Voltage Sensing for Balmar MC624 Regulators

When charging batteries, the charge voltage really matters.  Being off by one or two tenths of a volt can result in batteries that never get fully charged, lose capacity, and serve a shorter life.  Most charge controllers have a remote battery sense to address this.  The sense wires carry essentially no current, so experience no voltage drop along their length and can accurately sense the voltage.  You can connect them directly to your batteries, or to your battery bus bar, and get a really accurate reading that isn't impacted by the load or charging current in or out of the batteries and through the main battery cables.

Balmar makes a good voltage regulator for engine alternators, and there are 12V and 24V versions.  The 12V version (MC-612) has a remote voltage sense, but for some reason the 24V version (MC-624) does not.  Instead, it senses the battery voltage through its main power and ground wires.  This creates some interesting challenges in getting a good battery voltage reading.

The diagram below shows a typical regulator installation.  In this example, the batteries are in the laz about 15' away from the main engine and alternator, and the regulator is mounted in close proximity to the alternator.  While underway with electronics running, ventilation fans running, and other power loads, the alternator current can be 100A or more, even as the batteries approach full charge and are not accepting much charge current.  The regulator senses voltage at the alternator, but even with 4/0 cable as shown, there will be a 0.2V or larger drop to the batteries, which is too much for accurate charging.  It really should be a tenth of a volt or less.

Typical regulator installation near engine and alternator

One way to solve this problem is to wire the MC-624's power and ground wires directly to the battery or battery bus bar.  This brings the battery voltage directly to the regulator.  On the surface this would seem like a good solution, but you actually end up with the same problem, just in a different way.

The diagram below shows an installation with the regulator power wired directly to the battery.  But the power wires both sense the voltage and carry the current to power the regulator itself, and more importantly, it's the source of power for the field wire that causes the alternator to product power.  In this example the regulator is drawing 10A, 1A of which powers the regulator, and 9A of which power the field on the alternator.

The power wires need to cover the 15' distance to the batteries, and when you run the calculations to figure out what wire size is required to carry 10A with no ore than 0.1V loss, it's #4 wire which is pretty darn big wire.

But there's another way to solve the problem.  Rather than bring the battery voltage to the regulator, you can instead bring the regulator to the batteries.  Instead of mounting the regulator next to the alternator, mount it right near the batteries or battery bus bar.  That makes the power wires to the regulator very short, yielding excellent voltage sensing with the 12 ga wires supplied with the regulator.  Then make the longer 15' run back to the alternator with the Ignition, Field, and Stator/Tach wires.  Of the three wires, only the Field wire carries much current (9A in our example), and it is relatively insensitive to voltage drop.  Even with a half volt drop in the Field wire, there is no noticeable reduction in alternator output.  I have been using 14 ga, 3 conductor cable and it works great with no loss of alternator output.

Regulator relocated to DC power panel and bus bar

Short wiring distance to DC bus bar

Water Maker Media Filter

I have had two water makers, one operated primarily in New England waters, and  one operated primarily in the Pacific Northwest.  Both areas have very nutrient-rich waters, and this tends to plug up the water filters on a water maker.

The sea water has to be carefully filtered before being run through the reverse osmosis membrane at very high pressure (around 800 psi).  This typically happens in three stages, each with progressively finer filtration.

• The first stage is a simple sea strainer to keep out large debris, jelly fish, etc. 
• The second stage is a standard 2.5" x 10" cartridge filter using a 20 micro fabric filter element.  
• The third and final stage is a similar cartridge filter, but with a 5 micron element

I was typically only able to get 8-12hrs of run time before needing to change the filters.  The filters can be cleaned and reused a few times, but we were always changing filters, cleaning filters, and buying more new ones.  The good news is that the filters were doing their job, but all the filter maintenance sure was a deterrent to making water.

Standard 2.5" x 10" 20 and 5 micro filters

On our last boat I replaced the standard 2.5" x 10" filters with larger 4" x 10" versions.  These worked much better because the larger filter elements can hold more material before plugging up, but were still not what he hoped for.

Talking to people and doing some research suggested two additional things we could do.  The first is a plankton filter which is like the standard cartridge filters, but uses a fine mesh stainless steel basket.  What's plugging up the filters is microbial life in the sea water, and the plankton filters do a much better job of catching it.  And the stainless basket can be rinsed clean and doesn't need replacement.  That sounded like a big improvement, but it still sounded like I was going to be opening sea strainers and cleaning them all the time which is not something I was looking forward to.

The other alternative sounded even better, and involved adding a media filter.  A media filter is a very simple filter that runs the water through some sort of "media" like sand or gravel.  Whatever is in the water gets trapped in the sand, and clean water comes out the other side.  This is exactly the principal that makes ground water safe to drink when surface water is not.  The ground water starts out on the surface, but gets filtered as it soaks through the ground.  Media filters are used for pool filtration, and a wide range of drinking water filtration.  They work equally well for filtering sea water for a water maker.

One of the best parts about a media filter is the way you clean it.  All you need to do is back flush it, which is simply running water through it in the reverse direction, and dumping the outflow overboard.   The reverse flow dislodges all the trapped crud and flushed it away.  There are no filters to open, baskets to clean, and no sea water getting splashed around inside the boat.  It's just a matter of switching a valve and running the flush pump.  Not THAT sounded appealing, so this is the direction I went.

At the Seattle Boat show I came across a vendor who had just the sort of filter I was looking on display, so I arranged to buy it from him at the end of the show at a discounted price, with "some assembly required".

Media Filter

With filter in hand, the next question was what to use for filter media.  Everyone I talked to said to use "pool filter sand", whatever that is.  I couldn't find anything that actually described how fine the sand should be, or what size material it would filter out.  As best I could tell, I should expect to filter out particles anywhere from 20-100 micron in size.  I honestly didn't know how much that would help the 20 micro filters already in place, but figured it could only help.  I called around and found a pool supply shop that despite it being February, was open and could sell me "pool filter sand", whatever that is.

The media filter is an 8”x30” fiberglass filter cylinder with control valve.  The valve has 3 plumbing ports; one for incoming sea water, one for waste discharge, and one for filtered water outlet.  The valve also has three control positions:

1. Forward Filter:  This is the normal operation position.  Water enters the inlet, flows through the filter media, then out the outlet to the rest of the water maker.   As material is filtered out and collects in the media, there is an increasing pressure drop across the filter, and eventually it needs to be cleaned with a back flush.
2. Back Flush:  In this position, water enters the inlet, but flows through the media in the reverse direction such that is dislodges the trapped material.  And the outflow is directed out the waste outlet and overboard.   All you need to do is run the filter in back flush for 15-30 minutes, and it cleans itself.  That's my kind of maintenance.
3. Forward Flush:  When you are back flushing, you do so with sea water.  As sea water is flowing through and flushing our previously trapped material, that same seawater is being filtered by the media as it flows through in the reverse direction.  Much less material is captured because it only runs for maybe 30 minutes, but there is still some.  When you resume normal operation for forward water flow, you are now essentially back flushing what you trapped during the backwash.  Make sense?  This is what Forward Flush is for.  It runs water through in the normal forward direction, but direct the outflow out the waste port and overboard.  So after a back flush, you do about 1 minute forward flush to clear out anything trapped during the back flush and get a clear forward flow.

Media Filter with Top-mounted Control Valve

In my quest for better filtering for the water maker, I was of a mind to hit things with the largest hammer I could find.  That was part of the thinking behind using a media filter rather than a plankton filter, and it also drove me to replace the 2.5" cartridge filters with the larger 4" x 10" cartridges.  I also wanted to relocate them below the water maker control unit to keep salt water away from it.  So I got two filter canisters and mounted them up in their new location where they had nothing but the floor drain to drip on.

New, Larger Cartridge Filters

And remember that large hammer that I've been swinging around?  Well, another frustration throughout my water maker ownership had been figuring out which filter or filters are plugged and which are not.  It's always been a guessing game, and has made if very difficult to figure out how to fix the filtration problems.

The solution was to add pressure sensors between each filtration stage so I could see exactly what the pressure drop is across each filter, and hence which filters were plugging up and which were fine.  I did this using our existing Maretron monitoring system, and pressure transducers.

Pressure Transducers

Getting back to back flushing.... to get a good back flush, there needs to be enough water flow to agitate the media to shake loose all the trapped material.  With too little flow, the media doesn't really get cleaned.  From the reading I could find, a back flush rate of 3-5 times the forward filtering rate is what was required, and I honestly didn't know if the pump that came with the water maker could adequately back flush, but figured I'd give it a try and see.

I ran for a year with this new setup, and wow, what a difference. The original 2” filters were only good for about 8hr of run time before cleaning was required. With the media filter and the big blue filters, I only changed the fabric filters as part of annually scheduled maintenance.   They never plugged up.  I back flush the media filter about every 8hrs of run time, and it’s really easy to do.  Just move the filter control valve to back flush, and turn on the pump.  I installed an “Auto/Manual On” switch for the boost pump that gets switched to “Manual On” to run the pump for back flush, and is left in Auto for normal operation.  I typically back flush for 15-30 minutes. You can hold a light up to the back side of the media filter and see the cloud of crud when you start the back flush, and see it clear over time.

This all worked very well, however by the end of the season I was having more and more trouble back flushing, so went back to the drawing board researching filter media and back flush flow rates.  The good news is that with the pressure gauges, I could tell that the problem was the media filter and not the fabric filters.  I was catching all the microbial material, but wasn't getting rid of it.

In my research I found a filter material called Micro-Z that is now part of Parker's product line. The material provides filtering down to 5 microns, and is much easier to back flush requiring only 1x the forward flow. So I gave it a try, and that was the key to success. I’ve been running for 3 years now with the stuff, and it still filters and back flushes great with the Spectra boost pumps.

Schematic of final system

Another challenge is fresh water flush. After running a water maker, you flush it with fresh water.  With the media filter, plus the big blue filters, there is a much larger volume to flush.  The flush water is first run through a charcoal filter to remove any chlorine which will otherwise ruin the water maker.  For proper filtering, run can only run water through this filter at a limited rate, and that constrains how fast you can flush the water maker.  What I discovered that there is a limit to the flush time in the Spectra controller, and it is actually too short. Even at max flush time, I was still getting salt content readings that were too high. So instead I set the flush time to 1/2 what’s needed, and after the automatic flush at the end of making water, I just manually trigger a second flush to finish the job.  It's not ideal, but works.

I would love to get Spectra to increase the allowed flush timer, and their new controller system may be able to do it.  I'm waiting for confirmation as I write this.

Rose Point Navigation's Nemo Interface

I've been using Rose point Navigation's Nemo interface for 2 years now, initially as an Alpha/Beta test customer, then as a full paying customer.  And I continue to test new releases since I seem to have special skills when it comes to breaking things...

- Nemo is essentially the interface to all your marine electronics. It connects to your marine electronics using the two standard interfaces (NMEA 2000 and NMEA 0183), and connect to your computer via ethernet.

- Because it connects to your computer via ethernet and not USB or serial ports, it actually works reliably. No drivers are required, which is the source of most evil in Windows.

- The NMEA 0183 interfaces are correct RS-422 interfaces, not RS232 which so many people use. The two are NOT the same, and are NOT compatible by design and by specification. Using RS232 to interface to marine electronics is just asking for a flaky, unreliable system.

- It supports four 0183 inputs, so you can bring in data from a number of sensors without requiring external multiplexers. This reduces complexity, reduces the number of failure points, and reduces cost.

- It supports two 0183 outputs. This lets you directly control an autopilot via 0183, and other devices as well.

- In addition to interfacing all your data to Coastal Explorer, Nemo also sends the data on ethernet using a de-facto UDP standard. Other devices on your network can pick up and use this data, like iPad apps, Coastal Explorer running on a laptop, etc.

- It has excellent NMEA 2000 support including superior source data selection. With N2K, all data from all sensors is on the network at the same time. As a result, it's up to each listening device to decide which sensors it wants to use. This is referred to as Source Selection, and some vendors do it a lot better than others. The simplest devices will automatically pick one of each sensor type, and typically they will fail over to another if the first fails. But you have no control over which it picks.

Better devices present you with a list of available devices and let you pick which you want to use. At least this lets you ensure the system is using the best of each device type that you might have. If the selected sensor fails, some will automatically switch to something else, where others require you  make a new choice.

The best devices present you with a list of available sensors and let you prioritize their use, picking which you want to be primary, which is secondary, and so fourth. Coastal Explorer and Nemo do this better than anyone else, allowing you to prioritize any arbitrary number of devices. The only other product I've encountered that comes close it the Furuno NavPilot which let's you prioritize up to three devices for each data type.

- Nemo passes "native" N2K data back and fourth between Coastal Explorer and the N2K network. This means that the actual N2K messages pass through to Coastal Explorer and it processes and interprets them. And Coastal Explorer sends N2K messages which nemo relays to the N2K network. LOTS of other N2K interfaces don't do this, but rather translate between N2K and 0183, communicating with the computer only via 0183. Translation between the two is NOT 1 for 1, and can sometimes be quite messy. Plus you lose all ability to do source selection for your N2K sensors. Nemo does this correctly.

- Nemo can also translate between its 0183 and N2K interfaces. This provides a bridging function between the two that otherwise requires dedicated converter devices. And it does this translation MUCH better than any other device that I've used, allowing you to select individual sentences/PGNs that you want translated, selecting the update frequency, and of course using proper source selection. I have not seen any other device that can do this.  With this translation function, I have eliminated four dedicated converters from my electronics suite.

- Nemo might seem expensive (list price $699), but when you look at what it replaces, it's a real bargain. First, if you are going to add proper RS-422 data interfaces to your PC, it's going to cost you.  Yes, you can get RS-232 USB adapters for about 5 cents each, but remember, they are not compatible with NMEA 0183, and although they will probably appear to work, it will be unreliable. I used one once in a pinch, and all seemed to work OK. But when I looked at the actual sentences being received by Coastal Explorer, a significant portion of them were corrupted. Enough made it through OK for things to mostly work, but do you really want corrupted data running around you navigation system? I sure don't.

Anyway, an ethernet-connected 4 port RS-422 interface can cost $300-$500. So right there is probably 60% of the cost.

Then an N2K interface will cost another $200 or so, and they are all USB connected. That means they only work with the computer they are plugged into, and you have the associated driver nightmares. Nemo gives you the same capability, plus more. The data is on the network, so any Coastal Explorer  (or other) systems can see it. And you are spared the windows driver fiasco. At this point you have pretty much covered the cost of Nemo.

Oh, and because Nemo puts the data on the network, it eliminates the need for one of the many devices that do that as a dedicated function. There's another couple hundred $$ saved.

Then, if you have any data translation conversion required between N2K and 0183, then you really start coming out ahead with Nemo. I have already replaced four $200 converters, and expect to replace two more. That's $800 worth of converters eliminated

- And in good Rose Point fashion, the product is reliable, simple to use, easy to update over the internet, etc.

When you consider what you get for the $$ you spend a dedicated chart plotter, or how much more software like MaxSea costs, and how it solves none of the data interfacing problems, I think Coastal Explorer + Nemo is a real bargain, and one of the best performing products I have used in a long time.

Adding 240V Inverter Service

As discussed in more depth in this article, we wanted to be able to do laundry while underway without running our generator.  Laundry can take hours, and it's not really enough load by itself for the generator.  To run it off the main engine alternator required two things; 1) 240V inverter power, and 2) more alternator capacity to handle the steady increased load.  The alternator update was covered in this article, and this current article covers the inverter service upgrade.

I went around and around in circles on how to add 240V inverter service.  As a retrofit, I had to work within the context and constraints of the existing boat, creating the following considerations:

• Any new equipment needs to fit somewhere, preferably with minimal relocation of existing equipment.
• Minimize long, fat wire pulls.  This means any large DC cables need to be confined to the laz near the batteries.  And wire runs from the laz to the main AC breaker and distribution panel in the pilot house should be kept to a minimum.  It's a long, circuitous route with very little extra space for fat cables.
• There should be minimal (preferably none) increase in idle background power loads.  There is a certain amount of wasted power in an inverter, and I wanted to keep that to a minimum.  Also, transformers consume power and I wanted to keep that to a minimum.  These losses are inevitable to some extent when actually powering a device, but when the device is turned off, I ideally wanted all those losses to stop completely.
• Our existing 120V inverter system is capable of 7kw, so if there is a way to leverage that equipment, it would help solve a number of problems.
• If new inverters were required, it was desirable to have them be the same make/model as our existing inverters.  This makes sparing easier, and utilizes the existing control panels.
• The amount of rework of our breaker panel should be minimized.  We had a single 240V service to begin with, and the idea was to split it into two separate circuits, one powered only from shore power or generator, and the other powered by 240V inverter service.  Also part of this was dealing with the circuits that are 120/240 split phase, vs those that are pure 240V.

Like I said this sent me around in circles for a while considering different alternative.  Here are some of them:

1. Add a separate, dedicated inverter for 240V service.  I could leave the existing 120V inverter service unchanged which was good.  It would also be an inverter that I could completely turn off when not used, so no background power drain.  But I would have to find space for the new inverter, necessitating fabrication of some sort of power panel in the laz.  Large DC cables would have to be run with fuses and disconnects.  And two 10/4 cables would have to be pulled from the laz to the pilot house (one for shore/gen input power, and one for output power), plus probably a control panel cable.
2. Reconfigure my 120V inverters for 120/240 service.  This initially seemed very attractive.  I could reuse the existing inverters so no new equipment would be needed, and no new large DC cables would be needed.  But it created complications elsewhere.  In their 120V configuration, one of the inverters goes into standby when power loads are low, thereby reducing the background power loss.  When configured for 240V service, both inverters would be on all the time, roughly doubling the background power loss.  Also, both the AC input and output cables would have to be replaced.  The existing input cable didn't have a neutral conductor, and the output only had one line conductor.  But the worst part was the subsequent need to rearrange the 120V power panel to balance it across two split phases.
3. Add an auto-transformer to create 240V off the existing 120V inverter service.  This seemed promising.  There would be additional power loss in the transformer which I wasn't thrilled about, but it would be limited to the 240V loads.  And as long as it could be turned off with a breaker, the losses would not be there all the time.  There would be no need to rewire the 120V panel, and if I could fit the transformer behind the pilot house console, there would be no need to pull wires back to the laz.  The only hang up was figuring out how to have breaker space for a master-disconnect for the auto-transformer and 240V service.

I ended up going with approach #3, using a 6kw auto-transformer from Outback Power, the same people who make my inverters.

Here is a before of the 240V breaker panel.  You can see all the 240V devices as part of a single service.

And below you can see the after picture.  The only things on the 240V Ships service are the water heater, battery charger, dive compressor, and input power to the inverter.  To the right is the 240V Inverter service with main disconnect at the top, and the load breakers below.  The washer, dryer, and oven can now be run while underway, or while on 50hz shore power.

And here's a wiring diagram.

So far we are really happy with the results.  It's great to be able to do laundry while underway without the generator, and the loads are comfortably within the capacity of the main alternators.

Electric Power Strategy

Since talking about the original design of our power system back when we were specifying the boat, I really haven't said much more about how it's working, what changes we are making, and why.
I've posted this article on our solar system, this article on reworking my main engine alternator system, and this article on the relative efficiency of generating power via the main engine vs a generator.  I have several more articles is process for other modifications, but realize I've never stepped back and provided an overview of what I'm doing, where it's heading, and why.  So here's the big picture.

Solar Panels

As you read through this, think in terms of cruising for extending periods of time, underway some days, but not all, and anchored at night or for several days.  Notably absent are marinas and shore power, so the boat needs to be completely self-sufficient from a power perspective.  Loads need to be powered, and batteries need to be recharged.

If you return to a dock each day and plug into shore power, this is a much easier problem to solve.  All you need is enough power to last until you return to the dock, and shore power can recharge everything.

Similarly, if you run a generator all the time, you will have plenty of power.  As boats get larger, this becomes the norm, and if you are in a climate where you want air conditioning all the time, a constantly running generator is the practical solution.

For other boats, the power we consume comes from an alternator on our engine while underway, from a generator run periodically, or from power stored in a battery bank which of course needs to be recharged periodically.  Increasingly common are solar arrays that generate power as well.  Operating comfortably like this is our goal.   We ran for over a month last summer, never tying up to a dock, and never plugging into shore power.

All of these elements interact in infinite ways depending on how the boat is used, and how it is equipped.  It's important to understand that there is no right answer here, however I always find it interesting to see how other people have solved the problem, and hopefully our approach will prove equally useful to others.

The diagram below shows our power system.  We have ways to generate power; a generator, main engine alternator, and a solar array.   We have a way to store power in the batteries.  And we have things that consume power, some more than others.  It's worth spending a little time on the things that consume power, since they have a large influence on the over-all power system design.

Here's what we have discovered and done so far.


First, we have and continue to pay close attention to our electric loads, and select appliances that are as power efficient as possible without giving up the features we want.  I think we have been pretty successful so far, and have an at-anchor battery load that 1/2 to 1/3 what other boats of the same model report.

Relatively small loads can run off the batteries.  This includes things like lighting, entertainment systems, and various boat systems.  Heavier loads can also run off the batteries just fine as long as they are not prolonged loads.  Our house water pump, microwave, and coffee maker are good examples.  They draw a lot of power, but only for a limited time, so the total power consumption isn't huge.  The greater the cumulative load of everything, the larger batteries you will need and/or the more frequently they will need to be recharged.  Everyone needs to find their happy compromise between convenience of having gadgets, and the inconvenience of powering them.

For heavier, longer lasting loads, running off batteries usually doesn't make sense.  The batteries get drained quickly and need recharging, so it tends to make sense to just run the generator from the onset, powering the load directly, and taking the opportunity to charge the batteries at the same time.

This raises a couple of interesting questions for future articles.  For example, what is the dollar cost of generating power from a generator?  This article compares the fuel efficiency of generating power from a generator vs and engine alternator, but it is comparative only and doesn't quantify the costs in dollars.  And a related question is the cost to store and retrieve power from a battery.  It's not free.  But that can be the subject of another future article.

Batteries eventually need to recharged, and larger loads need to be powered.  We have a solar system, and it contributes nicely, but is limited by the space we have available for solar panels.  On a sunny day it more or less covers our standby loads while at anchor, but there is nothing left over to contribute towards recharging our batteries from the night before.  That still requires some other form of charging, even though the solar makes it less frequent.

That leaves us with two forms of power generation;  alternators on our main engine, and a generator.  Of the two, the generator is the more powerful with a 20kw output.  The main engine alternator is about 5kw.  If we are getting underway, we will typically just let the alternator recharge the batteries.  It can almost always do it before we reach our next destination.  But if we are staying at anchor for the day, we will run the generator and take advantage of the available power to do laundry, heat water, and of course charge the batteries.

But the decision becomes more complex when we are both underway, and when we want to do laundry or run some other larger load like our electric oven or air conditioning.  One option is to run off the alternator and inverters to power these devices.  The other option is to run the generator to power the loads, removing all loads from the alternator.

This brings us to the first enhancement we wanted to make, and the ensuing ripple effect of changes.

While underway, it is a great time to do laundry.  But it required running the generator, yet it's a very light load for the generator.  And our inverter system was 120V only, so we didn't have the option of running off the main engine alternator.  This spawned a project that I will write about in the future where we expanded our inverter system to run a few select 240V appliances - the washer, dryer, and oven to be specific.

Our inverters are capable of powering 7kw which will run two out of three appliances at the same time, and perhaps all three at once.  But our main alternator is only about 5kw, so we needed more alternator output or heavy loads will drain the bateries.  This triggered the first ripple effect and a project to increase our alternator output to about 7kw to match the power of the inverters.

Many people, including me, have increased their alternator and inverter capacity to operate some of these larger loads while underway.  Our initial focus was on being able to do laundry while underway without running the generator.  Laundry by itself isn't enough of a load to keep the generator busy, and not too large a load to power with our existing alternators, so it seemed like a good starting point.   The end result has been quite successful, I think.
One thing I have been wondering through the whole process is how far to take the alternator+inverter approach vs just running the generator.  The next questions I was facing was whether to allow for powering some or all of my air conditioning units via the alternator+inverter.  This question brought about a set of experiments that I ran last summer and reported in this article comparing the fuel efficiency of generating power via alternators vs a generator.  As a result of the experiments, it's unlikely that I will expand my alternator capacity since larger loads are more efficiently powered by the generator.

One related side effect benefit of moving a few 240V appliances to our inverters is that we now have a way to run them while on 50hz shore power.  Our washer, dryer, and oven will only run on 60hz power, or at least that's what the manufacturer says.  I haven't tried it, so am going on what they say.  That created a problem when we get to locations with 50hz shore power.

Our 120V service is 100% serviced through inverters so that always runs at 60hz.  The 240V service, however, was direct connected to either the shore power or generator.  The generator is 60hz, so that would work fine, but not sure power.

By moving the laundry and over to our inverter service, it too now always run at 60hz, so it at least partially solves the 50hz problem.   However, on 50hz shore power our inverters never switch to charge mode and just keep inverting, drawing their power from the batteries.  So there is a need to keep the batteries charged and the inverters fed from shore power.

Back when we first designed our electrical system we considered this and added a universal shore charger.  It's a 100A charger that can run on either 50 or 60hz, and pretty much any input voltage that you throw at it.  So when on 50hz shore power, the charger provides DC that keeps the batteries charged and powers the inverter, and the inverter in turn powers the 120V loads and the 60hz sensitive 240V loads.  It works well, but with the increased inverter load from the laundry, there are times when the charger won't keep up.  So, in the same way we needed to increase alternator output to support the washer and dryer loads, a project for this year is to add a second charger to double its capacity and provide some redundancy when running on shore power.  That will happen sometime before we leave North America.

Hopefully this provides a little more context for some of my more recent projects, and a few articles that I have not yet written.

Engine Alternator or Generator: Which is More Efficient?

This question keeps coming up, and is often debated at length.  Like most endless debates, they go on and on because nobody really knows the answer.  Here's an attempt at getting to a real answer, but I'll spoil it for you - the answer still isn't super clear.....

The question, in a nut shell, is which is more efficient, generating electricity with a big alternator on your main engine, or running a stand alone generator?

One of the first misconceptions is that power from your main engine alternator comes for free because the engine is already running.  Unfortunately that's not true.  Generating that power puts an increased load on the engine which in turn burns more fuel.  It's that old conservation of energy law in physics.  It just won't go away.

Once past that first realization, the debate quickly turns to other power losses, and which power generation source has more losses.  Here's a little score card tabulating the most obvious losses for each generation source.  Once again, there is no clear winner.

	Generator	Alternator
Drive losses	There is virtually no loss in the direct drive between the engine and alternator	Belt drives definitely have friction losses.  How much?  I really have no idea.
Initial form of electricity	AC	AC.  Many people don't realize that your alternator actually generates 3-phase AC.  Inside the alternator it is rectified to DC, with associated losses
Engine efficiency	This will vary with the load.  At light loads, the efficiency is noteably lower	Your main engine will be propelling the boat, so likely operating in a favorable efficiency range.
Powering AC loads	The output is already AC, so no extra losses to power AC loads	The alternators DC output has to go through an Inverter to get AC at about 90% efficiency
Powering DC loads	The generator's AC output needs to go through a battery charger to get DC at about 85% efficiency	The output is already DC, no no extra losses to power DC loads

Of course we all know that running our generator burns fuel too, so the real question is which burns more, the increased load on the main engine, or the generator?  Sitting at the helm while cruising can sometime lead one's mind to wander, and mine always seems to wonder to technical things like this, so one day I decided to measure the fuel burn to generate power from my main engine alternator.  Generator manufacturers publish fuel burn rates, so that data is know.  With corresponding data for generating power from a main engine alternator, perhaps some light could be shed on the question.

To measure the fuel burn required to generate power, I logged my fuel burn for the first few hours after weighing anchor.  The whole time I ran the main engine at a constant setting, then logged the fuel burn as the alternator output went from full output initially, down to just maintaining the underway house loads.  Using the fuel burn rate and power load at the end of the experiment, I was able to tabulate the incremental fuel burn and corresponding power generation from each of the higher output stages.

Here's what the data looks like:

Main Eng burn (gph)	Alt output (A)	Power (KW)	Inc Power (KW)	Inc fuel (gph)	kWh/gal
7.25	258	7.33	4.91	0.55	8.9
7.15	210	5.96	3.55	0.45	7.9
6.90	150	4.26	1.85	0.20	9.2
6.80	115	3.27	0.85	0.10	8.5
6.70	85	2.41

So if you consider the last line as the baseline, each line above it represents some incremental amount of power generated (Inc Power column), and corresponding incremental fuel burn (Inc fuel column).  The fuel efficiency to generate that incremental power can then be calculated as show in the last column.  The resulting numbers jump around a bit, I think because the precision of my fuel burn numbers are limited, but they are clustered together enough to be generally believable, even if not exact.

With this data in hand, it can now be compared to the fuel efficiency of a generator.  My generator is a 20KW Northern lights, and unfortunately they only publish 2 fuel burn data points at 50% and 100% load.  I wanted more than that, so picked the published data from an Onan 21KW unit.

Power (KW)	Fuel Burn (gph)	KWh/gal
21.5	2.2	9.8
16.1	1.5	10.7
10.8	1.1	9.8
5.4	0.8	6.8

At higher power levels, you can see that the generator is more efficient than the alternator, but it's not a huge difference.  And very noteworthy is the significant drop in efficiency at lower power levels where the diesel engine is less efficient.

Plotting all this on a chart helps put it all in perspective.  This chart shows the fuel efficiency of generating power in each device's native form; AC for the generator, and DC for the alternator.

From this you might conclude that for any loads greater than around 35% of the generator's capacity, the generator is the more efficient way to generate power.  That's probably not a bad rule of thumb, but there is still a bit more to the story.

Depending on your generation source, and the type of load you want to power, there might still be another conversion step involved with associated losses.  Let's look at two cases to illustrate this.

First we can look at powering DC loads, including getting your batteries charged up after a night on the hook.  In this case, the power coming out of the alternator requires no further conversion to charge your batteries.  But to charge the batteries from the generator, you need to run the generator's AC power through a battery charger, and they are typically around 85% efficient.  This favors the alternator and handicaps the generator.  The graph below shows the adjusted fuel efficiency of charging batteries from each power source.

You can see that the generator needs to be loaded to around 50% (10KW in this example) of it's rated power to match the efficiency of the main engine alternator when powering DC loads.

The second example is looking at powering AC loads like air conditioning.  In this case, the generator's output is ready to use with no further conversion, but the alternator's power output needs to be run through an inverter at about 90% efficiency.  This favors the generator and handicaps the alternator.  The graph below shows the adjusted fuel efficiencies of powering AC loads.

Now you can see that the generator load only needs to be a bit above 25% to match the alternator efficiency, and quickly becomes a good bit more efficient.

So what's the take-away from all this?  I think two rules of thumb:

1) When powering DC loads, it only makes sense to do so from your generator if you can keep the total generator load up over about 35-40%.  Note that this is the total load on the generator, not just the DC loads.  So if you have your generator on to make water or run your air conditioning and you load is up over 35-40%, then by all means use it for your DC loads as well.   But if you are just plodding along with loads less the 35%, let your alternator do the work.

2) When powering AC loads, the generator pretty quickly becomes the preferred power source, starting at around 25% load, and quickly becoming far more efficient.  For those thinking that giant alternators and inverters might be a good idea to run air conditioning on your boat, think again.  That generator is most likely the more fuel efficient way to power it.  Only modest loads make sense to power from your alternator.

Caveat emptor:

This is just an analysis of one boat and one generator, and not even my own generator.  So please just take this as what it is; just one example and not an exhaustive study.  Maybe I picked a particularly efficient generator, or maybe it's particularly inefficient.  I think it's a representative example, but honestly don't know.  And I have no idea how the efficiency of power generation from my main engine alternators compares to other, but I again expect it's a good representative example.