Sunday, November 6, 2016

Alternator Reconfiguration for Improved Power

Like many boats, our main engine is equipped with two alternators.  One smaller 85A unit charges the starter battery, and one larger 190A unit charges the house battery.  Ours also came equipped with Balmar smart regulators controlling each alternator to provide 3-stage battery charging for each battery bank.  This is a good setup, and worked fine, but for a handful of reasons I wanted to make some changes.

Dual alternators

Dual Balmar regulators

The first motivation to make changes was because the 85A start battery alternator was getting very little use.  Under normal conditions, an engine start battery doesn't get used very much.  When you run the starter, it draws a huge amount of current, but the engine typically starts within a few seconds, so the total amount of power draw out of the battery is very small.  As a result, it takes very little time to recharge it, after which the alternator is just spinning and producing little to no power.  That's 85A of potential charge power sitting idle, and meanwhile the house alternator is running full tilt for hours on end recharging the house battery after a day and night at anchor.  So I wanted to use that alternator to help re-charge my house battery faster while under way.

The second motivation is that I also had plans to add 240V inverter service on the boat so we could do laundry while under way.  This will be the subject of another article.  Up until now we had to run the generator, which works, but with only the washer and drier running, it was a very light load for our 20kw generator.  So running off the main engine alternator(s) via an inverter makes sense in the long run.

All this led to the same conclusion; combine both alternators and direct them to the house battery, and charge the start battery some other way.  The 190A alternator puts out a total of about 5kw, and the 85A unit puts out about 2.2kw.  By combining them it's almost a 50% increase in charging capacity, and is a close match to my 7kw of inverter capacity.

Problem 1: How to charge the start battery?

If I take away the dedicated alternator charging the start battery and use it to charge the house battery, it's just a matter of time before the start battery would go dead.  So I needed to come up with a new way to charge the start battery.  The good news is that there are lots of ways to so it.  The bad news is that you need to pick one and live with the trade-offs.  If boats teach you nothing else, they do teach you about trade-offs.  Here are the different approaches and the associated trade-offs as I see them.

Options A: Parallel the start and house batteries while charging the house battery.

There are a number of ways to do this, but they all have the same effect.  When the house battery is getting charged, simply connect the start battery to the house battery and let it go along for the ride.

- It's a nice simple approach
- There are a multitude of devices to automate the operations
- As long as the house and start battery are the same type, i.e. AGM, flooded acid, Gel Cells, the house and start battery will share the charge pretty well.
- When on a shore charger, both batteries are maintained.  This is important when leaving the boat for an extended time to compensate for self-discharge in the battery.

- Doesn't work if you have different types of battery for house and start, e.g. flooded for one, and AGM for the other.  They won't share the charge properly.
- Even though the battery will share the charge, the charge cycle will be tailored to the house bank, not the start bank, and may not be all that good for the start battery.  This is probably the biggest issue, and is most prominent with 3-stage chargers.  Here's a good example of how it can be a problem.  Say you are cruising, anchoring every night, and running for 4-6 hours each day.  While at anchor your house battery gets drawn down, so the next day when you are underway your charging system will be operating at higher voltages for most of the cruise as it works to recharge your house battery.  Meanwhile, your start battery only got a slight discharge when you started your engine, but because it's along for the ride as the house battery gets charged, it's going to be charged at a higher voltage all day when all it really needs is a relatively short recharge.  As a result, your start battery ends up getting over charged.  It won't be sudden death, but it likely won't last as long as it would if it were charged according to its needs rather than the house battery's needs.

Lot's of devices are available to do this battery paralleling.  Some support very high charge current, and some support lower current, but they all have the effect of sending the start battery along for the ride as the house battery gets charged according to its needs.

Example products:
Xantrex Echo ChargeBlue Seas ACRs

Option B: Use a Battery Charger

The other approach is to connect a dedicated battery charger to the start battery.  Ideally it would be DC powered by the house battery, and automatically switch on only when the house battery is being charged.  If your house battery isn't being charged, you probably don't want the start battery charger to drain it down.

- The start battery gets charged according to its needs rather than the needs of the house bank.  This gives max life to the start battery.
- The only power consumed is what's needed to recharge the start battery.
- Start battery can be maintained at full charge while on shore power for extended periods of time.

- Dedicated charger are generally more expensive than battery paralleling devices.

Several devices are available, including:

Balmar Duo Charge
Mastervolt  MAC/Magic

I elected to use a battery charger to ensure that our start battery received a charge appropriate to its needs.  Our cruising often involves the exact scenario that leads to over charging if you use some sort of combiner, so this seemed to make sense.

I'm generally not a fan of Mastervolt.  Their products are pretty good, but poorly documented, ignore the needs of North American split phase power (only really applies to their inverters), and have little to no tech support in the US.  But in this case I picked them, and it's entirely because of what I consider to be a major flaw in the Balmar Duo Charger.

The Duo Charger is limited to 30A, which is fine.  But if the battery calls for more than 30A - say because you had to crank your engine a whole lot to get it started after changing the fuel filters - rather than limiting charge current to 30A and running for however long it takes to recharge the battery, the Duo Charger shuts down.  It periodically starts up again and retries, but unless the Battery Charging Fairy came and recharged your battery in the mean time, there will still be a call for more than 30A and it will shut down again, over and over, and never recharge your battery.  They do have a provision for an external bypass relay.  But it's manually operated, so no better than the manual battery parallel switch that I already have.  So I consider this to be a major design flaw, and instead used the Mastervolt Magic 24/24-20 which is a 24V to 24V, 20A 3-stage charger.  It works well, recharges the start battery quickly, then maintains it at a comfortable float voltage for the rest of the time.

Matervolt Magic 24/24-20 charging start battery bank

Now, with an alternate way to charge the start battery, I was free to move on to part 2 of the problem - how to parallel the two alternators.

Problem 2: Paralleling two alternators

How you regulate multiple alternators depends greatly on how the alternators, engines, and batteries are set up.  The alternators can be on the same engine or on different engines.  And the alternators can be connected to the same battery bank, or different battery banks.  Unless they are on the same engine, and connected to the same battery bank, the alternators need to be individually regulated.  This was the case originally on my boat where the two alternators were on the same engine, but connected to different battery banks.  As a result, each had its own Balmar regulator.

But with the reconfiguration, both alternators would now be connected to the same battery bank, making it possible to regulate with a single regulator.  It's still fine to regulate the alternators individually, just not required.  Some friends on MV Dirona elected to regulate them separately as described in this article.  I decided to regulate them off of a single regulator for two simple reasons.  First, I could stash the second now-unused regulator away as a spare in case the other failed.  Second, I wouldn't have to worry about balancing the alternator outputs since they are self-balancing with a single regulator.

You might be wondering how this self-balancing works.  If not, skip ahead.  But it's really pretty simple.  The Field terminal on the alternator controls its output.  If you apply 0 volts to the Field terminal, you get zero output.  If you apply full battery voltage to the field terminal, you get full rated output.  And it's proportional in between.  The regulator controls the alternators by varying the Field voltage.  So if the regulator applies half voltage to the Field terminal, each alternator will put out half of it's rated current.  That means the big alternator will put out 95A, and the small one will put out 42A.  Each is always doing it's proportional amount of work.  It works really well, but does require that they be driven by the same engine, and be connected to the same battery.

Rewiring was actually very easy.  All I did was move the small alternator's output over to the big alternator's output, connecting them together right at the current shunt that measures the big alternator output current.  Wired this way, the pilot house meter shows the combined current of the two alternators.  I also had to up-size a couple of fuses from 200A to 300A to handle the extra charge current.  Fortunately the cables carrying all this current were already oversized (4/0), so capable of carrying the increased power with only about 0.7% voltage drop.

This is probably a good place for a little caution.  It's really important to check the entire wire run from the alternator to the battery to be sure all the intervening wires are sized to handle any increased current.  And this includes checking the ground path as well as the positive path.  Electrical fires are not a good thing.  And also recalculate the voltage drop.  3% would be the max, and even that is a bit much when you are working with charge voltages where 0.1V can be significant.

Next, I removed the now-unused second Balmar regulator and stashed that in the spare parts bin, and jumpered the field connection between the two alternators.

Another lesson in boating it to always expect the unexpected, and guess what.... This was no exception.  Initial testing was not producing the expected current out of the smaller alternator.  But why?

While poking around, I noticed something odd on the small alternator.  In the picture below, the box on the back of the alternator has two wires coming out of it connected to the positive and negative posts.  That's the way built-in fixed voltage regulators are set up.  And you see the two small holes in the box?  That's where you would reach in with a small screw driver to adjust the fixed charged voltage.  This alternator looks like it has an internal regulator rather than using an external regulator.

Alternator with unexpected internal regulator

I sent this picture to Balmar and after a bunch of head scratching they agreed that this alternator was equipped with an internal regulator, even though the alternator part number was for an externally regulated model.  So my start alternator was internally regulated, even though it was wired to an external regulator that was actually doing nothing more than turning on the internal regulator.  How messed up is that?  It's unclear where this mix-up occurred between Balmar (supplier of the alternator and regulator) and Cascade (supplier of the packaged engine and charging system), but at this point it doesn't really matter.  Fortunately Balmar was only about a 45 minute drive away, so I packed up the alternator and took it to them, and they swapped out the internal regulator box for a box wired for external regulation.

Once reinstalled, testing now showed it working exactly as expected with each alternator putting out its portion of the charge current across different loads.  And I now have a whopping 275 of charging current to get our house battery up to full charge between anchorages.  And when the charge loads are more modest, the work is shared between the two alternators rather than concentrated on one while the other loafs around doing nothing.

Wow, that ended up being a much longer article than I expected, but hopefully it will help others who are evaluating alternative charging systems.


  1. I am generally leery of combining two sources of voltage and current in parallel to supply a common load, and particularly so when the two sources are not identical. DId BALMAR have any reservations about your plan to connect the two alternators in parallel?

    Have you made any measurements of how the current is distributed when charging? For example, if your HOUSE battery is being charged by the two alternators, have you actually measured how much current is being produced by each one individually? If your circuit works as you intend it to, then I would think the load on the alternators should divide according to their current rating. The 190-Ampere alternator should take on 69-percent of the load and the 85-Ampere alternator should provide the other 31-percent. But is this really happening?

    You mentioned, "...testing now showed it working exactly as expected with each alternator putting out its portion of the charge current across different loads."

    Can you pass along those results? Thanks.

  2. It was actually Balmar who recommended the approach I took, so I know they are good with it. And the "proportionality" works exactly as you describe. I should have explained it better. Each puts out according to its rated capacity. In my case the alternators capacities are about 1/3 and 2/3 of the total capacity. When I tested, I measured current from each alternator at different load levels and verified that it was approx 1/3 and 2/3 of total. These were spot checks, but enough to get comfortable it is working as expected.

    I hope that covers it?

  3. Peter,
    Excellent research and explanation. You make a complex subject very reasonable and realistic. I will definitely keep this for my project list.
    Thanks, Bob

  4. I am very interested to hear that the two alternator outputs when connected in parallel and pushing current into a common load are behaving just as you wanted. But I am more interested in understanding how this came to happen.

    Let's look at a simple analogy. We have two alternators, A1 and A2. They each make exactly 10.000-Volts at their output. We connect them, one at a time, to a load of 1-Ohm. Both A1 and A2 push 10-Amperes into the load.

    Now we connect them in parallel and to the load. There is still only 10-Volts across the load, so only 10-Amperes can flow. Since the two power sources are both exactly 10.000-Volts, they should each contribute the same current, so we expect that each deliveres 5-Amperes.

    But that is not the outcome on TANGLEWOOD. A1 delivers twice a much current as A2. (Here A1 is the 190-Ampere alternator that takes on 66-percent of the load and A2 is the 90-Ampere alternator that takes on 33-perecent of the load.) What is making this happen?

    I believe the answer is in the internal series resistance of the two power generating sources. Every source of electrical power has some small internal resistance. Typically this resistance is very low. But the actual output current that flows from the source is flowing through both its internal resistance and its external load's resistance. If you want to have an outcome like the one on TANGLEWOOD, the two sources of power have to have their internal resistance is a ratio of about 2:1, with the more powerful alternator having the lower resistance.

    I suspect that the 190-Ampere-rated alternator has an equivalent series resistance that is about half that of the 90-Ampere-rated alternator. This is why it tends to take on most of the load. I can't see another explanation--other than magic. If someone has a better analysis, I'd like to hear it.

  5. Jim, I honestly don't know the answer, despite a degree in electrical engineering. I believe is all comes down to field controlled alternators and how they work, and I expect they act more like current sources rather than voltage sources. More field voltage begets more output current. This makes sense when you think about a single alternator with external regulator. The load (in this case the battery) controls the voltage. If you remove the battery, the voltage spikes and fries the rectifier diodes.

    But maybe we have a reader who knows more about this? I never studied motors or generators.

  6. I should add that if Balmar had not suggested this as an option, it never would have occurred to me to do it.

  7. Great post Peter! Like the single regulator approach and start battery charger which seem to be slight improvements over Dirona's approach. I plan to copy your design using two 220 amp alternators for maybe 10kw. Heck we might be able to wash, dry and make water while underway, all off the main.

  8. Peter, I wonder if I could ask you to post a quick follow up on your 2 alternator, 1 regulator installation, 5 years later. Is everything still working as planned? It seems like a fantastic idea.

    Thank you,
    Colin (

    1. Hi Colin,

      Yes, the approached worked as expected, and I have done the same thing on the boat we are commissioning now. That said, you should keep a few things in mind if you take such an approach.

      First is that most regulators assume they are only controlling a single alternator, so only have a temp sensor for one alternator. If you are controlling two alternators you will need to pick one to instrument, and assume the other follows in lock step. If you think one of the alternators is more prone to high temps, that would be the one to instrument. And of course if the other overheats by itself, it will go unnoticed unless you have anther instrumentation. For me this isn't a big deal because I monitor the temps via Maretron with sensors on each alternator. So I will get alarms if anything is amiss. That leaves the regulator's sensor to only throttle the output, if necessary.

      Second is that this approach is really only good where the alternators are driven by the same engine, so running at the same speed. I wouldn't do it for twin engines where they could be running at different speeds..

      And last, the alternator outputs need to go to the same battery bank. Don't take this approach where one alternator charges one battery bank, and the other charges a different bank.

  9. Thank you Peter, that's encouraging. My circumstances mirror yours... 2 alternators, 1 engine, 1 battery bank. My combination of alternators will be overkill for my boat, but I'm considering adding a 12 volt gyro that pulls about 80 amps during the initial start up, for about 50 minutes. I'm thinking that might be a bit tough on a single 120 amp alternator, but a walk in the park for 2.

    Many thanks again, Colin


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