Comparative Safety: 12v v 24v v 48

[ramblinrod]

Originally Posted by weavis

Rod.

If you want to have a discussion, Then agree or disagree would be the way to go.

If you want to fight, then correct or incorrect is the way to go. It will result in a lot of wasted time and energy and bad feeling if others feel different and eventually be a contributing factor to the closure of the thread.

There are a number of highly qualified members in this discussion. On that basis, it does not single out one of you to the authoritative voice regarding matters electrical. Given the history of these exchanges, it would benefit the thread if a disagreement over protocols is acknowledged as a disagreement and NOT as correct or incorrect, even though sometimes it may be a correct understanding, or as has been seen, not as correct as a member would like it to be.

I have offered you considered advice from dealing with thousands of threads and personalities. Take it or leave it.

Its a Discussion Board with professionals and amateurs alike. The admin's job is to keep the forum running smoothly and harmoniously, NOT to judge who is right or wrong in a technical subject. Where we see issues in communication, we offer advice to help because the threads are helpful to a lot of people. Your fellow professionals will decide who is correct or incorrect collectively, and better to be a dissenting voice with disagreement than an inflammatory statement that detracts from the subject under discussion.

Thanks for the advice Weavis.

It is my opinion that if one declares, "I disagree" or "Incorrect", it means the same thing; in their opinion, the post they are responding to, is wrong.

Again, for everything anyone posts "In my opinion" is implied, and we should not have to state this preceding every post, and before every new thought within a post.

People just need to be a little less sensitive and refrain from becoming disrespectful, just because someone disagrees with them.

After all, if we all agreed on everything, there would be only one best brand of boat, and one best place to sail it.

I have no problem if someone disagrees with me, and explains why.

If I agree with their contradiction, I will say so; if I don't, I will explain why.

It has been my experience that it does not matter what verbiage one uses or how they try to sugar coat their "difference of opinion", there are a few individuals on this forum that will use a myriad of bullying tactics to attempt to stifle the person, so that they appear to be the ultimate authority on the subject, even when they are completely and utterly incorrect on the matter (in my opinion).

Notwithstanding I will follow your recommendation to see if it makes a difference.

I do hope so.

[ramblinrod]

Originally Posted by CatNewBee

Hey, rod,

Induction is caused by current, not by voltage. Just saying...

I disagree.

Induction is caused by a change in magnetic field, which produces a current.

But an arc is developed by a voltage across a gap.

No voltage, and it doesn't matter how much current is available, no arc.

When the voltage (potential) across the contacts exceeds the breakdown voltage of the space between, an arc is formed and current flows as electromagnetic discharge across the gap.

The higher the voltage, the wider the gap an arc can be formed across, so the longer an arc is sustained during a contact opening or closing.

The plasma arc has almost zero impedance, so it is the rest of the circuit impedance that defines the circuit current.

The energy dissipated by the arc in heat, is based on the voltage drop across it times the current through it, in accordance with Watt's Law.

The higher the voltage, the higher the current, the more energy dissipated.

This is why a 48 Vdc circuit is much harder on contacts than 12 Vdc.

[Dockhead]

Originally Posted by ramblinrod

. . .

This is why I suggest if one is considering a higher voltage DC system, just determine if the heavy loads (refrigeration, Air Conditioning, and Water Making) can be run off a common AC generator.

If so, this means a smaller inverter can be used, a smaller house bank maintained, etc, etc, etc.

Is a generator heavy? Yup. But so is a big 48 Vdc house bank, and quite frankly, a generator is likely going to be necessary to recharge it anyway.

This kind of misses the point of advanced DC systems on cruising sailboats.


Running all big loads off a constantly running AC generator is a sound approach for sure -- SOP for larger power boats and commercial vessels. So I'm not saying there is anything stupid about it.


But advanced DC systems on sailboats are intended to reduce IC engine running and efficiently harvest and store power produced by solar or as a by-product to propulsion. This requires a large battery bank, and in particular a large lithium bank. This is where a 48v "backbone" makes sense.




Does any of this make FINANCIAL sense? Probably not, but neither does sailing. For what I paid for my carbon laminate sails, I could easily buy 10 years worth of diesel fuel.

[Jdege]

Originally Posted by Dockhead

Running all big loads off a constantly running AC generator is a sound approach for sure -- SOP for larger power boats and commercial vessels. So I'm not saying there is anything stupid about it.

The book I've quoted - and the few commercial systems I've found online - weren't running an AC generator for everything on the boat. They seem to use a pretty typical DC battery bank, 48v, 72v, or whatever, sized to the motor, and a three-phase inverter dedicated to powering the motor.

Elco Motor Yachts

Electric Motor Sport

[ramblinrod]

Originally Posted by Dockhead

This kind of misses the point of advanced DC systems on cruising sailboats.


Running all big loads off a constantly running AC generator is a sound approach for sure -- SOP for larger power boats and commercial vessels. So I'm not saying there is anything stupid about it.


But advanced DC systems on sailboats are intended to reduce IC engine running and efficiently harvest and store power produced by solar or as a by-product to propulsion. This requires a large battery bank, and in particular a large lithium bank. This is where a 48v "backbone" makes sense.

Thank you for your contribution.

In my humble opinion (IMHO) your declaration is that "in your opinion" that because it is SOP for larger power boats and commercial vessels to have ICE generators, you are not saying there is anything stupid about that.

(But you are implying it IS stupid as pertains to sailboats?)

I disagree. Lots of very smart sailors have ICE generators aboard.

Hmmm. I am having a really hard time understanding the forum politically correct lingo. ;-)

Just kidding, I'm not offended at all.

Just making the point that someone can claim being offended by any wording if they are overly sensitive about someone being in "disagreement".

OK, lets get on with the meat of the matter. (Sorry if this offended any vegetarians, vegans, or fruitarians.) ;-)

One of the things that makes me chuckle a bit about this post, is that in prior discussions you have expressed being vehemently opposed to charging with solar panels and wind generators, and favourable to charging with ICE generation so huh?

Oh, you are speaking on behalf of others needs, hmmm...

I digress...

"Advanced DC Systems?"

IMHO, there is nothing "advanced" about 48 Vdc.

It is just "a" voltage level, and the concept, if not the actual value in use, has been around pretty much since the first 12 Vdc electrical system.

IMHO, all of the technological principles that may be applied to 12 Vdc, pretty much applies to 48 Vdc, with the exception IMHO that the latter is more prone to arcing, more likely to cause electric shock or electrocution, and to start a fire in the event of a short circuit.

IMHO, I disagree with the statement that storing large amounts of power requires a lithium bank. IMHO sailors have been storing large amounts of power in battery banks of other than Lithium technology, for many, many years. IMHO Lithium is merely a battery chemistry option that has been around about 10 years, about the same amount of time as carbon foam, vs AGM and Gel that have been around about 30 years, vs plain old FLA that has been around at least 100 years.

(If anyone wishes to challenge these dates with actual data please feel free to "correct" me, as they are just numbers I pulled out of a hat, that I believe are close enough to make the point, without having to refer to a scientific paper about the history of battery technology.)

So IMHO there is nothing really "advanced" about Lithium battery technology, and by most modern technology standards, should already be long obsolete and superseded.

So I have to ask the question?

If one installs electrical equipment on their sailboat (having an ICE propulsion auxiliary engine), which causes their DC electrical system to consume 20 kWh per day, this mean they will need 12 Vdc x 1666 ah or 48 Vdc x 416 ah of usable battery capacity to last 24 hours without any form of charging.

So if the large sailboat is like the vast majority (97% maybe), that sits at a slip with shore power, it would make sense instead to connect the high load appliances (especially A/C, electric ranges and ovens and refrigeration systems) to the AC electrical system, and then for the rare time the vessel may not be in a slip, just run the generator to power the AC appliances (or forgo them and use other means (natural ventilation, propane grill cooking, coolers packed with ice, etc.) Sorry to suggest "roughing" it boys. ;-)

OK, so lets consider the liveaboard sailor who rarely is connected to shorepower (1%?),

So if we connect all of these high loads to a DC system, to replenish this at anchor, on a sunny day and avoid running ICE generation, they will need, about 5 kW of solar panels (or about 320 sq ft) (about the entire deck area of a 40' monohull) or the entire surface area of a 40' catamaran, less the trampoline.

To accommodate days with no solar production (won't really happen but you get my drift), with 10 knots of wind avg over 24 hours, they will need about 50 D400 wind generators.

But wait, all of these wind generators in close proximity will block wind to each other and shade the solar panels, so I guess we'll actually need more, a lot more, of each. ;-)

And for those days where wind and sun are lacking, they will need a pretty significant ICE generator.

So if we need an ICE generator aboard to recharge batteries some days, why not just run the energy hogs, A/C, refrigeration, electric range and oven, washer and dryer, watermaker, and shore power battery charger off the ICE generator?

This will enable us to reduce the size of the battery bank to run some LED lights, instruments, and charge some mobile devices to about 1000 Wh.

For this, a 12 Vdc system with a 83 ah (usable bank) will easily suffice, (2 grp 24 FLAs DC) cost about $200, and no converter or inverter required to power anything.

If one battery fails, I just bypass it and run off the other one with half capacity until I pull in anywhere and pick up another FLA (actually, if the bank is more than 2 years old, I'd replace both at the same time).

To me, knowing that I am going to have to have an ICE generator aboard anyway to recharge high capacity battery banks powering a 48 Vdc system, why bother with the 48 Vdc system at all?

Why bother with a huge bank of batteries and all of the alternative energy charging systems (for the occasions where they fulfill a full day of energy needs), when I can skip all that cost and complexity and just run the generator I need to have aboard anyway to power these loads?

(I understand some boats have lead lined generator compartments that are amazingly quiet?)

The point is, to each their own, and anyone can concoct whatever kind of electrical system they wish; it's there boat, but in my mind this just doesn't make sense.

If one claims to be doing it for "environmental" reasons, I call "BS", just do a better job of conserving energy, and all of the carbon that goes into manufacturing all of this stuff isn't required, and you won't have to run the ICE generator at all.

For someone who just says screw it, this is what I want no matter the cost, please PM me; I can hook you up. ;-)

[ramblinrod]

Originally Posted by Jdege

The book I've quoted - and the few commercial systems I've found online - weren't running an AC generator for everything on the boat. They seem to use a pretty typical DC battery bank, 48v, 72v, or whatever, sized to the motor, and a three-phase inverter dedicated to powering the motor.

Elco Motor Yachts

Electric Motor Sport

Thank you for your contribution.

I certainly agree that a higher voltage DC system is required for electric propulsion.

However, I really question the idea of the validity of electric propulsion in a typical cruising boat or passagemaker wishing to cover around 150 nm / day, wind or not (especially to get out of the path of a whirly girl) and how they will recharge this bank for the motor (and all the other stuff they claim to need or want) without an ICE generator running almost constantly but that's another discussion.

[evm1024]

Originally Posted by Dockhead

This kind of misses the point of advanced DC systems on cruising sailboats.


Running all big loads off a constantly running AC generator is a sound approach for sure -- SOP for larger power boats and commercial vessels. So I'm not saying there is anything stupid about it.


But advanced DC systems on sailboats are intended to reduce IC engine running and efficiently harvest and store power produced by solar or as a by-product to propulsion. This requires a large battery bank, and in particular a large lithium bank. This is where a 48v "backbone" makes sense.


Does any of this make FINANCIAL sense? Probably not, but neither does sailing. For what I paid for my carbon laminate sails, I could easily buy 10 years worth of diesel fuel.

I quite well agree. We really are talking about the advancement of DC systems in cruising sailboats. Such advanced DC systems may well spill over into Dock Queens as such advanced systems become the norm.

An agile mind should pick up on this quickly.

In terms of Specific energy Lithium based battery chemistry's eclipse lead based ones. Li based come in around 130Wh/Kg and 333 Wh/l where Pb com in around 35 Wh/Kg and 70 Wh/l. That is 3.7 X advantage to Li based by mass and 4.7 X advantage to Li by volume.

In terms of voltage, 48 volts has 4 times the energy per unit of charge over 12 volts. Current is reduced for any given work to be done.

Advanced does not equal new.

I would consider a Higher voltage, higher energy density energy storage system as advanced when installed on a sailboat that is still basically using technology from sometime before the 1960's.

I can easily see the automotive and off-grid communities generating standardized "energy blocks". Want a 5 KWh block no problem. 10 KWh, 15Kwh etc., easy.

With integrated controls (read BMS and other advanced controls) it is a small step to adaptable charge modules. Adaptable charge modules accept any input voltage and buck/boost to meet the charge profile the BMS requests. IN fact why not have more than one adaptable charge module.

From the energy block a standardized distribution voltage backbone takes power to where needed and through a DC-DC converter supplies the load based voltage.

Is a 48 volt (or any higher voltage than 12 volts) Li based energy system right for everyone? Not today.

I have a question for those who are content with an electrical system based on 1960's tech - Why aren't you sailing a wooden boat?

[Dockhead]

Originally Posted by ramblinrod

Thank you for your contribution.

In my humble opinion (IMHO) your declaration is that "in your opinion" that because it is SOP for larger power boats and commercial vessels to have ICE generators, you are not saying there is anything stupid about that.

(But you are implying it IS stupid as pertains to sailboats?)

I disagree. Lots of very smart sailors have ICE generators aboard.

Hmmm. I am having a really hard time understanding the forum politically correct lingo. ;-)

Just kidding, I'm not offended at all.

Just making the point that someone can claim being offended by any wording if they are overly sensitive about someone being in "disagreement".

OK, lets get on with the meat of the matter. (Sorry if this offended any vegetarians, vegans, or fruitarians.) ;-)

One of the things that makes me chuckle a bit about this post, is that in prior discussions you have expressed being vehemently opposed to charging with solar panels and wind generators, and favourable to charging with ICE generation so huh?

Oh, you are speaking on behalf of others needs, hmmm...

I digress...

"Advanced DC Systems?"

IMHO, there is nothing "advanced" about 48 Vdc.

It is just "a" voltage level, and the concept, if not the actual value in use, has been around pretty much since the first 12 Vdc electrical system.

IMHO, all of the technological principles that may be applied to 12 Vdc, pretty much applies to 48 Vdc, with the exception IMHO that the latter is more prone to arcing, more likely to cause electric shock or electrocution, and to start a fire in the event of a short circuit.

IMHO, I disagree with the statement that storing large amounts of power requires a lithium bank. IMHO sailors have been storing large amounts of power in battery banks of other than Lithium technology, for many, many years. IMHO Lithium is merely a battery chemistry option that has been around about 10 years, about the same amount of time as carbon foam, vs AGM and Gel that have been around about 30 years, vs plain old FLA that has been around at least 100 years.

(If anyone wishes to challenge these dates with actual data please feel free to "correct" me, as they are just numbers I pulled out of a hat, that I believe are close enough to make the point, without having to refer to a scientific paper about the history of battery technology.)

So IMHO there is nothing really "advanced" about Lithium battery technology, and by most modern technology standards, should already be long obsolete and superseded.

So I have to ask the question?

If one installs electrical equipment on their sailboat (having an ICE propulsion auxiliary engine), which causes their DC electrical system to consume 20 kWh per day, this mean they will need 12 Vdc x 1666 ah or 48 Vdc x 416 ah of usable battery capacity to last 24 hours without any form of charging.

So if the large sailboat is like the vast majority (97% maybe), that sits at a slip with shore power, it would make sense instead to connect the high load appliances (especially A/C, electric ranges and ovens and refrigeration systems) to the AC electrical system, and then for the rare time the vessel may not be in a slip, just run the generator to power the AC appliances (or forgo them and use other means (natural ventilation, propane grill cooking, coolers packed with ice, etc.) Sorry to suggest "roughing" it boys. ;-)

OK, so lets consider the liveaboard sailor who rarely is connected to shorepower (1%?),

So if we connect all of these high loads to a DC system, to replenish this at anchor, on a sunny day and avoid running ICE generation, they will need, about 5 kW of solar panels (or about 320 sq ft) (about the entire deck area of a 40' monohull) or the entire surface area of a 40' catamaran, less the trampoline.

To accommodate days with no solar production (won't really happen but you get my drift), with 10 knots of wind avg over 24 hours, they will need about 50 D400 wind generators.

And for those days where wind and sun are lacking, they will need a pretty significant ICE generator.

So if we need an ICE generator aboard to recharge batteries some days, why not just run the energy hogs, A/C, refrigeration, electric range and oven, washer and dryer, watermaker, and shore power battery charger off the ICE generator?

This will enable us to reduce the size of the battery bank to run some LED lights, instruments, and charge some mobile devices to about 1000 Wh.

For this, a 12 Vdc system with a 83 ah (usable bank) will easily suffice, (2 grp 24 FLAs DC) cost about $200, and no converter or inverter required to power anything.

If one battery fails, I just bypass it and run off the other one with half capacity until I pull in anywhere and pick up another FLA (actually, if the bank is more than 2 years old, I'd replace both at the same time).

To me, knowing that I am going to have to have an ICE generator aboard anyway to recharge high capacity battery banks powering a 48 Vdc system, why bother with the 48 Vdc system at all?

Why bother with a huge bank of batteries and all of the alternative energy charging systems (for the occasions where they fulfill a full day of energy needs), when I can skip all that cost and complexity and just run the generator I need to have aboard anyway to power these loads?

(I understand some boats have lead lined generator compartments that are amazingly quiet?)

The point is, to each their own, and anyone can concoct whatever kind of electrical system they wish; it's there boat, but in my mind this just doesn't make sense.

If one claims to be doing it for "environmental" reasons, I call "BS", just do a better job of conserving energy, and all of the carbon that goes into manufacturing all of this stuff isn't required, and you won't have to run the ICE generator at all.

For someone who just says screw it, this is what I want no matter the cost, please PM me; I can hook you up. ;-)

I did not imply that it's stupid for sailboats. If a large sailboat spends most of its time in a marina berth, like most large power boats, then for sure it's a waste of money trying to implement an advanced DC power system.


My own boat, actually, was originally designed to have the generator running most of the time. She has a heavy duty prime power rated generator, which is so quiet that you can't hear it all outside, and barely hear it below, and the designer intended that the generator should be running most of the time when the boat was off shore power.



Accordingly, she was given a small house battery bank of only 220 amp/hours @ 24v, separated from another bank of the same size used for thruster, windlass, and winches.


But this is not everyone's use case. I used to spend most of the time off shore power, when I had a Hamble River mooring, and even now spend weeks or even months at a time off shore power. I spent last summer in the Arctic and didn't even see a shore power connection for probably two months.


Many people on here have similar use cases, and even if you already have the heavy duty generator, you don't necessarily want to run it for weeks on end, if you can arrange your system somehow so that you can "coast" on battery power for a day at a time or so. Not only is this more pleasant, and saves generator hours (which costs more in maintenance and amortization, than the fuel cost), it allows you to harvest power from sources like solar and alternator on the main engine, and might also help you use the generator more efficiently by charging intensively during shorter runs, then shutting it down.


Lithium offers some huge advantages for this type of use -- very high acceptance rate means you can use a very large alternator, large enough that if you have to run the main engine without propulsion, it will have a healthy load, but also so that you can harvest the energy you need over relatively short episodes of motoring. Also the cycle life. Also, and not the least of the advantages, the fact that you don't need to worry about finishing charges, nor is it harmful to leave lithium batts at a low state of charge -- a lithium bank is much more like a fuel tank, where you just dump fuel in when you have it, and then draw it out, when you don't, which is an enormous advantage for an off grid boat. And then yet another advantage is much less voltage sag for really big consumers like bow thrusters, which can greatly increase their efficiency. Lithium is also considerably cheaper than lead, if you calculate the real cost of a kW/h stored and then used.



Downside is that you can't just drop them into a system designed for lead -- lithium is so different from lead, that it needs a rather different system architecture with different control systems and infrastructure.


My next boat, which will be a metal boat designed for extreme latitudes, will have a large lithium bank, either 24v or 48v, with dual alternators on the main engine including a very large domestic power one (probably 5kW or so), and a very small emergency generator (and I would do without the generator if I were cruising in less remote places). Either the smallest Northern Lights job, or perhaps a NexGen. The yard which will make my next boat, which specializes in expedition vessels, rarely installs lead batteries any more.



Lead is great for marina queens or power boats, but does not stack up well against lithium for more demanding usage, at least not if the boat can be designed from scratch for lithium.






As to my preference for charging sources -- your memory is not serving you well. I love solar power and had great luck with it on my last boat. It's the best power source for those with space to install enough of it. Present boat however is a dedicated sailing machine and no place to put solar which wouldn't compromise upwind sailing ability. So I'm stuck with generator charging, which is nothing against those who have solar -- in fact, I am somewhat envious. The next boat will have solar panels built into the pilothouse roof, but will be electrical intensive enough (no gas on board) that I wouldn't count on living off solar power.





As to "incorrect" vs. "I disagree", this is not political correctness. Saying "incorrect!" or "wrong!" is saying something fundamentally different from "I disagree". Saying "incorrect!" asserts authority to judge the validity of another's opinion. That might not be unjustified in all cases, but in this thread, for example, the assertion of alleged authority has been massively irritating and has degraded the quality of the discussion. "I disagree" allows that there may be different opinions, and is much more respectful. To say "incorrect!" you'd better really be sure you know what you're talking about, and besides that, be really sure that you're not dealing with people with equal (or better) knowledge than your own, and even then, it's probably impolite in most cases.


It's not impossible that an electrician might understand some basic principle of electricity, better than an actual electrical engineer, but it is unlikely enough, that the electrician ought to be prepared with some really well prepared arguments, expressed in a really respectful way, and not simply bombastic assertions of one's own authority.

[ramblinrod]

[QUOTE=evm1024;2839557]

I quite well agree. We really are talking about the advancement of DC systems in cruising sailboats. Such advanced DC systems may well spill over into Dock Queens as such advanced systems become the norm.

Of course this is only correct if one considers there to be anything advanced about a 48 Vdc system. (I personally don't.)

In terms of Specific energy Lithium based battery chemistry's eclipse lead based ones. Li based come in around 130Wh/Kg and 333 Wh/l where Pb com in around 35 Wh/Kg and 70 Wh/l. That is 3.7 X advantage to Li based by mass and 4.7 X advantage to Li by volume.

Depends. Often we are talking around 1/3 the weight and 1/2 the volume for prismatic or cylindrical cells, and about 1/2 the weight and same volume for drop-ins.

In terms of voltage, 48 volts has 4 times the energy per unit of charge over 12 volts. Current is reduced for any given work to be done.

I disagree. The amount of energy stored is not directly related to voltage, it is directly related to VA (Watts). 48 Vdc at 100 Ah, represent the same stored energy as 12 Vdc at 400 Ah, both being 4800 W-h.

Advanced does not equal new.

In my opinion, that is normally what is meant when on refers to "advanced technology". Nothing new or advanced about 48 Vdc in that light. "Advanced technology" is really just a marketing term. (IMHO).

I would consider a Higher voltage, higher energy density energy storage system as advanced when installed on a sailboat that is still basically using technology from sometime before the 1960's.

I think you are confusing 2 separate factors. Whether a system is 48 Vdc is independent of whether it incorporates Lithium Ion battery technology.

There is nothing advanced (IMHO) about 48Vdc.

L-ion can be applied to any dc voltage within a reasonable range, and certainly form 12 to 48 Vdc, pretty much equally, if one so desires (except for the higher rated components, costs, and risks associated with the higher voltage.

Is a 48 volt (or any higher voltage than 12 volts) Li based energy system right for everyone? Not today.

IMHO, not likely ever, for the vast majority of boaters. Just not needed, and no real benefit, unless high energy consuming devices are desired, especially electric propulsion, which is not the end all be all some are currently touting IMHO.

I have a question for those who are content with an electrical system based on 1960's tech - Why aren't you sailing a wooden boat?

IMHO, some people are and absolutely love them, even though they represent 5000 year old technology or older.

The age of the technology does not make the continued application invalid.

Quite the opposite, the length of time a technology remains valid is a testament to how sound the technology really is.

People are still making boats out of wood.

However most people today seem to prefer the lower cost, reduced maintenance, and ease of access to production fibrelgass boats vs wood comparators.

These attributes together, do not seem to pertain to 48 Vdc vs 12 Vdc systems on boats.

In fact a few posts back I detailed all of the pros and cons of each, and at least in my opinion, and as supported by some others, there were some very good points that detailed why the pros of the 48 Vdc system would not outweigh the cons, for the vast majority of boaters today, (or likely even in my lifetime).

I'm not against 48 Vdc.

After a vessel design and configuration review and an owner needs and wants assessment, I would be the first to recommend it to a customer if I felt that was the best solution for them.

I just had a similar conversation with a customer moments ago whether to go with a single outboard or a dual outboard arrangement.

I advised the pros and cons of each option (IMHO) and he could choose which ever suited him better.

I believe this is the way, every major modification should be addressed.

To claim "48 Vdc is the way of the future" is just false in my opinion. I suspect, there will be more boats with 12 Vdc systems that 48 Vdc systems 50 years form now, (if humans still boat here).

[transmitterdan]

Originally Posted by ramblinrod

I disagree.



Induction is caused by a change in magnetic field, which produces a current.



But an arc is developed by a voltage across a gap.



No voltage, and it doesn't matter how much current is available, no arc.



When the voltage (potential) across the contacts exceeds the breakdown voltage of the space between, an arc is formed and current flows as electromagnetic discharge across the gap.



The higher the voltage, the wider the gap an arc can be formed across, so the longer an arc is sustained during a contact opening or closing.



The plasma arc has almost zero impedance, so it is the rest of the circuit impedance that defines the circuit current.



The energy dissipated by the arc in heat, is based on the voltage drop across it times the current through it, in accordance with Watt's Law.



The higher the voltage, the higher the current, the more energy dissipated.



This is why a 48 Vdc circuit is much harder on contacts than 12 Vdc.

Rod,

The arc that happens when interrupting a DC motor is not caused by a change in magnetic field. It’s the exact opposite. The arc happens because the magnetic field does not change rapidly.

You seem to be mixing up the concepts of back EMF with inductive “kickback”. Back EMF is created by the motor turning. But arcing of contacts is not caused by back EMF. The arc energy is caused by the energy stored in the magnetic field.
The idea that inductive arcing is related to system voltage is not mathematically correct.
Energy stored in a capacitor is a function of voltage squared. Energy stored in an inductor is a function of current squared. Capacitor energy has nothing to do with current and inductor energy has nothing to do with voltage. These facts are based on fundamental laws of physics known as Maxwell’s equations and Faraday’s law.

You can say you don’t agree with these laws of nature but you would be disagreeing with the all of the fundamental theories of electromagnetism.

Here is a fun problem. What is the maximum voltage you can get by interrupting the current in a big windlass powered by a 12V battery? If you say it is the system DC voltage of 12V that is the wrong answer. The voltage will rise until there is an arc or other type of voltage breakdown. If the contacts can withstand 1,000V before arcing then that is how high the voltage will go.

The only way to limit the voltage to the battery voltage is to install a diode that lets the current flow back into the battery or continue to flow in the motor. Either method will work. It is much cheaper to do this in 48V systems where the current is 25% that of a 12V system of equal kW rating.

Contact arc damage is not higher for 48V systems. It is a function of energy which is related to motor power not voltage. The same power motor causes the about the same arc damage to contacts regardless of system voltage. And 48V systems are easier to stop all arcing with a simple diode because it is 1/4 the size compared to a 12V diode.

[evm1024]

Originally Posted by ramblinrod

SNIP

I disagree. The amount of energy stored is not directly related to voltage, it is directly related to VA (Watts). 48 Vdc at 100 Ah, represent the same stored energy as 12 Vdc at 400 Ah, both being 4800 W-h.

Rod, take a look at what I wrote again - you are misunderstanding the meaning of "per unit of charge". Simply stated you increased the unit of charge 4 times to make your 12 volt example work for you.

And that is what I said. You need 4 times the unit of charge at 12 volts (in this case going from 100 AH to 400 AH).

The unit of electric charge is the coulomb (ampere-hour). And clearly an AH at 48 volts is more energetic than an AH at 12 volts.

I think that we can skip energy vs. power for that part of the discussion as that it detracts.

Originally Posted by RR

In my opinion, that is normally what is meant when on refers to "advanced technology". Nothing new or advanced about 48 Vdc in that light. "Advanced technology" is really just a marketing term. (IMHO).

SNIP...

Advanced DC systems is what was referenced not advanced technology. Not really the same thing. Apples to apples please.

Heck by your logic putting a SNAP generator on a sailboat would not be an advanced power system because SNAP has been around since the 1960's.

I guess we will have to disagree on this one. I think that having a SNAP generator on my sailboat would be very advanced.

[ramblinrod]

Originally Posted by Dockhead

I did not imply that it's stupid for sailboats. If a large sailboat spends most of its time in a marina berth, like most large power boats, then for sure it's a waste of money trying to implement an advanced DC power system.


My own boat, actually, was originally designed to have the generator running most of the time. She has a heavy duty prime power rated generator, which is so quiet that you can't hear it all outside, and barely hear it below, and the designer intended that the generator should be running most of the time when the boat was off shore power.



Accordingly, she was given a small house battery bank of only 220 amp/hours @ 24v, separated from another bank of the same size used for thruster, windlass, and winches.


But this is not everyone's use case. I used to spend most of the time off shore power, when I had a Hamble River mooring, and even now spend weeks or even months at a time off shore power. I spent last summer in the Arctic and didn't even see a shore power connection for probably two months.


Many people on here have similar use cases, and even if you already have the heavy duty generator, you don't necessarily want to run it for weeks on end, if you can arrange your system somehow so that you can "coast" on battery power for a day at a time or so. Not only is this more pleasant, and saves generator hours (which costs more in maintenance and amortization, than the fuel cost), it allows you to harvest power from sources like solar and alternator on the main engine, and might also help you use the generator more efficiently by charging intensively during shorter runs, then shutting it down.


Lithium offers some huge advantages for this type of use -- very high acceptance rate means you can use a very large alternator, large enough that if you have to run the main engine without propulsion, it will have a healthy load, but also so that you can harvest the energy you need over relatively short episodes of motoring. Also the cycle life. Also, and not the least of the advantages, the fact that you don't need to worry about finishing charges, nor is it harmful to leave lithium batts at a low state of charge -- a lithium bank is much more like a fuel tank, where you just dump fuel in when you have it, and then draw it out, when you don't, which is an enormous advantage for an off grid boat. And then yet another advantage is much less voltage sag for really big consumers like bow thrusters, which can greatly increase their efficiency. Lithium is also considerably cheaper than lead, if you calculate the real cost of a kW/h stored and then used.



Downside is that you can't just drop them into a system designed for lead -- lithium is so different from lead, that it needs a rather different system architecture with different control systems and infrastructure.


My next boat, which will be a metal boat designed for extreme latitudes, will have a large lithium bank, either 24v or 48v, with dual alternators on the main engine including a very large domestic power one (probably 5kW or so), and a very small emergency generator (and I would do without the generator if I were cruising in less remote places). Either the smallest Northern Lights job, or perhaps a NexGen. The yard which will make my next boat, which specializes in expedition vessels, rarely installs lead batteries any more.



Lead is great for marina queens or power boats, but does not stack up well against lithium for more demanding usage, at least not if the boat can be designed from scratch for lithium.






As to my preference for charging sources -- your memory is not serving you well. I love solar power and had great luck with it on my last boat. It's the best power source for those with space to install enough of it. Present boat however is a dedicated sailing machine and no place to put solar which wouldn't compromise upwind sailing ability. So I'm stuck with generator charging, which is nothing against those who have solar -- in fact, I am somewhat envious. The next boat will have solar panels built into the pilothouse roof, but will be electrical intensive enough (no gas on board) that I wouldn't count on living off solar power.





As to "incorrect" vs. "I disagree", this is not political correctness. Saying "incorrect!" or "wrong!" is saying something fundamentally different from "I disagree". Saying "incorrect!" asserts authority to judge the validity of another's opinion. That might not be unjustified in all cases, but in this thread, for example, the assertion of alleged authority has been massively irritating and has degraded the quality of the discussion. "I disagree" allows that there may be different opinions, and is much more respectful. To say "incorrect!" you'd better really be sure you know what you're talking about, and besides that, be really sure that you're not dealing with people with equal (or better) knowledge than your own, and even then, it's probably impolite in most cases.


It's not impossible that an electrician might understand some basic principle of electricity, better than an actual electrical engineer, but it is unlikely enough, that the electrician ought to be prepared with some really well prepared arguments, expressed in a really respectful way, and not simply bombastic assertions of one's own authority.

We seem to be confusing 48 Vdc with Lithium here too. Unrelated.

[Jdege]

Originally Posted by ramblinrod

Thank you for your contribution.
However, I really question the idea of the validity of electric propulsion in a typical cruising boat or passagemaker wishing to cover around 150 nm / day, wind or not

They are called sailboats for a reason.

There are a number of liveaboards who seem happy motoring only into and out of harbor, and if there is no wind, simply waiting.

Where they are typical, or not...

[brownr377]

So I can't back away from this thread. The thing the thing that is bothering me about this thread is that posing the question which is safer 12V or 48V is not a useful question. Either one can be perfectly safe, and either one can be dangerous. An electrical installation cannot be evaluated for safety based on a single parameter, it's just more complicated than that. To make an adequate safety assessment the entire system must be evaluated.

On a boat at low voltage, I consider anything less than 60 Volts low voltage for these purposes, the shock hazard is practically nonexistant. Yes, I suppose if you put your mind to it, you can figure out a way to get current across your heart, but you really really really have to work at it. There is however a real risk of fire hazard. Fires on a boat scare the bajesus out of me. In a 12V system load currents will tend to be higher, that is where the fire risk comes into play. If continuous load currents can be safely reduced with a higher supply voltage that seems like a good thing. If you were designing a system without the historical constraint of 12V dc being ubiquitous I doubt very much that would be your ideal voltage.

There seems to be disagreement on the fire hazard from a 48V battery bank vs 12V battery bank. But that's a different question than the risk from the supply voltage, either one of those banks can light your boat on fire, and requires appropriate fusing.

Now add lithium battery tech into the mix and there's a whole new set of risks involved, not insurmountable but different from lead acid, my only point here really is that to reduce a complex system down to a single parameter is not going to get you anywhere when trying to actually design a safe installation. Sorry that's a whole lot more words than I meant to write.

[AKA-None]

I admit I scrolled through the long recent posts but I don’t see any mention of the charge voltage for a 48 volt bank.