Comparative Safety: 12v v 24v v 48

[Dockhead]

Originally Posted by ramblinrod

Yes, that thread is as bad or worse than this one for misinformation.

So lets look at the physical reality.

Human skin resistance averages around 5 kohms.

It can be much higher.

Between the fingertips of a dry person who works with their hands and has tough callouses, it may be 100Kohms.

On just about anyone who is sweaty the inside of the forearm may be only 2 kohms.

For someone who is sweaty and has cuts on their hands, it may only be 1000 ohms hand to hand (current path across the heart).

Now, if we search the internet, including all extremely questionable sources, we can find all kinds of different current thresholds capable of stopping the human heart. I have personally seen as low as 10 mA, to one case as high as 500 mA. I use the threshold taught to me in Grade 9 "Theory of Electricity" class, 80 mA. (Before anyone gets their nighty in a knot, it really doesn't matter what the actual value is for this discussion.)

So in that ridiculous thread on that other forum, there was one valid statement (IMHO).

"48V is reasonably safe for most people under normal conditions."

I absolutely agree with this statement.

My position has never been that 48 Vdc is extremely dangerous under all conditions.

It was simply, "The risk of danger increases with voltage". It does.

Earlier in this thread I gave a perfect example of how one who works with electrical wiring can easily have lower than normal skin resistance.

a) They are often working hard in hot, cramped quarters, with little ventilation, where they will become very sweaty, shirt and shorts soaked sweaty.

b) They are working with the ends of cut wire, twisting the ends between their fingers so they fit in the end of a crimp connector. If they forget or skip the gloves (because they keep catching the wire strands) it can cut their fingers.

c) They may be working in an area with a bilge, that may have accumulated bilge water, which is where anything dropped will try it's worst to fins it's way. Could be a crimp connector, nut off a battery terminal, tool, what have you, it WILL end up in the bilge.

OK, so now lets look at what happens with extremes of skin resistance, at various voltages, and assume a 100 mA threshold is capable of stopping the human heart.

1. 12 Vdc

a) 100 kohm skin resistance (about the most it can be without gloves).
I = E/R = 12 Vdc / 100,000 ohms = 0.12 mA. No electrocution.

b) 10 kohm skin resistance (actually pretty common across the hands).
I = E/R = 12 Vdc / 10,000 ohms = 1.2 mA. No electrocution.

c) 1 kohm skin resistance (not normal, certainly possible).
I = E/R = 12 Vdc / 1,000 ohms = 12 mA. Risk of electrocution, only if we take the lowest threshold purported.

2. 24 Vdc

a) 100 kohm skin resistance (about the most it can be without gloves).
I = E/R = 24 Vdc / 100,000 ohms = 0.24 mA. No electrocution.

b) 10 kohm skin resistance (actually pretty common across the hands).
I = E/R = 12 Vdc / 10,000 ohms = 2.4 mA. No electrocution.

c) 1 kohm skin resistance (not normal, certainly possible).
I = E/R = 12 Vdc / 1,000 ohms = 24 mA. Greater risk of electrocution, at the lowest threshold purported.

3. 48 Vdc

a) 100 kohm skin resistance (about the most it can be without gloves).
I = E/R = 48 Vdc / 100,000 ohms = 0.48 mA. No electrocution.

b) 10 kohm skin resistance (actually pretty common across the hands).
I = E/R = 12 Vdc / 10,000 ohms = 4.8 mA. No electrocution.

c) 1 kohm skin resistance (not normal, certainly possible).
I = E/R = 12 Vdc / 1,000 ohms = 48 mA. Greater risk of electrocution yet.

Clearly this shows, that under typical conditions, while one may not be electrocuted (and perhaps most likely won't) "The risk (or danger) of electrocution increases with voltage."

Now lets take a look at the lowest skin resistance I have seen, hand to hand, on my own body, after a hard day at work, with many cuts. 500 ohms.

At 48 Vdc, I = E/R = 48 Vdc / 500 ohms = 96 mA. This is damn close to that 100 mA, that I believe is capable of stopping a human heart.

Now lets look at the worst case scenario, one touches both battery terminals, at the same time, (on purpose (dumb), or accidentally (plausible), when the charger is on.

At 60 Vdc (charge voltage), I = E/R = 60 Vdc / 500 ohms = 120 mA. Pretty good chance of electrocution.

If we were talking a 12 Vdc system in this scenario...

At 15 Vdc, I = E/R = 15 Vdc / 500 ohms = 30 mA. Not much likelihood of electrocution.

So far so good. Up to here, all that seems pretty reasonable. So if "48V is reasonably safe for most people under normal conditions", a statement you agree with, which presumably also means that it can be made without too much trouble quite safe indeed, why would anyone NOT use 48v if it offers material advantages in use?

Originally Posted by ramblinrod

Based on this, I conclude, that "Danger increases proportionally with voltage".

Even if we don't get to the actual threshold of electrocution under normal circumstances, we are working our way closer to it, and under worst conditions, we may already be there.

What?? This is just totally illogical. So the danger at 6v is double the danger at 3v? No, it's not, because the risk of electrocution is zero in both cases, even if you are sweaty and cut and are at 500 ohms. Actually depending on the currents involved, 3v is likely to be MORE dangerous than 6v, because high currents create other risks -- the wrench or wristwatch across the terminals. "Working closer" to a threshhold which is still far away, is not equal to danger increasing "proportionally".



Furthermore, who ever said electrical systems are "safe" only if you can grab them with wet, cut hands? If we are taking normal precautions to prevent that from happening, precautions anywhere close to what we take with AC power, then we just don't grab the terminals. Do we know of a single case of a person electrocuted on a boat with DC power of whatever voltage? I think we have not. So a non-material risk is non-material.

Originally Posted by ramblinrod

So this is just a brief [sic!] explanation of why I choose the lowest DC voltage possible that will satisfy loads, and believe it is the best policy to adopt for my business, to protect myself from possible liability claims, and my customers from possible danger.

But anyone who enjoys living on the edge and taking unnecessary risks can choose a higher electrical system voltage.

So it's a choice of either using the lowest possible voltage system, or "living on the edge and taking unnecessary risks"? No, it's not. There are reasonable balances of risks, and in any case, the risks of shock with DC systems on boats are so small, that the difference between them in terms of risk is simply not material.



If you really believed that "danger increases proportionately with voltage every single time", you would have 6v on your own boat, and not 12v, because you can certainly transport all the electrical power you need on a small boat at 6v, but you don't, do you? Even though according to your "rule", 12v is twice as dangerous as 6v, "every single time"? Or hell, let's make it FOUR times as safe, and go to 3.2v! And you don't, because there is simply no material difference in shock risk between 3.2v and 6v and 12v, or for that matter between 12v and 24v. And that is why no one accepts this "rule".

[TeddyDiver]

We had similar discussion some time back about paralled vs serial banks. In that thread RR refused to acknowledge any danger with large paralled banks, which are inevitable with larger hotel loads in 12 volts systems. Now he tries to claim higher voltages being more dangerous.

Allmost like he gets royalties of 12 volts

Anyway, what I learned from that thread RR holds his ground despite the facts so there's no way to educate him. However as some other poster pointed earlier false claims like RR's must be revealed everytime.

BR Teddy

[Q Xopa]

Originally Posted by ramblinrod

Yes, that thread is as bad or worse than this one for misinformation.

So lets look at the physical reality.

Human skin resistance averages around 5 kohms.

It can be much higher.

Between the fingertips of a dry person who works with their hands and has tough callouses, it may be 100Kohms.

On just about anyone who is sweaty the inside of the forearm may be only 2 kohms.

For someone who is sweaty and has cuts on their hands, it may only be 1000 ohms hand to hand (current path across the heart).

Now, if we search the internet, including all extremely questionable sources, we can find all kinds of different current thresholds capable of stopping the human heart. I have personally seen as low as 10 mA, to one case as high as 500 mA. I use the threshold taught to me in Grade 9 "Theory of Electricity" class, 80 mA. (Before anyone gets their nighty in a knot, it really doesn't matter what the actual value is for this discussion.)

So in that ridiculous thread on that other forum, there was one valid statement (IMHO).

"48V is reasonably safe for most people under normal conditions."

I absolutely agree with this statement.

My position has never been that 48 Vdc is extremely dangerous under all conditions.

It was simply, "The risk of danger increases with voltage". It does.

Earlier in this thread I gave a perfect example of how one who works with electrical wiring can easily have lower than normal skin resistance.

a) They are often working hard in hot, cramped quarters, with little ventilation, where they will become very sweaty, shirt and shorts soaked sweaty.

b) They are working with the ends of cut wire, twisting the ends between their fingers so they fit in the end of a crimp connector. If they forget or skip the gloves (because they keep catching the wire strands) it can cut their fingers.

c) They may be working in an area with a bilge, that may have accumulated bilge water, which is where anything dropped will try it's worst to fins it's way. Could be a crimp connector, nut off a battery terminal, tool, what have you, it WILL end up in the bilge.

OK, so now lets look at what happens with extremes of skin resistance, at various voltages, and assume a 100 mA threshold is capable of stopping the human heart.

1. 12 Vdc

a) 100 kohm skin resistance (about the most it can be without gloves).
I = E/R = 12 Vdc / 100,000 ohms = 0.12 mA. No electrocution.

b) 10 kohm skin resistance (actually pretty common across the hands).
I = E/R = 12 Vdc / 10,000 ohms = 1.2 mA. No electrocution.

c) 1 kohm skin resistance (not normal, certainly possible).
I = E/R = 12 Vdc / 1,000 ohms = 12 mA. Risk of electrocution, only if we take the lowest threshold purported.

2. 24 Vdc

a) 100 kohm skin resistance (about the most it can be without gloves).
I = E/R = 24 Vdc / 100,000 ohms = 0.24 mA. No electrocution.

b) 10 kohm skin resistance (actually pretty common across the hands).
I = E/R = 12 Vdc / 10,000 ohms = 2.4 mA. No electrocution.

c) 1 kohm skin resistance (not normal, certainly possible).
I = E/R = 12 Vdc / 1,000 ohms = 24 mA. Greater risk of electrocution, at the lowest threshold purported.

3. 48 Vdc

a) 100 kohm skin resistance (about the most it can be without gloves).
I = E/R = 48 Vdc / 100,000 ohms = 0.48 mA. No electrocution.

b) 10 kohm skin resistance (actually pretty common across the hands).
I = E/R = 12 Vdc / 10,000 ohms = 4.8 mA. No electrocution.

c) 1 kohm skin resistance (not normal, certainly possible).
I = E/R = 12 Vdc / 1,000 ohms = 48 mA. Greater risk of electrocution yet.

Clearly this shows, that under typical conditions, while one may not be electrocuted (and perhaps most likely won't) "The risk (or danger) of electrocution increases with voltage."

Now lets take a look at the lowest skin resistance I have seen, hand to hand, on my own body, after a hard day at work, with many cuts. 500 ohms.

At 48 Vdc, I = E/R = 48 Vdc / 500 ohms = 96 mA. This is damn close to that 100 mA, that I believe is capable of stopping a human heart.

Now lets look at the worst case scenario, one touches both battery terminals, at the same time, (on purpose (dumb), or accidentally (plausible), when the charger is on.

At 60 Vdc (charge voltage), I = E/R = 60 Vdc / 500 ohms = 120 mA. Pretty good chance of electrocution.

If we were talking a 12 Vdc system in this scenario...

At 15 Vdc, I = E/R = 15 Vdc / 500 ohms = 30 mA. Not much likelihood of electrocution.

Based on this, I conclude, that "Danger increases proportionally with voltage".

Even if we don't get to the actual threshold of electrocution under normal circumstances, we are working our way closer to it, and under worst conditions, we may already be there.

Now if we go way back in the thread, I proposed the phenomenon exists I referred to as "desensitization", or same may wish to consider it "complacency".

We hear these stories where residential electricians stick their finger in a socket to determine if it's live.

I have done that too.

When I was young and stupid and thought I was invincible.

Even with the experience of feeling this sensation many times, on one occasion of receiving an unexpected 120 Vac, I instinctively/involuntarily withdrew may so quickly and with such force, that shattered a window made of tempered, re-inforced safety glass, that was beside my work bench.

Now that I have tons and tons more experience, I am more careful, because I know what can happen when one becomes desensitized to or complacent with the danger of electrical current, even if it insufficient to even be felt, in the human body.

Because now I know, that I may contact 48 Vdc (or 60 Vdc charging) 1000 times and 900 times never even feel a tingle (and even this can cause some physiological damage), but it will only take that once, when conditions are working against me, that could be the last time.

Now, on the AC side, where the voltage is even higher yet, (and yes I do understand that we have a sinusoidal peak voltage that may more easily reach the skin resistance breakdown threshold), ABYC standards compliance requires that a GFCI receptacle be used, for any outlet where there is a good chance of moisture, such as a head, galley, weather deck, or anywhere water is likely to be in close proximity, (such as above a bilge, though not specifically stated).

The threshold for tripping to prevent electric shock is 5 mA.

As mentioned, there may be a difference between AC and DC voltage regarding the electrocution threshold, but since we are not sure what that is, or if it is even true, if look solely at the difference in voltage, the same level of protection for 12 Vdc would be calculated by direct assimilation...

GFI threshold 48 Vdc = 120 Vac / 48 Vdc * 5 mAdc = 12.5 mA

Even if we account for the difference between Peak AC voltage and constant DC voltage with respect to skin resistance breakdown potential (which we don't really know is a factor), then the difference would be 12.5 mA DC * 150% = 18.75 mA.

GFCI rely on hall effects sensor technology, that unfortunately will not work on DC.

So as it stands, we do not have an "always on" integral safety device to prevent people from being electrocuted by DC voltage.

If we did, then I would consider that a satisfactory "Mitigating Factor", to overcome the increased risk of electrocution by 48 Vdc than by 12 Vdc.

Now this is just the electrocution angle, then there is also the increased danger due to burns (one example illustrated by Growly Monster earlier in this thread) and yes of course, under less that ideal conditions one could be burned by 12 Vdc but because P (energy) = E (voltage ^ 2) / R, I know that mathematically (and barring any mitigating factors in practice) that the risk of a burn from 48 Vdc, is EXPONENTIALLY greater, than from 12 Vdc.

This same principle also applies to the risk of sufficient heat being produced due to a short circuit to start a fire.

So this is just a brief explanation of why I choose the lowest DC voltage possible that will satisfy loads, and believe it is the best policy to adopt for my business, to protect myself from possible liability claims, and my customers from possible danger.

But anyone who enjoys living on the edge and taking unnecessary risks can choose a higher electrical system voltage.

It is certainly possible.

But just because it is possible and may save some copper (usually minimal in the grand scheme), does not make this a wise choice, because, wait for it, here comes the generalization again, that so many love...

Danger increases proportionally with voltage (barring any mitigating factors).

The only reason I even care, is because desensitization to the risk, and a moment of complacency, could result in personal injury, death, or property destruction, that may negatively impact the individual making this decision, or their loved ones, or even total strangers (e.g. slip neighbours).

Of course anyone can knowingly or unwittingly disregard these risks, and go ahead and do it.

But, is it the right thing to do?

In my opinion, and perhaps solely in my opinion and nobody else's, I believe not.

I have not heard one argument nor seen one shred of evidence in this entire thread, that proves the facts, theories, and experiences that I have relied on to develop this position and policy over the last 40 years (actually 50 if we consider that I first learned of Ohms and Watts Law when conducting a research project in and advanced science class in Grade 6.)

I also learned in Grade 8, after conducting research into "The Candu Thermo-Nuclear Reactor", to never trust anyone who claims...

"Don't worry, I am more highly educated than you, and I declare it safe, therefore it is."

(Meanwhile, 50 years later, we are living near huge pools (25 years beyond life expectancy, and well beyond original design capacity) of untreated nuclear waste, just waiting for that one brief moment of complacency to unless its "gotcha".

Of course the consequence would likely be far greater, but the principle is the same as choosing a higher than necessary marine electrical system voltage.

Now some people have gone to all kinds of extremes to present "strawman arguments" (restating my position, or including mitigating factors, to intentionally change the meaning or veracity of my position), or challenge my education, experience, or very authority to even evaluate the risks.

But I know this fact won't change, when I evaluate and electrical system design, "I will choose (or recommend) the lowest possible system voltage to support the required loads, that is commonly in practice and for which parts are readily and economically available."

For a marine DC electrical system, to support loads up to about 6 kW for ~ 5 minutes, or ~ 3 kW for one hour, this is currently 12 Vdc.

For higher loads, I will choose 24 Vdc for loads up to about 12 kW (brief) and 6kW (extended).

For even higher loads, I will choose products that can be powered by the 120 or 240 Vac to be operated by that system that will already normally be present to handle higher loads than the lowest possible voltage DC systems.

(Note that by doing so, I have now introduced a mitigating factor (GFCI's and ELCI) to help negate the increased risks associated with higher voltage.)

If powering high load devices with the AC electrical system is not practical, I will chose (recommend) 48 Vdc for up to 24 kW or 12 kW respectively, and so on and so on and so on.

And this policy (rule of thumb if that makes some more happy) is because I know (absolutely) that barring the presence of any mitigating factor, danger increases proportionally with voltage EVERY SINGLE TIME. It has to, it's as simple as Ohm's and Watt's Laws that I learned 50 years ago, and have held true throughout my entire 40 year career, testing, troubleshooting, designing, prototyping, manufacturing, repairing, marketing, teaching, electronics and electrical products and systems.

One can try to justify going to a higher voltage than necessary any which way from Sunday, but it will be based on flawed arguments, willingness to accept higher risk than necessary, or to employ a higher degree of mitigating safety factors.

It's there choice.

My choice, and recommendation, is to not do so.

What's the problem people?!?!?!?!?

I have to admit when I see your long winded posts, I just skip over. Its good to see others still continue to point out how you are incorrect

[AKA-None]

My point was that sometimes voltage induces a muscle flex which can cause injury. The odd of that occurring go up with voltage and reduced skin resistance.

[evm1024]

Many great posting have been made but starting here:

Originally Posted by Dockhead

So far so good. Up to here, all that seems pretty reasonable. So if "48V is reasonably safe for most people under normal conditions", a statement you agree with, which presumably also means that it can be made without too much trouble quite safe indeed, why would anyone NOT use 48v if it offers material advantages in use?

SNIP %<

So it's a choice of either using the lowest possible voltage system, or "living on the edge and taking unnecessary risks"? No, it's not. There are reasonable balances of risks, and in any case, the risks of shock with DC systems on boats are so small, that the difference between them in terms of risk is simply not material.
SNIP

I went off and took another look at how we got there. Many of RR posts stand out but I was trying to see if I could identify a single point of departure from reason.

This post might qualify

Originally Posted by ramblinrod

This is an important point that deserves a bit of focus and direct response.

1. Electricity and water don't mix.

2. The marine environment is extremely harsh for electrical systems.

3. 55% of the fires on boats are caused by an electrical fault. (USCG report).

4. Of these, 54% are related to 120 Vac circuits. (Ref: HowtoMarine.com).

5. Only 10-25% of circuits aboard boats are 120 Vac (my WAG).

6. Conclusion: Higher voltage is more dangerous than lower voltage. (So the actual data proves the theoretical greater risk.)

The reason we don't perceive the enormity of the danger present?

De-sensitization.

We are exposed to it every day.

It looks pretty typical. Throw some data out, look at only one aspect, ignore other viewpoints, make a sweeping conclusion, defend that conclusion to the death. Blah, blah, blah.

As opposition to the sweeping conclusion grows expand defense to other areas e.g. from fire risk add electrocution.

Double down to a strawman i.e higher voltages are more dangerous. Try to prove it with cherry picked parameters e.g 10 ohms series fault resistance rather than 0.01 ohm or 100 ohms.

Mislead with EEE where everything else is not equal. Simply stated, using 48 volts is transformative. You would not install a 48 volt system as if it was a 12 volts system running at a higher voltage. If for no other reason than to take advantage of the increase in power.

The reality is that some of us see the spectrum of risk/rewards at various voltages. Others appear to be more one dimensional in their thinking regarding risk only and in a very limited scope at that.

Broaden your horizons! include I^2*R in your evaluation.


Get ready - 48 volts is a game changer!

[evm1024]

48 volts is a game changer (another long winded essay)

48 volts is a game changer. I say 48 volts but in reality I am speaking about any voltage between oh say 36 volts and 60 volts. High enough to pack a lot of energy and yet low enough so that the risk of electrocution is minimal should someone come in contact with a bare conductor.

Are you working on your electrical system while drenched in salt water? Who would do that? Tell you what go soak your hands in salt water, Nix that. Go soak your forearm in salt water then lay it across your 12 volt bus and ground bus. May be nothing happened. May be you get a good burn. Basic rules….

Currently (no pun etc) for many, many boats the generation and consumption of electrical power are on par. There is no excess to speak of and we are always counting AH to be sure we don’t run out. In the past because AH were hard to come by and hard to store we were hoarders of energy.

Contrast that with water. In the old days (and for any boat without a water maker) we were hoarders of water. We were always trying to keep our usage to a minimum and were on the lookout for the next fill. Enter water makers and daily showers became possible. Use based on need rather than availability became the norm.

With 12 volt lead acid systems we could add storage, and then add generation, better regulators to overcome “issues” with alternators and batteries. Solar became more commonly available and gensets. But we were (are?) still basically hoarders of power. Need more power? Add more batteries and all the weight and other issues that are involved. At 12 volts the currents involved can and do exceed our ability to manage them effectively. In effect 12 volts has set the upper limit of the energy budget.

Enter LiFePO4. Moving from Lead Acid to new battery technologies is an enabler that should not be underestimated. This is the storage portion of the equation. Close to 100% charging efficiency, Close to 100% discharge efficiency, 80% plus usable capacity range, high acceptance rate. You can add in Firefly and the like to these new battery technologies. The point is that we are becoming free of the limits that FLA has bound us to.

Back to 12 volts, this is a limit. As we have seen many times in this thread P= I^2R. For any given resistance if you double the current you get 4 times the power. Why is that a problem?

I have a bad connector that I pulled out of my boat. One of the lugs on a 2/0 cable that I have has some series resistance. I really should go measure the resistance to see what it is. It is not 10 ohms for sure…. It will likely be somewhere in the 0.01 ohm to 0.005 ohm range as a wag (wild ass guess). How do I know it is bad? Easy, it gets hot. This cable ran between the inverter/charger and the inverter disconnect switch. And when running the inverter or charger the lug gets hot. I do use an infrared pyrometer to check all my connections under load and you should too. It would be even better to have one of the infrared cameras designed to show hotspots but they are spendy and I digress.

Let’s run some numbers. For sake of argument let’s say that the series resistance was 0.01 ohm. If we are charging the batteries at 100 amps or using the inverter at 100 amps then the power dissipated in this bad lug would be (100*100*0.01) 100 watts. I^2*R at work. OK, what happens at 200 amps? My inverter can draw 200 amps so that is not unrealistic. We end up with (200*200*0.01) 400 watts dissipated in the bad lug. Four times the power for double the current. OK, how about 400 amps? My inverter does not draw 400 amps but we do get 400 amp loads on our boats at times. And I will weave the 400 amps back in later. Anyway that 400 amp current dissipated (400*400*0.01) 1600 watts in that bad lug. 1600 watts is your typical space heater here in the USA. I imagine that that lug would get really hot at 400 amps. Of course the resistance of the bad lug is in reality unknown but the math is real. If the resistance was 10 time smaller at 0.001 ohm then we would end up with 10, 40, 160 watts dissipation with the currents specified. As I said I don’t know the actual resistance of the bad lug. But it appears reasonable to guess that it is between 0.01 and 0.001 ohms. I really will have to measure it.

The point is simple, 12 volts has limits of usable power and that the limit is set by the effects of increasing currents. This limit may not show up in a 10 second burst of 400 amp use but any fault such as this lug will be a problem at 400 amps.

Wait, where is the voltage in all this? It is not there, it does not need to be there. Current through a resistance sets the power dissipation without regard to voltage. Sure, voltage, current, resistance and power are all related. But we do not speak of a voltage flow; we speak of a current flow. Let that sink in.

I^2*R is a curse at lower voltages and a gift at higher voltages. Case in point, Bonneville Power Administration (BPA) ships power from the Pacific North West with all of its cheap hydro power to Los Angles. The power line is running 1,000,000 (one million) volts DC for the roughly 1000 mile journey. Why 1,000,000 volts? To minimize I^2*R losses, DC rather than AC to minimize losses as well.

Enter 48 volts - the game changer. Just like having a water maker on board frees us of a number of limits having a higher voltage energy storage solution frees us on a number of limits imposed by lower voltage systems.

On my boat for example I have 700 AH of LiFePO4 battery at 12 volts. For sake of argument let’s just call that 6 KWH of power. I also have a 200 amp alternator on the engine and I tend to run that at 120 amps for a number of reasons. Reasons that include the I^2*R issues in the cabling. If I were to draw the battery most of the way down I would need to run the engine for 6 or 7 hours with a 120 amp charge current to get that capacity back. Any ohmic connector, lug wire will heat up at 120 amps. But not too bad.

Say I wanted to recharge that bank in half the time. That is a big win I would say. So now I need to run my alternator full bore. It is a 28SI so it could do near 200 amps for a while (or perhaps 180 amps but let’s say 200). Now I get to run the engine for only 2.5 or 3 hours. Half the time but double the current and 4 times the dissipation (and heat in proportion) at any bad connection.

What is I wanted to charge my batteries in one fourth the time? OK here’s where the 400 amps comes back. With 400 amps charging I could fill up that (mythical) 700 AH bank in less than 2 hours. But, But the I^2*R is a killer. With 400 amps going through that bad cable lug I could easily see smoke, a lot of smoke.

[evm1024]

Let the magic begin – When we decouple storage and generation of power from use we end up with a number of possibilities.

Let’s take a look at some numbers again. We will start with a 48 volt nominal (51.2 @ 3.2 VPC) house bank composed of 16 LiFePO4 180 AH cells. This bank is close to the 700 AH 12 bank capacity (720 AH @ 12 volt) but it runs at 48 volts nominal. Great, so our 400 amp 12 volt charge rate becomes 100 amps at 48 volts. This means that we get to charge the house bank in less than 2 hours with 1/4 the current and thus the risk of I^2*R heating greatly reduced. (400 amps vs. 100 amps)

Bruce Schwab sells and installs American Power Systems 44i-110act-56v alternators. These alternators are 110 amps at 56 volt units with a rated power of 6 kW. The future is here.

Is this for everyone? Hardly. Is it Mainstream? Not yet.

[Dockhead]

Originally Posted by evm1024

. . . Enter LiFePO4. Moving from Lead Acid to new battery technologies is an enabler that should not be underestimated. This is the storage portion of the equation. Close to 100% charging efficiency, Close to 100% discharge efficiency, 80% plus usable capacity range, high acceptance rate. You can add in Firefly and the like to these new battery technologies. The point is that we are becoming free of the limits that FLA has bound us to.. . .

This is all a SYSTEM, and you can't really look at just one part of it in a vacuum and have a useful idea.


I don't like the hackneyed phrase "game changer", but if anything is that, it's lithium, and the availability of this type of storage feeds right back into the other discussion about system voltage.


Lithium makes it possible to use mammoth alternators, and frees us from the necessity to tend the Victorian science experiment which is lead acid batteries, requiring "finishing charges", nursing them through long charge cycles because of the low acceptance rate, worrying about let them get drawn down too far, and all that stuff, which someday soon, we won't believe we ever put up with. Lithium is like a tank -- just dump it in when you have it available, then use it as you like, including very high power applications because lithium doesn't care. At the end of the day, with power so easy to store, it makes much more power available to us off grid.



So with that we can use our boats differently and other parts of the system look different to us, too. Suddenly we're handling a lot more electrical power, with mammoth alternators and big inverter banks, and then taking advantage of abundant power, we can do stuff like electric cooking, which we might not otherwise have considered, so demand grows with supply. It's all a system, and it's changing.



Handling all that power, all of a sudden, means handling huge currents, the likes of which we haven't seen, if we don't get the system voltage up. I do not believe that a huge current is safer, just because it's at a lower voltage. In fact I think it's mostly the other way around.

[evm1024]

Originally Posted by Dockhead

This is all a SYSTEM, and you can't really look at just one part of it in a vacuum and have a useful idea.

SNIP

I agree that it is a system.

Because it is a system calling 48 volts a game changer is an overstatement without Lithium. By the same token Lithium is not a game changer at 12 volts. It is a great leap but because of the exponential increase in power dissipation with increasing currents at 12 volts the advantages of LiFePO4 are real but limited.

And thus I amend my statement to:

48 volts is a game changer when coupled with LiFePO4 batteries.

Originally Posted by Dockhead

Handling all that power, all of a sudden, means handling huge currents, the likes of which we haven't seen, if we don't get the system voltage up. I do not believe that a huge current is safer, just because it's at a lower voltage. In fact I think it's mostly the other way around.

Very well said. Who could argue with that?

[ramblinrod]

Originally Posted by TeddyDiver

We had similar discussion some time back

Strawman Alert: I made no such claim.

Irrelevance Alert: Not related to this subject.

These childish games just have to stop.

I do not wish to re-argue every debate I have ever been in, in my entire life, because you (nor anyone) can find error in my position for this debate.

(After all, it becomes a bit embarrassing for a person as humble as I, to win so many different debates.) ;-)

If you think ANYTHING I have posted in THIS thread (and there is a lot here to choose from) is incorrect, please:

1. Pick the easiest incorrect statement (of mine) for you to argue.
2. Indicate why you feel it is wrong, using verifiable facts, logic, and reasoning.
3. If I believe your claim is correct, I will admit error.
4. If I believe your claim is incorrect, and you raise an issue I have not already addressed in this thread, (because even I am tiring of having to repeat myself time and again to address the same falsehoods) I will post why, using logic, facts, and reasoning.
5. If you have no valid rebuttal, that can be supported by logic, facts, and reasoning, I expect you to admit error.
6. If you choose not accept this challenge, or you cannot satisfy step 5, your position shall be considered, to be erroneous.

[Dockhead]

Originally Posted by evm1024

. . . .Who could argue with that?

Do you really want me to answer that question?

[ramblinrod]

Originally Posted by Dockhead

So far so good. Up to here, all that seems pretty reasonable. So if "48V is reasonably safe for most people under normal conditions", a statement you agree with, which presumably also means that it can be made without too much trouble quite safe indeed, why would anyone NOT use 48v if it offers material advantages in use?

Simple.

Everything else equal, they would be increasing danger, for little to no benefit.

As I've said many times, there comes a point where the supporting cable or equipment becomes inavailable, or just too expensive. Then and only then, can a higher voltage be justified (by any reasonable person).

What?? This is just totally illogical. So the danger at 6v is double the danger at 3v?

Strawman Alert: I made no such claim.

However, based on the premise, barring any mitigating factors, danger increases proportionally with voltage, then 6 Vdc is more dangerous than 3 Vdc.

This is supported by the examples I gave, of the power dissipated by a fixed short circuit resistance for two voltages.

I fully acknowledge that at this low voltage, the danger of electrocution is minimal.

Unless there is a mitigating factor (such as a current limiting method) the energy dissipated in heat by the circuit having the higher voltage, will be EXPONENTIALLY higher.

If we accept the premise, that fire requires heat, fuel and oxygen,
and the greater the heat, the greater the danger of fire (also barring any mitigating factors) then we must accept that danger increases proportionally with voltage (BMF).

Furthermore, who ever said electrical systems are "safe" only if you can grab them with wet, cut hands?

Strawman Alert:

Nobody.

If we are taking normal precautions to prevent that from happening, precautions anywhere close to what we take with AC power, then we just don't grab the terminals.

We can take normal precautions not to release natural gas into our house and blow it to smithereens. That doesn't change that natural gas in a house can be dangerous (BMF).

Do we know of a single case of a person electrocuted on a boat with DC power of whatever voltage?

I don't know of a single case of a person being electrocuted by 240 AC; that doesn't mean it hasn't happened, or the risk is not present.

If you really believed that "danger increases proportionately with voltage every single time", you would have 6v on your own boat, and not 12v, because you can certainly transport all the electrical power you need on a small boat at 6v, but you don't, do you?

I do believe that. However, as I stated, I used the lowest voltage practical. Many of the loads that I need or wish are not available in 6 Vdc version, and this deviation from the norm could reduce asset value, so I would choose not to.

Even though according to your "rule", 12v is twice as dangerous as 6v, "every single time"?

Strawman Alert:

I never made this claim. I merely stated danger increases proportionally with voltage (BMF). It does. It did at the start of this thread, and it still does, despite every irrational claim that it doesn't.

I am sorry, if it disturbs anyone for any reason, to accept that the statements I HAVE made are correct, but that doesn't change that they are.

As I've indicated many times, reasonable precautions should be made when handling 12 Vdc. Greater precautions should be taken when handling 24 Vdc, and even greater yet precautions should be taken when handling 48 Vdc.

Why?

Danger increases proportionally with voltage (BMF).

By inference, introducing mitigating factors can help reduce the danger of higher voltage.

[transmitterdan]

What is BMF?

[OldMan]

Originally Posted by transmitterdan

What is BMF?

After 40+ years, EMF or BMF, same thing!

[ramblinrod]

Hmm, pretty typical.

Originally Posted by evm1024

Throw some data out, look at only one aspect, ignore other viewpoints, make a sweeping conclusion, defend that conclusion to the death. Blah, blah, blah.

Incorrect statement # 1.

Actually, here is what happened.

1. I made a statement, "Danger increases proportionally with voltage.
2. Tou and others challenged that posting invalid justification for your arguments.
3. I rebutted with facts and examples proving my position correct and your wrong.
4. You posted repeated attempted character assassinations, questioning my credentials, and carrying on in a disrespectful manner post after post.
5. Note that you have yet to prove my initial statement incorrect, or disprove any of my corrections to your subsequent flawed arguments.

Why?

I'm correct, on all counts.

Sheesh.

As opposition to the sweeping conclusion grows expand defense to other areas e.g. from fire risk add electrocution.

Error #2.

Ummmm, those were all logical and accurate defences to the primary position that you were challenging.

I am sorry if you found them impossible to shoot down because they are true.

I guess that is the way it goes when you are wrong.

Sheesh.

Double down to a strawman i.e higher voltages are more dangerous.

Error # 3.

Please look up the definition of a "strawman argument".

You have used this term incorrectly throughout this thread.

I provided a correct definition a while back in one of my posts (after you and others had posted many strawman arguments, and you still get it wrong.

Sheesh.

Try to prove it with cherry picked parameters e.g 10 ohms series fault resistance rather than 0.01 ohm or 100 ohms.

Error # 4.

In post number 1, NUMBER 1, yes the very first post, I posted the results for a series fault resistance for each of 12 Vdc, 24 Vdc, and 48 Vdc, for resistance value of 0.001, 0.01. 0.1, 1, 10, and 100.

There was no cherry picking as you claim. You are ragging on about my not doing something, that I did at the very start of this thread.

Sheesh.

Mislead with EEE where everything else is not equal.

Error # 5.

That was not a mislead.

That was a misinterpretation on your part.

EEE meant for the same load (in Watts) at each respective voltage, no mitigating factors, and no other changes.

Of course I know that for every thing else (including P) to be equal, that the current would be reduced by the exact same factor the voltage was increased, and I have stated this again and again and again.

Sheesh.

The reality is that some of us see the spectrum of risk/rewards at various voltages.

OMG, correct, correct, a beautiful, glorious correct statement.

This makes me so happy.

Finally you are almost there.

If I interpret this statement correctly (and please correct me if I'm wrong) you are stating that..

You acknowledge that danger (risk) increases proportionally with voltage, but that you are willing to accept that risk.


Am I correct that you are finally acknowledging that my statement, "Danger Increases Proportionally with Voltage" is correct?

Whether ones desire for the benefit of a choice or modification, enables them to accept higher risk, does not make the risk go away, it's still there.

Thank you so much.

Not so hard was it?

Broaden your horizons! include I^2*R in your evaluation.

Error # 6

Ummm, I have.

I have referred to the formula P=E*I (Watt's Law) and its derivatives subsitituting terms using Ohms law (E=I*R), to produce, P=E^2/R, and P = I^2*R throughout this thread and applied it to every example I have posted (as appropriate).

Sheesh.

I think I have figured out the problem.

You simply have not read what I posted, concocted some other position that you claim is mine, (but isn't) so that you can launch an argument that you think you can win.

What is that called again?

Oh yeah, that is a "STRAWMAN ARGUMENT".

Sheesh

Get ready - 48 volts is a game changer!

Ummm, the ability to use 48 Vdc has been around for umm ever.

Did you read any of the posts that declared that cars were supposed to go to 48 Vdc (or thereabouts) around 40 years ago, as lead by a group at MIT.

But they didn't do it.

And do you know why?

Because Danger Increases Proportionally with Voltage.

Too funny.

So there are a few people who are converting boats to electric propulsion.

As I said, for those high loads, 48 Vdc (or even higher) may be justified.

And with the advances in technology, reduction in cost, and real estate on some boats ideally suited to the installation of large solar arrays, this has made energy production at higher voltages more practical.

I also suspect that 48 Vdc alternators (and other marine electrical products) will become increasingly prevalent. (There are enough people out there trying to make a buck, doing their best to convince people to do something different.)

There are enough people on forums like this one, who have increased their system voltage and claimed, "this is great", while not honestly reporting the problems encountered, and needless costs.

So I don't doubt for one minute, that we will see more and more vessels with higher voltage DC electrical systems.

I WILL BE INSTALLING THEM (when appropriate to meet the customers energy demand).

But do you know the conversation we will be having?

<<<<<<<< >>>>>>>>>>>

My review of your energy needs based on the loads you desire will demand the incorporation of an X Vdc electrical system. (X represents the appropriate voltage level.)

Compared to a traditional 12 Vdc electrical system, this will increase cost by $Y, and has inherent increased risks that include electrical shock and possible electrocution, and an increased risk of fire, should the system develop a short circuit.

We will employ best industry practices to mitigate these risks, but they will still be present.

Here are some options:

A) Proceed with this X Vdc system?

B) Reduce loads to enable use of a lower voltage DC electrical system?

C) Transfer some loads to the AC electrical system, to be powered by shore power or a generator, to enable use of a lower voltage DC electrical system?

Please advise which option you prefer, and I will draw up the plans with estimates for your final approval.

<<<<<<<<<< >>>>>>>>>>

Why would I do this?

Because "Danger increases proportionally with voltage", and I do what ever I can to keep my customer's boats, and their precious cargos, safe.