Comparative Safety: 12v v 24v v 48

[TeddyDiver]

Originally Posted by ramblinrod

After all the time I have spent trying to correct the incorrect statements posted in this thread by others, that could cause forum members harm...

I am not about to waste time searching for studies to prove something that I have known to be correct for over 40 years.

A good contact or connection (switch, breaker, wire connector, etc.) can be near zero resistance.

A bad contact or connection can be any resistance value above design, up to and including "open" circuit.

This is pretty fundamental.

That kind of attitude remainds of the infamous Brent Swain. Sad actually..

[Dockhead]

Originally Posted by ramblinrod

I would not present a position and defend it, if I didn't know it to be correct.

If one does present a position and defend it, when they know they are incorrect, that would be stupid and self-defeating.

I suspect I am neither. . . . .

Just a point of order . . .



Arguing something one knows to be correct and arguing something one knows to be incorrect -- are not the only choices.


You left out -- presenting a position and defending it, when you THINK you know it to be correct, but in fact you might be mistaken, or might be only partially right, and someone else might be telling you something which might add to your knowledge, if you would only listen to it and think about it.



Aristotle said -- wisdom begins when you start to understand what you don't know. Unfortunately that is kind of like, you know, a word to the wise

[Dockhead]

Originally Posted by ramblinrod

Not surprisingly, yours is the size of vessel and your thruster is the size of load, where I prescribed that an increase from 12 Vdc to 24 Vdc (for those loads) may be warranted for practical reasons.

Make no mistake, 24 Vdc is more dangerous, but it's also more practical for a short duration 7.5 kW load.

PS, an unacceptable voltage sag in a DC system powered by FLA batteries can be resolved by increasing capacity. It can also be resolved by a change in battery technology less prone to sag, but that kettle of fish is unrelated to the subject of this thread, so lets keep it separate please.

But why is sending 400 amps of power, 30 feet to the bow, at 24v, more dangerous than sending 800 amps (eek) 30 feet to the bow, at 12v?


I am not an electrical engineer by any stretch, neither am I ABYC certified , and have a completely open mind about this, but I haven't heard any coherent argument of why this should be so. On the contrary, I've heard a lot of seemingly technically sound arguments that 24v is more safe, for this particular very high power circuit (10kW according to the manufacturer, not 7.5 -- motor efficiency losses, bevel gears etc.).



As to voltage sag -- this is a really important issue for thrusters, because the actual thrust falls drastically with voltage. Just a couple of volts can reduce the thrust nearly in half. I guess I could nearly double the real thrust of my thruster with lithium and either heavier cables or higher voltage. That is a massively important thing, and on topic of this thread. Bigger capacity of course helps, but I already have a quarter of a metric tonne of lead batteries on board and no place or desire to add more.



When designing a new boat -- it's not as simple as just putting the next sized thruster in. Tunnel thrusters cause drag which hurts sailing performance, and cause exponentially increasing drag with tunnel size. So you really, really want to get every kg/f of thrust you can, out of given sized thruster, so dealing with voltage sag is critical.


This is really relevant to this thread -- because it shows that there are applications where higher voltage can bring very significant benefits, and so you don't want to casually dismiss higher voltages out of some long held belief or superstition -- because there is a lot at stake. So in making these design decisions, you really need to get to the bottom of the question which is the subject of this thread.

[CatNewBee]

In your application, higher system voltage is not only beneficial, but a must have.
The alternative - if you insist on 12V - would be to place a lithium buffer battery next to the bow truster, but with 10kW you may need at least a capacity of 800Ah to have the required 800A continously usable (1C discharge current). even with 48V, there are still 200A flowing and you need a 48V 200Ah at least to handle the current, would also be good to have the battery in close proximity.

You can use LFP on 2C for a short time, even up to 10C for some seconds, but for safety and having some reliable energy in dangerous situations, I would not exceed the 1C limit that allows permanent discharge over the whole capacity without overheating.

[Dockhead]

Originally Posted by CatNewBee

In your application, higher system voltage is not only beneficial, but a must have.
The alternative - if you insist on 12V - would be to place a lithium buffer battery next to the bow truster, but with 10kW you may need at least a capacity of 800Ah to have the required 800A continously usable (1C discharge current). even with 48V, there are still 200A flowing and you need a 48V 200Ah at least to handle the current, would also be good to have the battery in close proximity.

You can use LFP on 2C for a short time, even up to 10C for some seconds, but for safety and having some reliable energy in dangerous situations, I would not exceed the 1C limit that allows permanent discharge over the whole capacity without overheating.

Yes, I think this is right. I don't think that this thruster is even feasible on 12v at all (actually I don't think it's even made in 12v).


Dedicated bow thruster batteries are silly in my opinion -- wasted capacity, and extra weight in the worst possible place.


Hydraulic thruster is one intriguing option, but this doesn't really make sense unless you're doing all the high power gear in hydraulic. I think I like electric better than hydraulic for all this gear, and the next boat will likely have powered furlers as well, so even more high power gear.

[CatNewBee]

You have so much weight in your keel anyway, placing the lithium battery midships or in the front cabin can help a lot with efficient wiring of all this power hungry gear. Lithium is half the weight of lead acid and it yelds more amps on discharging, so you save a lot of weight anyway. keeping the stuff midships / at the front of the keel is not that bad, you can balance out the boat by water and fuel tanks, and storage aft. A 1000Ah cell weights only 42kg, a 12V 1000Ah battery is 170kg, not too heavy, 400Ah cell is about 13.5kg, 48V would be around 220kg.
http://www.evlithium.com/Winston_Battery.html

I have to revise my previous post, according to the specs, you can continously discharge them at 3C, so even a smaller pattery can do the trick.

[Dockhead]

Originally Posted by CatNewBee

You have so much weight in your keel anyway, placing the lithium battery midships or in the front cabin can help a lot with efficient wiring of all this power hungry gear. Lithium is half the weight of lead acid and it yelds more amps on discharging, so you save a lot of weight anyway. keeping the stuff midships / at the front of the keel is not that bad, you can balance out the boat by water and fuel tanks, and storage aft. A 1000Ah cell weights only 42kg, a 12V 1000Ah battery is 170kg, not too heavy, 400Ah cell is about 13.5kg, 48V would be around 220kg.
Winston Battery - Evlithium

I have to revise my previous post, according to the specs, you can continously discharge them at 3C, so even a smaller pattery can do the trick.

Well, I am absolutely not going to have a separate battery bank just for my thruster. That's nuts. Especially not lithium. Using a whole battery bank, even a small one, just for one device which might not be used more than a few times a week -- bah.


No, the my thruster work ok now. My battery bank is over my keel, midships, so the weight is not as terrible as it would be on a catamaran, but there is no more space for more lead batteries.


With lithium and 48v (if that's the way I go), it's just not that big a deal to size the cables to provide minimal voltage drop over such a cable run (about 9 meters on my boat), even for a 10kW load.




Next boat might have an even longer cable run to the thruster. One of the points of the design brief is that there should be a proper electrics space, dry and well ventilated, with watertight doors, where I can easily reach the inverters, batteries, main breakers, and other electrical gear. I don't know where the designer will find the volume for this -- it might be further aft.



Much better technical (and storage) space is the main reason why the accommodation volume of the next boat will be less than my present boat, but with 10 feet or so more LOA.

[CatNewBee]

If you have the design choice, you can make this compartment somewhere not too far from the heaviest loads and put your house bank there.

Amel has a nice center engine room design, where all technical equipment is concentrated, i like this, but I would place the batteries in a separate area to keep them cool and away from the engine / generator. It is really no big deal wit LiFeYPO4 batteries, they are sealed and maintenance free, you need a mechanical fixing around to prevent deformation, but thats all.

Because they also are not very heavy, you can place them easily under a seating bench or something similar, they do not get warm in operation, cooling is not necessary if you not operate them at 3C currents, my bank runs under load at 0.5C (up to 500A) and the battery cells stay at ambient temperature, the inverter is more of a problem regarding airflow. I would put the inverter into the vented engine room and keep the batteries separate if designing it from scratch. Some big manufacturer like Lagoon put the house bank back into the engine compartment nowadays, I consider this a drawback. You gain some storage at the cost of quicker dying batteries.

[Dockhead]

Originally Posted by CatNewBee

If you have the design choice, you can make this compartment somewhere not too far from the heaviest loads and put your house bank there.

Amel has a nice center engine room design, where all technical equipment is concentrated, i like this, but I would place the batteries in a separate area to keep them cool and away from the engine / generator. It is really no big deal wit LiFeYPO4 batteries, they are sealed and maintenance free, you need a mechanical fixing around to prevent deformation, but thats all.

Because they also are not very heavy, you can place them easily under a seating bench or something similar, they do not get warm in operation, cooling is not necessary if you not operate them at 3C currents, my bank runs under load at 0.5C (up to 500A) and the battery cells stay at ambient temperature, the inverter is more of a problem regarding airflow. I would put the inverter into the vented engine room and keep the batteries separate if designing it from scratch. Some big manufacturer like Lagoon put the house bank back into the engine compartment nowadays, I consider this a drawback. You gain some storage at the cost of quicker dying batteries.

Well, the biggest load, and the one used most often, is the inverter bank, and the other crucial connection is to the alternator, so my batteries will be located near the engine room and together with the inverter bank, and near the main distribution panels, definitely not under settees. I don't think distance to the bow thruster will be such a big deal if it's 48v.


I also think that eliminating (nearly) voltage sag at the batteries will have a huge beneficial effect. I haven't measured voltage sag at the thruster while it's being used (ought to do that), but system voltage goes down from say 25.2 volts (pretty full batteries light loads) to about 22 or so, if the engine is not running, and with the engine running (so school bus alternator contributing a couple of kW to support system voltage), maybe 23 or 23.5v. So that means potentially a couple of volts benefit from lithium.

[transmitterdan]

1 and 1 is 3 in binary.

[CatNewBee]

Originally Posted by transmitterdan

1 and 1 is 3 in binary.

nope, 1 and one is 1 in binary.

[Q Xopa]

Originally Posted by ramblinrod

I would never claim a "fact" is incorrect.

By definition facts are correct.

When an "incorrect statement" is posted, to help forum members separate facts from incorrect statements, I sometimes post that it is "incorrect" and explain why with "facts", including examples that support them.

I hope this and every explanation I post, is clear enough that it is obvious which "incorrect statement(s)" it is referring to.

I think if one reads the original post (or quote) I have referenced, and then reads the correction I post carefully, it should be obvious which incorrect statement the facts I post correspond to, even for those without any engineering background at all.

You have certainly provided for interesting thread reading baiting all the other posters. Im suprised they are still falling for it.

Either that, or I am impressed with your conviction of your beliefs in the face of such overwellming oppostion to your position.

Geniuses are mostly first ridiculed.

Either that or... Nah I cant think of any other explanation.

Carry on.

[evm1024]

Originally Posted by CatNewBee

nope, 1 and one is 1 in binary.

Oh so true

1 and 1 is 1 in binary

1 plus 1 is 10 in binary

Just ask any computer.

[ramblinrod]

Originally Posted by evm1024

In other words you have no data to back up your opinions.

Requesting "proof" of this basic principle, is akin to asking someone to provide scientific evidence that the sun rises in the east and sets in the west.

If one does not already know that a contact or connection resistance can be 10 ohms (or any value between near zero and near infinity), I recommend they take a class, "Basic Fundamentals of Electricity".

If they have already taken such a class, and still don't know this basic fundamental, sorry, I doubt I or anyone can help.

[transmitterdan]

I like Peggy’s tag line: if you can’t explain it to a 6 year old ....