Switching from 12V to 24V

[coopec43]

Originally Posted by rom

It would be interesting (for Clive at least) to know how Amel does that. Which components they use and if there is a 12V subsystem, how is it connected to the 24V system. Does the engine start on 24V ?

I have been thinking about that.

I know a fair bit about electricity - red wires are always positive and black wires are always negative (or is it the other way around?)

I think it would be far safer to start a motor using a 12v starter motor on 24V than using 12v on a 24v starter motor? Is my logic correct?

I would imagine the windings on a 24v starter motor would be lighter than on a 12V starter motor and therefore could burn out. Am I wrong?

Clive

[rom]

Originally Posted by coopec43

I would imagine the windings on a 24v starter motor would be lighter than on a 12V starter motor and therefore could burn out. Am I wrong?
Clive

So in the end you don't know if it works... And it seems no one here can tell you for sure. Well, you may want to google 24V starter for your engine. Might exist, might not be expensive. I don't know. But I know I would not plug a 24V battery on my 12V starter unless I am 100% sure it works. Good luck.


Edit; FYI, that is what prevented me to go 24V three years ago when I heavily modified the electrical system on my boat. I sadly kept 12V. I am still interested to see how it works for others.

[noelex 77]

Originally Posted by rom

Edit; FYI, that is what prevented me to go 24V three years ago when I heavily modified the electrical system on my boat. I sadly kept 12V. I am still interested to see how it works for others.

Keeping the engine 12v is the most common solution. The engine battery system and the house battery system are normally seperate anyway, with their own battery, battery switch etc, so different voltages here are not a problem.

The 12v start battery can be charged from a 24/12v converter or you can keep the systems completely independent by using the original small engine alternator for the start battery and fitting a second large 24v alternator for the house bank.

The reduced wire costs in switching to 24v will more than likely mean you can fit the second large alternator and have plenty left over for several 24/12 v converters, although unless you have older marine equipment you will find there is very little that needs 12v these days.

[coopec43]

I found a forum for guys restoring antique engines

Originally Posted by Ozlander View Post
My Ford NAA still has the original 6 volt starter, starts good on 12v, lasts long time

Yep, I've converted some of the old rigs to 12v and just leave the 6v starter.
6v on 12v will last a long time if a Bubba or Bertha (most generally Bertha or Bessie) don't just keep over cranking if engine hesitates on starting.

But vice versa 12v on a 6v is not a good thing.
I seen a 6 volt pos ground restored V8 Flathead Ford car that had slow cranking issues and hard to start and when I pulled the starter it was very faintly in big numbers stamped into the metal case of the starter 12V

[rom]

Originally Posted by noelex 77

Keeping the engine 12v is the most common solution. The engine battery system and the house battery system are normally seperate anyway, with their own battery, battery switch etc, so different voltages here are not a problem.

The 12v start battery can be charged from a 24/12v converter or you can keep the systems completely independent by using the original small engine alternator for the start battery and fitting a second large 24v alternator for the house bank.

The reduced wire costs in switching to 24v will more than likely mean you can fit the second large alternator and have plenty left over for several 24/12 v converters, although unless you have older marine equipment you will find there is very little that needs 12v these days.

Yes but I chose a very simple design: "everything on the house, plus a safety batt." So only 2 batteries (on a cat). How many batteries would I need to achieve the same redundancy if start and house have different voltage ? Simple answer, 4. Plus some clever DC/DC converters to charge the start batt, otherwise it will permanantly suck a few amps from the house to remain floated. I may have missed something but I could not figure how to keep a simple design with a 12V starter.

[coopec43]

Well this guy must know what he is talking about because he agrees with me

[rom]

Originally Posted by coopec43

Well this guy must know what he is talking about because he agrees with me

Oh boy, I hope you are not really about to follow the advice of some random forumer just like that ! IMHO those guys don't know what they are talking about. Here on CF there are only a handful of cruisers I would trust on that particular problem and I haven't yet seen their answer.

[coopec43]

Originally Posted by rom

Oh boy, I hope you are not really about to follow the advice of some random forumer just like that ! IMHO those guys don't know what they are talking about. Here on CF there are only a handful of cruisers I would trust on that particular problem and I haven't yet seen their answer.

Every auto mechanic and diesel fitter I have spoken to has stated there is no problem.

"IMHO those guys don't know what they are talking about
Can you say why!

Yes I would like to see the forum's experts opinion too and their reasoning. (just hope they do)

Clive

[john61ct]

Sounds like you're dead set on 24V as your default.

Do your field testing close to home, carry spares of stuff you're not dead sure about.

Use converters to get you 12V where needed, and carry spares of those.

Besides the bigger pricey Victron ones, OzeFridge recently posted a specific example of a little cheap one he trusts for his fridge electronics.

[john61ct]

Originally Posted by Cadence

I hope you have verified what will and won't run on 24V verified closely.

+1 critical bit for success there

[noelex 77]

Originally Posted by rom

Yes but I chose a very simple design: "everything on the house, plus a safety batt." So only 2 batteries (on a cat). How many batteries would I need to achieve the same redundancy if start and house have different voltage ? Simple answer, 4. Plus some clever DC/DC converters to charge the start batt, otherwise it will permanantly suck a few amps from the house to remain floated. I may have missed something but I could not figure how to keep a simple design with a 12V starter.

No, you don’t need any more batteries (or battery banks) for a 24v house system than you do with a 12v house system. So just a 12v start battery and 24v house bank for my boat, as would be the case if it was all 12v.

If the start battery fails it is perfectly OK to start the engine in a emergency from a single (12v) battery in the house bank. Deriving 12v from a 24v bank like this is not ideal as a long term prospect, but it is fine as a short term measure (for lead acid batteries). The AHr draw starting an engine is only small, so any imbalance will not take long to self correct even if this was used for reasonable period.

Of course, with a boat that has more than one 12v start battery such as cat, or a monohull with a generator, you could elect to use the other start batteries as the means of starting a 12v engine with a defective battery, or you could cover all options and also include 12v from the 24v house bank.

Once again, this is no different to a boat with a 12v house bank.

[rom]

Originally Posted by noelex 77

No, you don’t need any more batteries (or battery banks) for a 24v house system than you do with a 12v house system. So just a 12v start battery and 24v house bank for my boat, as would be the case if it was all 12v.

Did you understand I am comparing single voltage (be it 12 or 24) versus bi voltage (12 plus 24), not 12 vs 24 ?

If the start battery fails it is perfectly OK to start the engine in a emergency from a single (12v) battery in the house bank.

Not very practical but ok, let's admit you have a safety starter battery. Now what if the house fails ? Doesn't it lack the redundancy of a single voltage system with 2 batteries ?
(FYI cats do not necessarily have 2 starter batteries. Some cats e.g older FP start one engine with a dedicated starter batt and the other with the house battery)

[rom]

"IMHO those guys don't know what they are talking about
Can you say why!

Because of a rather typical forumer's way of unsientifically proving something they think is right and giving it to you as an undeniable fact. And also because I have the feeling that I=V/R, that R remains about the same, and that doubling the voltage would then double the current, not the other way round. But that could be completely wrong and they could be right. As I said, don't trust anyone until he/she can give a comprehensive explanation.

[ka4wja]

Rom,
Well, yes you caught me being loose with words...please accept my apologies..
Please understand it was not my intention to mislead...
And, you are correct I should check my postings more carefully...

Yes, many of the larger autopilot corepaks do run on either 12vdc or 24vdc (most will use from 10vdc to 32vdc), but as you write the smaller units (used on smaller or mid-sized boats) are usually 12vdc only...(and, of course the drive units are either / or, 12vdc or 24vdc, not both)

BTW, I have an S3G corepak and yes, I am aware that it will run on either 12vdc or 24vdc....




Not sure I should even write this....as it might be taken the wrong way??
But, there is much piece-meal work going on in this thread...which is fine...
but in my opinion, Clive would be better served with recommendations to look at things as a system, rather than a group of individual pieces...
I was trying to emphasize the advantage of designing a system, rather than just trying to "make things work"...
And then I got off onto tangents that diluted my point...

Yes, I'm fully aware that nothing is perfect, and most (me included) need to both live within a budget AND make things work in the real world, it is just that in my decades of working in electronics there are few absolutes, with one of them being those that take a systems approach for their specific needs/application almost always have better results AND save money!
Take that for whatever you may think it's worth, but these words I stand behind, 100%!


Fair winds and again accept my apologies for being careless with some words..

John

[noelex 77]

Originally Posted by rom

Did you understand I am comparing single voltage (be it 12 or 24) versus bi voltage (12 plus 24), not 12 vs 24 ?

Yes, I replied on that basis. To reiterate, if you have a 24v house bank and a 12v engine system you do not need any more batteries, or battery banks than a boat with a single voltage.

Originally Posted by rom

Now what if the house fails ? Doesn't it lack the redundancy of a single voltage system with 2 batteries ?

As on most boats, my start battery is small (and is not a deep cycle battery) so it would not be much use in the event of house bank failure. Also, I would not want to compromise the ability to start the main engine by using it to drive house loads.

If the 24v house battery bank failed, I would isolate the best house batteries, and use these together with solar panels/alternator to power the house loads. I think this is a better plan than using the start battery, irrespective of the voltages. It is not wise to turn one problem into two by flattening the start battery and disabling the engine.

Originally Posted by rom

(FYI cats do not necessarily have 2 starter batteries. Some cats e.g older FP start one engine with a dedicated starter batt and the other with the house battery)

Yes, understood. The configuration does not matter. It would only need a slight alteration with both engines started from the same start battery with the house bank as the emergancy start. The number of batteries or battery banks needed does not increase with a mixed voltage configuration. You are perceiving complications that do not exist.