Installing a battery monitor.

Project date: 2010

For this project, I am installing a nifty DC battery monitor DMM-4, manufacturered by MicroLog, Inc (website). This device has the capability of monitoring 1 to 3 batteries, as well as charge/discharge current in one of several modes; system charge/discharge, specific battery charge, or specific battery discharge using an optional shunt. The mode of operation depends on how the shunt is wired. A two battery monitor, DMM-3 is also available if you do not need monitoring for 3 batteries.

Since I have a battery dedicated to house current, my preference is to monitor house discharge current. I can fairly determine that when the engines are shut down, little discharge current is present; however, the house battery discharge current is often an unknown value. The instructions allow for monitoring of various discharge currents, and you can purchase a 100A or 500A shunt, depending on your discharge monitoring needs. Since I am monitoring house current - and that the house system has a 50A main breaker, the 100A shunt is sufficient.




The block diagram shown here is the basic connectivity of the battery monitor. The battery sense leads should go all the way to the battery. If you connect anywhere else, such as at a battery switch, engine alternators, or other locations, you may introduce a read error as a voltage drop can be introduced due to current flow across the line. If you are "sharing" the voltage sense with active loads, the active load will change the voltage reading. For this reason, the most accuracy will be found if you run dedicated voltage sense wires from the monitor to each battery.

Since the current used to power the unit and sense voltage is only a few millamps, 16AWG wire (the minimum wire the USCG allows for individual circuits) with even 20 feet of wire from the battery to the monitor, the voltage drop at the expected current demand required by the monitor should be less than 0.0002 Volts. This is accurate enough for battery monitoring purposes. For more on the voltage drop phenomenon, consult my article on voltage drop issues found here.

The MicroLog module simply sticks to a surface with double-sided tape. I really didn't like that configuration all that much, so I decided to improve on it by making a semi-flush mount panel for the module. Also, I wanted the ability to turn the monitor off, so I am mounting a 3PST switch next to the module as shown by this conceptual assembly.


While I typically use another source for making panels, I wanted to try a new company I found; and to provide the reader of this project a source of custom panels. The company I used is called Front Panel Express (website), located in Washington state. When you access their website, you can download a panel creation drawing program which has the pricing for each hole, cutout, and lettering engraving you need.

You can even order the panel right from the drawing application. However, I highly recommend printing out your panel and test-fitting it to make sure it is what you want, as the panels are not cheap.

Front Panel Express will usually custom machine and mail your panel within 5 business days. For an additional cost, you can get one machined faster.


When the panels arrived, I was pleasantly surprised. The panels were well protected with a heat-applied plastic sheet over a piece of cardboard.

I actually made three panels; the main battery monitor panel, the back mounting panel for mounting the module using it's double-sided tape approach, and a blank panel, well, just in case something went wrong. It is a bit cheaper to buy unfinished panels, so the back mounting panel was ordered that way.

I was especially pleased with the engraved, and paint-filled lettering, which was top-notch.

The entire cost of this custom panel set was around $70, including shipping. While not exactly cheap, it is certainly within the realm of most small projects. And you have to admit - it does make the project look a whole lot nicer.


The Assembly Begins


The assembly of my customized panel is pretty easy. To begin, attaching the front to the rear panel is done with #4-40 hardware and 1/2" standoffs, available from any electronic supply house (Mouser, Digi-Key, Allied, etc).

For a truly customized look, I used socket head cap screws for the front mounting screws; partly for looks, but partly as they were smaller, and I had to make everything a bit "tight" to fit into the location I wanted. I also added equipment handles, which I love to do whenever I can. The equipment handles actually hide the mounting screws that mounts the panel to the electrical panel on the boat.


When mounting the panel, simply remove the protector for the double sided tape on the back of the unit, then press it in place. As it turned out, I made a slight mistake when I "mike'd" the dimensions of the Microlog module, and didn't provide enough clearance for the panel to fit all the way into the panel. But it fit about 75% of the way, so it was close enough for a semi-flush look.

You can see how far the Microlog module gets pushed into the mounting panel by looking at the seam on the module. That is about as far as it would go, due to my measuring mistake. Its nice when your mistakes don't ruin the job - and in this case, a $70 custom made panel.

The equipment handle to the right is temporarly screwed into the mounting panel from behind. The panel will mount into the boat's electrical enclosure with these screws put in from the back side.


From the backside of the Microlog module, all of the wiring is accessable. You might notice that I had to cut the little green loop wire for use with the 100Amp shunt. If you are using the 500Amp shunt, don't cut this wire. Cutting this wire provides a 0.1A resolution on the readout.

The orange thing to the left is the 3PST switch. I should have allowed a bit more clearance room, but I wanted to make the mounting panel as small as practical, as I may have to install other equipment in the electrical cabinet in the future.


On to the boat's electrical panel. Here is where it gets scary as I am cutting into boat parts that cannot be easily replaced. After much decision, I am mounting the battery panel into the boat's AC electrical panel (it's DC panel is directly below the AC panel). While this may not be quite a best practice, there is no alternative, as the DC panel was just too full. The area I am using was to be used for a generator, and since the boat doesn't have a generator, this area of the panel will not be used.

To cut out the panel, I used my pneumatic nibbling tool. It is an inexpensive Harbor Freight tool, but sometimes cheap works. The thickness of the boat's metal panel is about 0.08", which is just a bit more than the rating for the nibbling tool. But it worked OK if I forced it easily.

The cutout where I started the nibbling tool is where the breaker for the generator would have been.

It should go without saying that "measure twice - cut once"; or "think twice - cut once" applies here.

I was not all that happy with the outcome of the nibbling process, but I was able to clean it up a bit by "chucking" an end-mill bit into my drill press and run the panel across the drill's fence.

Again, you have to be darned careful here, as the bit can easily get away from you and go marching right across the front of the panel. In hindsight, I probably should have done this from the backside, but after holding onto the panel for dear-life, the bit didn't jump any.


While not perfect, the cutout is flat and all burrs have been removed. Since it will be hidden by the monitor panel, its going to be good enough.


The boat's AC electrical panel with the battery monitor panel installed. I think after all of the work so far, things are starting to look good.


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