Alternator Positive Wire Sizing

[goboatingnow]

[

The voltage regulator on a traditional alternator, is not actually a voltage regulator. It is actually a voltage sensing current regulator, technically speaking.

glad to see someone is thinking correctly.

This is why alternators or any current or voltage source do not set battery terminal voltage unless the " battery lets them". At low SOC the battery equivalent impedance is so low compared to the output impedance of the charge source that the battery terminal voltage is purely a function of the battery

people assume regulation means a fixed output voltage. Next time take a 1 watt , 12 V power supply feed it into a 1 ohm resistor, measure the actual voltage across the resister, ( it may have crowbar protection of course) , is the voltage 12V at that point ??

regulator only set the terminal voltage when they "can" , ie when the battery impedance has risen to a point where the charger is "more dominant" ( sorry trying to avoid specific EE terms here)

Everyone uses teh term " teh charger sets the voltage to xx V", in practice in a big battery bank nothing like that actually happens.

Dave

[pbiJim]

Hellosailor, I see that you & I have been down some of the same bumpy roads in the past. The only thing that I could possibly disagree with in this last post is the idea that getting rid of the sense wire would be more cost effective because hardware costs money. From what was posted earlier, it appears that many people are running extra heavy charge wires on alternators that don't have sense wires & I suspect that the cost of the extra heavy wire may exceed the added cost of the thin sense wire.

I do understand that simple can be a good thing on a boat though. An extra wire is an extra thing to corrode & go bad & stop a gizmo from working. Failed gizmos do not improve the quality of anybody's day.

[pbiJim]

Originally Posted by goboatingnow

[
regulator only set the terminal voltage when they "can" , ie when the battery impedance has risen to a point where the charger is "more dominant" ( sorry trying to avoid specific EE terms here)

Everyone uses teh term " teh charger sets the voltage to xx V", in practice in a big battery bank nothing like that actually happens.

Dave

This is true if someone is suffering from a case of small alternator syndrome. For the rest of us, it works as was otherwise described.

Please do feel free to let loose with the EE terminology. I understand the difference between a Laplase transform & a Fourier transform. I'll probably be able to comprehend at least a little piece of what you have to say.

[goboatingnow]

Originally Posted by pbiJim

This is true if someone is suffering from a case of small alternator syndrome. For the rest of us, it works as was otherwise described.

Please do feel free to let loose with the EE terminology. I understand the difference between a Laplase transform & a Fourier transform. I'll probably get at least a little piece of it.

The characteristic impedance of LA batteries during recharge at low SOC is such that most alternators will full field . But not regulate ( you may have protective regulation due to heat etc , but lets leave that out. This is true especially of AGMs and GELs etc.

Depending on the power of the charge source , its equivalent output impedance , once it has sufficient power and the load impedance is such then the terminal voltage will rise, but only to a point as in going beyond that point will cause large current flows and thermal runaway of the battery , which itself prevents the fixed power source from raising the voltage any more.

Ie a regulator simply can't decide to regulate to a given output voltage unless it has the power to do so in respect of the load impedance , its not a perfect source.


In a typical say 400-500 AH bank with a 50 A alternator , the battery for the majority of its charge cycle is determining the terminal voltage

That all I was trying to say on this ( small ) point.
Dave

[pbiJim]

Dave,
Thank you for that most eloquent description. I now have a much better understanding of the point you were trying to make.

You did however, touch on one issue that I am not particularly familiar with. Could you please expand on what you mean by thermal runaway of the battery?

Thanks,
Jim

[hellosailor]

Dave-
"take a 1 watt , 12 V power supply feed it into a 1 ohm resistor, measure the actual voltage across the resister,"
Is that a trick question? Really? What is a one-watt 12v power supply supposed to mean? A traditional transformer-based wall wart, that puts out up to 17 volts with no load, and has such a tiny transformer in it that the transformer saturates and can't deliver more than one watt at 12 volts, so the voltage just tanks after that? Or a nice new little digital one, which will attempt to put out 12.00 volts under all conditions?

There used to be a rule book. In the old analog world. These days? Stuff follows Robert Heinlein's paradigm, i.e. "any sufficiently advanced technology will be indistinguishable from magic". Yup, we're there now, there's no telling wtf those boxes are going to do.

Jim-
Whether the charge output wire is big or small, has nothing to do with the relative cost of running an extra battery sense lead. Big or small, whatever that wire costs you, a second wire costs you more. And building the extra terminals for it, crimping it, physically installing it, running it, connecting it, all again WAY exceed the cost of the wire. Regardless of the size of the output charging wire. Two totally separate costs, totally unrelated.
Once you've got a piece of silicon inside the alternator making decisions, the cost of adding another grain of sand and telling it "while you've got nothing better to do, ping the battery, will you?" becomes cheaper than the zip-tie that is holding that voltage sense wire to the bulkhead.

[goboatingnow]

Originally Posted by hellosailor

Dave-
"take a 1 watt , 12 V power supply feed it into a 1 ohm resistor, measure the actual voltage across the resister,"
Is that a trick question? Really? What is a one-watt 12v power supply supposed to mean? A traditional transformer-based wall wart, that puts out up to 17 volts with no load, and has such a tiny transformer in it that the transformer saturates and can't deliver more than one watt at 12 volts, so the voltage just tanks after that? Or a nice new little digital one, which will attempt to put out 12.00 volts under all conditions?

There used to be a rule book. In the old analog world. These days? Stuff follows Robert Heinlein's paradigm, i.e. "any sufficiently advanced technology will be indistinguishable from magic". Yup, we're there now, there's no telling wtf those boxes are going to do.

There's no such thing as a" digital" one that can put out 12v under all conditions.

F the source has the power to feed current to the load in sufficient quantities it can determine the load voltage. If it hasn't the power the load voltage is simply ( Max I * load resistance) or the units crowbars and shuts down

You cannot maintain a fixed regulated voltage into a decreasing equivalent load impedance. If you could you would maintain 12V into a short , which violates ohms law

Dave

[goboatingnow]

Originally Posted by pbiJim

Dave,
Thank you for that most eloquent description. I now have a much better understanding of the point you were trying to make.

You did however, touch on one issue that I am not particularly familiar with. Could you please expand on what you mean by thermal runaway of the battery?

Thanks,
Jim

LA batteries if over driven in absorption mode by a powerful enough charger , ie by allowing super high absorption voltages , will induce a failure state where the battery rapidly heats up uncontrollably and charge current ( now in destruction mode) begins to rise again uncontrollably , rupture can occur at this point.

Dave

[pbiJim]

Dave,
So then, are you saying that when a battery heats up from high charge currents, it looses it's counter EMF?

[goboatingnow]

Originally Posted by pbiJim

Dave,
So then, are you saying that when a battery heats up from high charge currents, it looses it's counter EMF?

http://www.sbsbattery.com/PDFs/VRLAT...erySystems.pdf

Dave

[pbiJim]

Dave,
Thank you for the link. That technical paper was certainly well researched & explained quite a bit in good detail.

Unfortunately, that paper only seems to cover valve regulated lead acid batteries. Do you think that the same attributes apply to more traditional flooded cell lead acid batteries as well?

[hellosailor]

Dave, does that paper meander on at some point to actually say there is thermal runaway in a VRLA battery?

Thermal runaway as commonly defined refers to what happens in a lithium battery, i.e. once you get it hot enough, a nasty fire starts and it feeds itself without any outside source of fuel or air. Or in some nuclear reactors, thermal runaway will bring on the China Syndrome as the core slags down, increasing its own density and heat. (A pebble bed reactor, for example, just can't do that.) In both cases, the runaway continues absent of outside actions.

That paper fails to make it clear whether a VRLA battery will continue to overheat once the outside source of power has been removed, it seems to be discussing a condition where outside power is always present. Which could be explosive but ain't thermal runaway. (Or if it does make that clear, it doesn't do so up front.)

[goboatingnow]

Originally Posted by hellosailor

Dave, does that paper meander on at some point to actually say there is thermal runaway in a VRLA battery?

Thermal runaway as commonly defined refers to what happens in a lithium battery, i.e. once you get it hot enough, a nasty fire starts and it feeds itself without any outside source of fuel or air. Or in a nuclear reactor, thermal runaway will bring on the China Syndrome as the core slags down, increasing its own density and heat. In both cases, the runaway continues absent of outside actions.

That paper fails to make it clear whether a VRLA battery will continue to overheat once the outside source of power has been removed, it seems to be discussing a condition where outside power is always present. Which could be explosive but ain't thermal runaway. (Or if it does make that clear, it doesn't do so up front.)

Well I suppose you could argue about the presence or absence of power sources in a definition of thermal runaway. In electronics thermal runaway , is where increasing temperature causes a situation to get worse. ( like for example in power transistors ) ie a positive feedback loop is created. , whereas devices that do not exhibit thermal runaway , have a negative feedback loop ( like say power mosfets ).

LA batteries do have a positive loop once certain conditions are breached .

See http://en.wikipedia.org/wiki/Thermal_runaway

Hello sailor your description , I suppose could be rightly called self sustained thermal runaway !

Dave

[sam123]

Originally Posted by Pelorus32

(nips)

I'm replacing the Victron with a CTek D250S Dual and a SmartPass. I already have them to hand ....

(nips)

As long as you assemble CTEK D250S near to your house bank -> you are fine. That Swedish device will make 5 stage charging from anything over 13 volts from your starting battery. CTEK charge max. 20A, so no need to go any overkill wires.

Cheers, Sam

[Pelorus32]

Originally Posted by sam123

As long as you assemble CTEK D250S near to your house bank -> you are fine. That Swedish device will make 5 stage charging from anything over 13 volts from your starting battery. CTEK charge max. 20A, so no need to go any overkill wires.

Cheers, Sam

The problem is that the Smartpass will deliver up to 80A if it's available and if the batteries will accept it. Later it falls back to the D250S Dual at up to 20A. So that suggests slightly bigger wires 😎

Regards
Mike