Charging 24v Bank from 12v Alternator

[Dockhead]

Originally Posted by edmundsteele

Only cheap and nasty way is to get a 12 volt inverter to 110 volt A/C and run it from your engine battery (it doesn’t matter what your house A/C system is, 12 v / 110 v will be the cheapest option, ditto the extension cord). Run a 110v extension to either your house battery charger or buy a 24 volt battery charger from NAPA (or any place that sells 24v truck parts) and charge up the house batteries. Not an ideal solution but a cheap work-around.
Ed

Yes, I thought of that! I actually have all that gear on board and was ready to use it if my batteries got too low.

My 12v inverter is marginal for my spare 24v charger. The cheapest solution of all would be for me to simply upgrade the 12v inverter.

It's "cheap and nasty" but you know -- it's not actually all that nasty. It will actually work just fine as long as it's all set to draw less than the alternator's full output. Some loss of efficiency. But for a backup to a backup for dire emergency use -- might really be the right approach.

[Thuid]

All failures are a game of probabilities and risks. You can never remove the single point of failure or simultaneous failures, all you can do is manage the risks until you are comfortable with the compromises you have to make.

Too bad you got rid of your suitcase generator, as it was probably the best bang for your buck backup solution, as it can be used to charge the batteries or run devices directly. It also has a separate fuel supply (if being gas powered, spare dingy fuel could be used). And the power out vs. space taken on a boat cannot be beat.

[Dockhead]

Originally Posted by JEschman

Would something like this Sterling B2B charger (12v in, 24v out) connected between the start battery and house bank, do the trick?

Bingo!

Yes, that would be perfect. I looked but wasn't able to find that for some reason. Thanks.

[David B]

Originally Posted by Dockhead

Electrical power on board most cruising boats is mission-critical. I think I've even heard of boats being abandoned because of loss of power.

I have two fully redundant means of producing bulk quantities of electrical power -- a heavy duty externally regulated school bus alternator on the main engine, and a heavy duty low speed generator (6.5kW Kohler), both devices very reliable. I have backup inverters and battery chargers to provide redundancy in the AC-DC conversions.

I was so paranoid about electrical power that for years I even carried a Honda suitcase generator as a backup to the backups. It was in the way, however, and I finally got rid of it last year.

Well, last week I suffered failures of BOTH systems within about a week of each other. What are the odds?! It was quite disturbing. There was a thread on my travails with the generator. I did get it running within a few days (with a lot of help from knowledgeable people on here -- thanks), and with parts and tools I had on board, but the whole incident has left me dissatisfied with my setup.

An easy fix would be to carry a spare alternator. It's not that expensive, but the large frame device is a bulky, heavy beast. I'm debating whether to bite the bullet on the storage issue, or just carry a full set of bearings, brushes, etc.

But as someone on the other thread pointed out -- I have TWO alternators on the main engine -- what about the other one?

Well what indeed? The problem is that the other alternator, used exclusively to charge the engine start battery, is 12v, and my system is 24v.

I looked for a battery-to-battery charger which would let me charge a 24v bank from a 12v one, and couldn't find anything.

This is not electrically complex. I suppose I could just get a large DC-DC converter and run a B2B charger from that.

Anyone have any tips?

I suppose the easiest would be an inverter running across your 12V circuit ONLY when the engine is running, with switching to your normal AC circuit so that the regular battery chargers can operate. Not super efficient, but with care, it would work.

Another option is to replace that 12V alternator with another 24V one (perhaps a high-output one, which you then use as your main alternator). Then with the other 24V alternator, you run that through a 24V - 12V converter (commonly available), while you have the regular 24V output available to be switched in, if the main alternator fails.

That to me is the cleanest and most sensible option, other than changing your starter motor to 24V, and therefore eliminating the 24V - 12V converter.

There are of course ways to relay switch for 24V charging to 12V batteries and vice versa, but that is in your case going to get quite messy. For me, change the 12V alternator to 24V, and either a 24V - 12V converter, or a 24V starter on your engine.

I hope that helps.

David

[bobnlesley]

Originally Posted by Dockhead

Not a "54 ft boat" but a "54 foot sailing machine without a scrap of unnecessary windage"

Then how does electricity, or the lack of it become 'mission critical'? Hull, rig and sails are mission-critical, the lack of a couple of buckets full of sparks will mean the steaks thaw out but it isn't going to sink you!
The weight of all those generators and batteries that you're carrying around might perhaps slow you down as much as the windage from a few solar panels; if you don't want goal-posts, then replace a couple of guardwire sections with fixed rails and hang/hinge them on there instead.

Sorry, given it more thought and realised the beer'll get warm too, so I guess that does make those sparks mission-critical

[Kenomac]

So.... If you swap out your 12v alternator for a 24v alternator plus add a few bits so it can charge the 12v batteries, and assuming you don't bin your perfectly good 12v alternator and choose to include it in the spares inventory....

How has that saved you any weight over obtaining a 24v spare alternator in the first place? Does 30 pounds of alternator really make any difference on a 25 ton cruising boat? Why not just empty out 12 liters of water from the tank, it's the same weight? Come to think of it... why do you carry around a full water tank weighing thousands of pounds, instead of having a watermaker to produce water as needed... if weight is so important to you?

[GILow]

Dockhead, I have exactly the same problem, but in reverse. My house bank is 12 volts and the engine is 24 volts. My emergency engine start solution is a custom made "jumper cable" (made from left over cable after gutting the boat electrics) that is long enough to run between the two battery banks (about 3 meters) and a set of smaller interconnects to turn part of the house bank into 24 volts to start the engine or to tap power from one of the engine batteries if the house bank fails. It's an extension of what I had to do on our delivery trip back in 2012 when it turned out the house bank was toast, the solar was Stone Age toast, the wind gen brushes were fried and the 12 volt alternator died on the first day. It got us home over 500 miles then, using bits of crap wire I scrounged together, so I figured it would do to have a proper setup for the future.

Biggest advantage is that it contributes very little wind drag.

[smac999]

If you can't fit solar, get a efoy.

[Dockhead]

Originally Posted by Thuid

All failures are a game of probabilities and risks. You can never remove the single point of failure or simultaneous failures, all you can do is manage the risks until you are comfortable with the compromises you have to make.

Too bad you got rid of your suitcase generator, as it was probably the best bang for your buck backup solution, as it can be used to charge the batteries or run devices directly. It also has a separate fuel supply (if being gas powered, spare dingy fuel could be used). And the power out vs. space taken on a boat cannot be beat.

Yes, it's true that the suitcase generator is terrific from the point of view of total independence from other systems.

But it is a significant chunk of bulk and weight.

Also it is really hard to store as you can't keep gasoline/petrol inside the passenger space. So in my present boat, I can't even keep it in the lazarette without thoroughly draining the fuel.

And the carb tends to gum up with disuse.

A propane powered one would actually be more practical -- easier to get rid of the fuel, and no gumming up. But still a lot of space and weight for something which is theoretically never used.

[Dockhead]

Originally Posted by GILow

. . . on our delivery trip back in 2012 when it turned out the house bank was toast, the solar was Stone Age toast, the wind gen brushes were fried and the 12 volt alternator died on the first day. . . .

Your cascade of failures has my cascade of failures beat by a mile!

Your resourcefulness in overcoming it is impressive!

[Dockhead]

Originally Posted by bobnlesley

Then how does electricity, or the lack of it become 'mission critical'? Hull, rig and sails are mission-critical, the lack of a couple of buckets full of sparks will mean the steaks thaw out but it isn't going to sink you!
The weight of all those generators and batteries that you're carrying around might perhaps slow you down as much as the windage from a few solar panels; if you don't want goal-posts, then replace a couple of guardwire sections with fixed rails and hang/hinge them on there instead.

Sorry, given it more thought and realised the beer'll get warm too, so I guess that does make those sparks mission-critical

It's an absolutely fair question.

I suppose it's possible of course to go on without any electrical power, but a few functions are really seriously needed, if maybe not necessarily a matter of life and death. In order of priority:

1. Radios

2. Nav lights

3. Autopilot

4. Electronic navigation

5. Refrigeration on an ocean crossing



Possible to cross an ocean without any of the above, but I wouldn't want to.

Next boat will have electric cooking, so electrical power will be even more important.

[sstuller]

Sometimes the only practical answer is to reduce your demands. Trying to satisfy ever increasing demands becomes an exercise in absurdity. Reducing your "requirements" is much less costly than increasing them. Trying to live with less is more rewarding than a box of bonbons. Thanks.

[GILow]

Originally Posted by Dockhead

Your cascade of failures has my cascade of failures beat by a mile!



Your resourcefulness in overcoming it is impressive!

... but the jumper cable solution ain't doin it for ya?

[Dockhead]

Originally Posted by GILow

... but the jumper cable solution ain't doin it for ya?

Jumper cables are my secret weapons! One of the first thing I put on my boat!


But yes -- in a real emergency, I would connect engine start and generator start batteries in series and use those.

[Dockhead]

Originally Posted by sstuller

Sometimes the only practical answer is to reduce your demands. Trying to satisfy ever increasing demands becomes an exercise in absurdity. Reducing your "requirements" is much less costly than increasing them. Trying to live with less is more rewarding than a box of bonbons. Thanks.

Hmmm. So I guess you wouldn't approve of the convection oven, induction cooker, and dishwasher I plan to have in my next boat?