Friday, March 9, 2018

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


  1. Thanks for another excellent post Peter. As a now retired teacher and VocEd instructor I really appreciate all the time and effort you put into these learning resources. Prior to serendipitously meeting 5 years ago my wife and I were full time single handed sailors but now after over 2 years working on the design we are about to push the button on the CNC cutter and begin construction of our 24m/78ft all aluminium Extreme Exploration Passagemaker. We've been following your adventures for several years and are particularly voracious students looking to learn and benefit from all the many lessons people like you can provide as we make the transition from sail to power.

    The Hi Amp DC power panel you showed is a thing of beauty in itself and if you've not already done so and I missed it as well as when you get to planning the new electrical panels for your new build we would really appreciate your thoughts and ideas on layout and building of the electrical systems.

    Quite selfishly we do hope you will be able to somehow fine the time to keep up your great work on the blog if at all possible as you too dive back into the deep end of the boat building pool with your new boat.

    Two of your many appreciative learners,

    Wayne & Christine

  2. Thanks for the note, Wayne and Christine. I'd love to hear more about your new build. Do you have a blog? Or if you prefer, contact me through the "Contact Me" form here on my blog.

    PAE (Nordhavn) do a really good job with the electrical systems on their boats. That DC distribution panel is 100% their doing - I had no role in it. And the main breaker panel and electrical cabinet are works of art.

    I do plan to write a lot about the new build, but am first trying to get caught up on the backlog of cruising adventures and technical articles. I'm nearly there, so will shift focus to the new build pretty soon.

  3. Great post Peter! Seems like Balmar would want to include this tip as the optimum MC-624 installation. See you on AIS in PE. Orlando's is great food if you get to Loretto. Also Hotel Tripui about 1/4 mile walk from PE is good too. Alex

  4. Also, I am curious why in the battery switch photo the generator battery is paralleled with the house bank? I'm assuming that's not the normal position?

  5. Thanks for the post. How did you deal with the Alternator temp sensor? Was this something that you had on your install?

    1. Good point. You would need to run a cable for it. I had been running with these alternators for a lot of hours and knew they weren't running too hot, so I didn't have a temp sensor connected to the Balmar regulator. That said, I did have temp sensors that were monitored and alarmed in teh pilot house.

      I would not recommend leaving out the regulator temp sensor unless you have carefully monitored the alternators running at full load for extended time to know how hot they get. This is especially true for smaller alternators that can really only put out a small fraction of their rated output on a continuous basis.


Make comments here