s still not adjusting the current; that is set by the battery. 

But it remains incorrect to suggest that the regulator adjusts the current because it doesn’t. It adjusts the voltage. 

The regulator controls the power output.  Power is amps multiplied by volts.  You can not adjust volts without changing amps and likewise you can not adjust amps without changing volts.  The current is the result of a voltage across a resistance.  So to control power you control volts and the amps must follow.

[WotEver]

20 minutes ago, dmr said:

During bulk charge the alternator is running flat out, current (or power) is limited by fundamental design, the regulator is not limiting things

Yes, correct. Chewy led me off down a garden path by suggesting that the regulator was reducing the current. 

 

This Glenrothes is rather good...

3 minutes ago, Chewbacka said:

The regulator controls the power output.  Power is amps multiplied by volts.  You can not adjust volts without changing amps and likewise you can not adjust amps without changing volts.  The current is the result of a voltage across a resistance.  So to control power you control volts and the amps must follow.

Yep, but all the regulator does is limit the voltage to the design value - 14.4V or whatever. It doesn’t control the current flowing into the batteries, they do that. 

[dmr]

10 minutes ago, WotEver said:

Yes, correct. Chewy led me off down a garden path by suggesting that the regulator was reducing the current. 

 

This Glenrothes is rather good...

I find that the whiskey stuff is dangerous so usually stick to the beer, but it was very posh beer tonight

 

https://www.vocationbrewery.com/pages/vocation-co

 

.................Dave

 

[WotEver]

Just now, dmr said:

I find that the whiskey stuff is dangerous so usually stick to the beer, but it was very posh beer tonight

 

https://www.vocationbrewery.com/pages/vocation-co

 

.................Dave

 

Oooh, you posh git.

 

Must see if we can check it out the next time we visit the missus’s relations up in Keighley. 

[dmr]

1 minute ago, WotEver said:

Oooh, you posh git.

 

Must see if we can check it out the next time we visit the missus’s relations up in Keighley. 

Yes, we are staying up here till late April/early May (if CRT don't shout at us) then down to Bristol via Liverpool, but are thinking we might now make the Rochdale our winter home and just pp down south for the summers.

 

Drove through Keighley a couple of weeks ago, it has a railway.

 

..................Dave

[WotEver]

2 minutes ago, dmr said:

Drove through Keighley a couple of weeks ago, it has a railway.

Remember the film ‘Yanks’?  Remember the bit where the girl ran through the crowd at the train station shouting “Let me through, I’m pregnant!” And someone responded “So’s half the bloody town love!”...

 

... that was shot at Keighley station. 

 

Eta it has an Aldi too

Edited February 24, 2019 by WotEver