Terminal fuse blocks saved my batteries

After installing the final piece to my MasterVolt DC and AC power project (more on that in a huge post coming soon) I began having issues with battery capacity and engine starting. After a lot of investigation, and the decision to re-cable most of the original (old) and hand made (me) cables, I found the real reason that things were awry.

I had added a BlueSea Terminal Fuse Block to every battery when I installed my new Firefly batteries a few months ago. I had used these on my previous boat Jammy, and loved how compact and useful they were, while providing excellent fusing right at the battery terminal.

After removing some of the old cabling, and removing the fuse blocks, I wondered if part of my problem might be the blocks. After doing some quick voltage tests, I realized that two of the three batteries appeared to have blown fuses!

Blue Sea Terminal Fuse Block Fuses

Fuses compared

I had oriented the fuse blocks as best as possible for the cable runs from the batteries, not for viewing. After removing the fuses, I compared one bad one (bottom) to a good one (top).

Zoomed in closer...

Zoomed in closer…

Zooming in you can see that the bottom one has blown – the two smaller sections of the square nearest the center have melted away.

MasterVolt MasterShunt connections

MasterVolt MasterShunt connections

I know how this happened – there are two very close together connection points on the MasterVolt MasterShunt which are connected to the main positive and negative blocks for the boat. While I was reconnecting everything, I did one step out of order, and these were energized when I installed them. The negative lead just barely touched the positive one as I was connecting it, and of course a wonderfully big spark and nasty noise occurred.

So the terminal block fuses did what they were supposed to, which is prevent it from hurting the batteries which were “down stream” of this point.  The third battery is further away cabling-wise, so I suspect that’s why its fuse didn’t blow, although it could have been a number of reasons.

While I am glad they worked, I really wish that there was an easier way to see that one of these have blown. Most people, like me, have these in battery locations which are not necessarily the easiest to get to or see in sometimes.  It is also important to point out I’ve been using these for 5+ years and never had one blow, so perhaps it doesn’t happen that often?

Glad I found the issue, and that my beautiful Firefly batteries are still happy.  More to come on the full power system replacement shortly!

About Steve Mitchell

I live in Seattle, WA and love sailing, technology, & playing and composing music. I started playing the piano when I was 3, and ended up figuring out many other instruments along the way. I’m an avid sailor and have a 2000 Beneteau 311 named Grace, and sail it whenever and wherever I can.

  • Wouldn’t a blown fuse mean zero voltage? It sounds like you’re saying house power still worked, sort of, and starting worked maybe sometimes? Presuming your house batteries are connected in parallel, I guess house power fed from the one non-blown battery (which explains lower capacity), but if you have an isolated starting battery, how did the starter get power at all?

    • Hi Patrick,
      I do not have an isolated starting battery – I chose to have three house batteries instead. There was only one of three that was active, and it was using existing cabling which was undersized. I just finished replacing that cabling, and testing each battery in isolation, and no longer have the problem. Of course, it would be best if I didn’t blow the fuses at all 🙂

  • Jaime Fischer

    Hello Steve!
    You can place a 12 VDC indicator LED in parallel right across the fuse.


    If the fuse opens, the LED will light and indicate an open circuit at the fuse.

    Jaime Fischer

    • Thanks Jaime! That’s a pretty cool idea, I just may have to do it!

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