Mystery major short circuit

[Captain Lee]

I have blown 2 100A battery terminal fuses on two batteries, and it's a mystery to me how it happened.

Hooked up stern nav light yesterday and worked fine. Added bow nav lights to same circuit, they didn't work. Returned today with multimeter to troubleshoot.

Found the 100A fuses on positive battery post of each of two batteries was blown.

Why? Boat started and ran on battery just fine 2 days ago, with AC shore supply disconnected. At least one nav light worked yesterday. The large 4 ga starter circuit wires all look fine, and everything seems to work fine after I replaced fuses (at $17 each).

I was unaware of causing any short circuits while I was doing my wiring, and was using 16 ga wire. The only thing I can recall that was different from my usual routine: After I had locked cabin to leave at end of day, I realized I had left the 110 V AC radio on. Not wanting to re-open the boat cabin, I simply disconnected my shore power 110 V AC cable from its cockpit receptacle. Ths shut off the radio, but also the battery charger.

Could interrupting the AC supply to the battery charger in this non-standard way have somehow caused a very large DC surge through the positive battery terminal? If not, anyone have any other ideas?

I have a new Blue Sea AC panel with reverse polarity protection, new "smart" wall-mounted Xantrex charger, new batteries, new DC panel with new battery switch, old battery cables. The blown fuses were new, Blue Sea rectangular battery terminal fuses.

Thanks to all for any ideas or theories!

[btrayfors]

Originally Posted by Captain Lee

I have blown 2 100A battery terminal fuses on two batteries, and it's a mystery to me how it happened.

Hooked up stern nav light yesterday and worked fine. Added bow nav lights to same circuit, they didn't work. Returned today with multimeter to troubleshoot.

Found the 100A fuses on positive battery post of each of two batteries was blown.

Why? Boat started and ran on battery just fine 2 days ago, with AC shore supply disconnected. At least one nav light worked yesterday. The large 4 ga starter circuit wires all look fine, and everything seems to work fine after I replaced fuses (at $17 each).

I was unaware of causing any short circuits while I was doing my wiring, and was using 16 ga wire. The only thing I can recall that was different from my usual routine: After I had locked cabin to leave at end of day, I realized I had left the 110 V AC radio on. Not wanting to re-open the boat cabin, I simply disconnected my shore power 110 V AC cable from its cockpit receptacle. Ths shut off the radio, but also the battery charger.

Could interrupting the AC supply to the battery charger in this non-standard way have somehow caused a very large DC surge through the positive battery terminal? If not, anyone have any other ideas?

I have a new Blue Sea AC panel with reverse polarity protection, new "smart" wall-mounted Xantrex charger, new batteries, new DC panel with new battery switch, old battery cables. The blown fuses were new, Blue Sea rectangular battery terminal fuses.

Thanks to all for any ideas or theories!

If you're starting your engine from those batteries, there's your answer. Starters even on small diesels can draw 150A or more and, very briefly, much more amperage.

I'd use nothing smaller than a 200A or even 300A fuse in a starting circuit.

And, no, disconnecting the AC power would not blow those 100A MRBFs.

Bill

[jeepbluetj]

If it happened while you were working on the wiring, you would have known it. 100A will make a pretty dang big spark. 16G wire would get red-hot immediately with 100A through it (may even burn in half before you blow the fuse)

I'd assume you've got smaller breakers/fuses to everything else on the boat except the starter, right? Nothing wired to the battery except the charger, starter (A/B switch likely) and a feed for your electrical panel? If so it's in all the bigass wires or in the starter. (or just need a bigger fuse to start the boat)

[Captain Lee]

To both Bill and Jeep,

Bill, I like your theory, but did not start engine between when DC circuits worked and when they didn't. (I guess I could have burned the fuses 99% through, then time did the rest.) My engine is a M3 20B 20 hp Universal diesel, 190 CCA. I have moved up to a 150 A fuse, so we'll see how that does. Don't want to go higher without new thicker cables ( My roundtrip circuit to batt switch is 20', 4 gauge wire). I do use only battery #1 to start engine, and fuses to both batts are fried.

Jeep, I have only large4 ga cable to batt selector switch on batt #1, and the 6 ga cable to the charger. On batt 2, same 2 wires, plus a fused bilge pump wire

[geoffr]

Originally Posted by Captain Lee

To both Bill and Jeep,

Jeep, I have only large4 ga cable to batt selector switch on batt #1, and the 6 ga cable to the charger. On batt 2, same 2 wires, plus a fused bilge pump wire

Lee,

If I understand your set-up correctly you have:
2 batteries:
#1 - AWG4 to battery selector (position #1)
- AWG6 to battery charger
#2 - AWG4 to battery selector (position #2)
- AWG6 to battery charger
- AWG? to bilge pump
On both batteries the wires are connected to the battery via a 100A terminal fuse.

You found that both 100A fuses (battery #1 and #2) were blown.

Questions:

1- The only way I can see that the both fuses were blown is that you had the battery selector in the combine/both position - can you confirm this?

2- Where is your engine starter connected? (battery selector or direct to battery?)

3- Where is the alternator connected?

4- I assume as jeepbluetj noted you have a wire from the battery selector to a fuse/breaker panel - were any of these fuses/breakers blown?

[Cadence]

I would have to agree it is nothing you did wiring with 16 awg.. Blowing 100
A fuses would have probably crisped 16s. Do you not have fusing for the lighting circuit that would have gone first?

[Scrimshaw4]

You might want to check around your boat to see if you took a lightning strike. otherwise check your ac and charging circuits. Good luck

[Maine Sail]

Originally Posted by Captain Lee

I have blown 2 100A battery terminal fuses on two batteries, and it's a mystery to me how it happened.

Hooked up stern nav light yesterday and worked fine. Added bow nav lights to same circuit, they didn't work. Returned today with multimeter to troubleshoot.

Found the 100A fuses on positive battery post of each of two batteries was blown.

Why? Boat started and ran on battery just fine 2 days ago, with AC shore supply disconnected. At least one nav light worked yesterday. The large 4 ga starter circuit wires all look fine, and everything seems to work fine after I replaced fuses (at $17 each).

I was unaware of causing any short circuits while I was doing my wiring, and was using 16 ga wire. The only thing I can recall that was different from my usual routine: After I had locked cabin to leave at end of day, I realized I had left the 110 V AC radio on. Not wanting to re-open the boat cabin, I simply disconnected my shore power 110 V AC cable from its cockpit receptacle. Ths shut off the radio, but also the battery charger.

Could interrupting the AC supply to the battery charger in this non-standard way have somehow caused a very large DC surge through the positive battery terminal? If not, anyone have any other ideas?

I have a new Blue Sea AC panel with reverse polarity protection, new "smart" wall-mounted Xantrex charger, new batteries, new DC panel with new battery switch, old battery cables. The blown fuses were new, Blue Sea rectangular battery terminal fuses.

Thanks to all for any ideas or theories!

You'll want 250-300A MRBF if you are starting that engine, 200A on the very low end.

[Cheechako]

would a shorted battery, with the switch in "all" mode blow a fuse like that?

[senormechanico]

Time to get out a multimeter.
With batteries disconnected, use the ohms scale to find the short (if there actually IS one).

[Maine Sail]

Originally Posted by Cheechako

would a shorted battery, with the switch in "all" mode blow a fuse like that?

Not usually it would just get warm, then hot from the 12V battery trying to charge the 10V battery......

[randoo]

Careful, blowing 100 amp fuses, to me, suggests a dead short. That short will create a serious spark and burning. Can you access the wiring and look for the "spark" i.e. evidence of a spark, blackened wires, etc? I bet you can, that kind of amperage can weld things!

Good luck, there is a lot of help here but you might consider a trusted marine electrician.

[Cheechako]

Yeah... potential major fire....

[Stu Jackson]

Maine Sail already explained it. The fuses were too small.

[W3GAC]

100a is a BIG load/ short. As said, hard for normal 16-14ga branch circuits/ wiring to pull 100a w/o melting in half... and that would happen only once.

I would attack two ways:

Using Ohm Meter
- Disconnect batteries
- Turn off all battery switches & panel
breakers
- Connect ohm meter across the
now pigtailed + & - battery wires
- Should read infinity (no meter
deflection)
- Starting with each battery SW if
more than one... Slowly cycle
through each position while
watching for short (full meter
deflection)
- If all OK, leave on and move on to
panel & flip breakers one by one
watching Ohm meter for short

If a short still exists this should reveal which circuit has issue

If you don't have Ohm meter or if this hadn't revealed any issues, connect the negative battery lead to the negative battery terminal on only one battery. And temporary wire a 12v filament type light bulb between the position battery lead and the positive battery post. The higher wattage the 12v bulb, the better... like a spare 55w 12v spreader light. If you trigger a short/ high load the bulb (now in series will glow dim to bright depending on load/ full short circuit. This will save blowing a costly $17 fuse as you progress through finding
the short with the boat's 12v system softly energized.

- With the 12v system now being activated, but with limited current availability because of the series 12v bulb, repeat the above sequence. I wouldn't expect any difference, but if you have to do some/ all this working alone... it's a good technique to be at two places at the same time because you can position the bulb to be seen as you go to other location on the boat.

Assuming you haven't found the short yet, now go back to your most recent projects (statistically new problems are usually triggered by new work... directly/ or indirectly by misplaced pliers left behind fallen and shorting something unrelated to the project at hand) and shake, rattle, roll the wiring/ new device you just installed. It won't be the first time a new light fixture came with shorted bulb base, or a wire got pinched, hidden wire got drilled through,... doing an otherwise simple project. Stop, look, listen, smell around each job site for any clues. While it's more likely that your project somehow triggered the current issue, it's possible that long term chaffing of wiring just picked this time/ or so slight nudged and failed. So grab and tug on any/ all wiring harnesses/ bundles whether you were around them or not.

As part of this process, pay particular attention to/ trace (every inch) of each of your high current battery cables to their destinations.

While further down the list of probable cause, if your batteries are wired in parallel or the battery SW was left in the Both position and one battery developed an end of life shorted cell it could possibly trigger a catastrophic event leading to a battery-to-battery high amp short through your fuses, which is one reason why they are there! So while the batteries are separated check their standing voltages (no charger/ loads) to see that both are in the range of 12-13v'ish.

Many boats, despite ABYC guidelines run their bilge pump wiring directly to a battery to ensure it never accidentally gets flipped off with the rest of the breakers on the panel. Of course, even wired this way there is usually a 'twist & lock' glass cartridge fuse next to the ubiquitous ''Auto-Off-Run Bilge SW to protect the boat from direct shorts that might cause a fire/ dead batteries. But it is possible that the bilge pump wire from the battery somehow got pinched/ shorted before it got to that protective fuse. Again, I doubt that the typical bilge wiring gauge could cause a 100a fuse to blow... but if you haven't found the issue by now, it's worth tracing that bilge wire from the battery to its destination. We should all do these checks once in a while anyway, including the shake, rattle, roll of wiring harnesses. Better to find issues at the dock than coming into a marina chased by a storm and the lights all go out.

Circling back... I'd really especially check all the big cables from the battery... and really check the ones to to the starter and the electrical panel... because generally they are the only big red wires that end up going to places where there is a clues by negative terminal/ metal that could cause such a significant 100a short if it got loose/ came off its stud.

Most boats now used switching technology based chargers... lighter because no big heavy iron transformer. But w/I that isolation transformer, when switching power supplies fail it can result in the 120v AC it's connected to at least temporarily connected to your battery. Again, I sorta doubt that this could fry a 100a fuse, but if you have exhausted other options, it's worth disconnecting the charger wires and checking it's output voltage.

I saved this last one for last because it's very unlikely, and I hope it isn't the cause, and because if it caused a 100a fuse to blow, there could be a serious safety issue. Our boat are all usually 'grounded' to the salt water via shaft/ prop mechanically connected to the engine which in turn almost always has a big negative battery cable bolted to it to complete the starter circuit. Also many boats have their safety green ground wire from their shore power cord eventually also finds itself connected to the shop's 'ground.' If your dock ground (green) or neutral (white) wiring has lost its continuity your boat may have temporary taken the dock's return path to ground when you re-plugged in your boat. Again, this is highly unlikely, as by now many boats would be suffering some all kinds of low voltage, hum in electronic audio circuits, erratic operation of most electronics equipment. But if this has occurred, you should disconnect your shore cord until the issue is resolved by a professional.

Give us a status update/ what you found, or if you need more ideas.


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