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Discussion Starter · #1 ·
A couple days ago, I noticed the voltmeter in the cockpit reading low (below 12 volts), then I noticed that battery 1 (which I use to run lights, electronics, etc.) was quite low (11.8 volts on the multimeter).

Upon inspection, I found that the fuse on the line between the alternator and battery had blown or somehow smashed, as the glass was broken. I replaced the fuse (30 amp, 32 volt) and turned on the engine. The voltmeter on the cockpit jumped to just over 13 and seemed to be slowly rising for ~10 seconds, and then the fuse blew again. I cleaned and tightened the contacts on the wires. Put in a new fuse, tried it again - same exact thing happened.

I did the alternator test shown here (
), and it seems to indicate that the diodes are fine.

If I put a wire with clamps from the alternator post to the battery (small wire, not doing it for any extended period of time, just to get readings), I get 13.1 volts at the batteries, 13.1 (approximately) at the cockpit voltmeter, and 15.3 volts at the post on the alternator using the multimeter.

The alternator is rated at 51 amps, and is fairly new, but was on the boat when I bought it. Is it possible that the 30 amp fuse is a remnant of the old system, and I need a 50 amp fuse for this alternator?

Recently I've been using the electric system more (just installed pressure water, anchoring out more w/ anchor light on, running an inverter to charge a drill occasionally, etc.), so maybe I've just drawn down the batteries to where the alternator is kicking into higher gear and blowing the small fuse? Not sure if that's how the regulator works, but it could explain why it's been fine and hasn't blown the fuse until recently.

Any help appreciated!
 

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Running the engine with the fuse blown on the alternator output is the same as running with the battery switch in OFF. That would usually mean blown diodes.

With the battery low, it would cause the regulator to put out maximum current. so that could be blowing the 30 amp fuse.
 

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Discussion Starter · #3 ·
Hmm, that makes sense. Is there a good way for me to test whether or not the diodes are blown?

Like I said above, I tried the test demonstrated in the youtube video and it was fine, but I don't know if that's conclusive. Also, if the diodes are blown, wouldn't that mean the that the alternator wouldn't work? Just not sure why I was seeing 15.3 volts at the alternator while running.
 

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Master Mariner
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I'm confused. Why would you think a 30 amp fuse would be sufficient for a 50 amp alternator? Most fusing is a percentage ABOVE the unit's rating, not below it.
For instance, a Rule 2000 pump: Fuse Size: 15 amps, Amp Draw @ 12 Volts: 13.6 Amps. Amp Draw @ 13.6 Volts: 12.0 Amps.
 
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15V+ on the alternator output post might indicate a damaged regulator. Rashly assuming the whole system is wired correctly and there is a voltage sense lead that is properly tied back ("1-wire") or connected to the batteries ("3-wire") the alternator should never be putting out more than 14.4 in conventional charging, plus a bit more depending on the wiring losses and where that sense lead is. So you might want to make real sure that alternator and regulator are good. Pull them out and have it bench tested if you are unsure of it. (Not that all the people testing them know what they're doing, or are honest, anyway.)

You need to identify the alternator to find out about fusing. There should be a model number and capacity plate on it or a stamping on it, somewhere.

Alternator outputs used to be fused with a "fusible link wire", i.e. the output wire itself was supposed to burn up as a fuse. Because this was a fire hazard (duh?) they went to using a fusible link wire with a fire-resistant fiberglass woven cover. Then, to "fusible link blocks", a special type of fuse. A glass fuse sounds like a kludge job. And if it is just swinging in the breeze, it is likely to fail just from the motion.

So no matter how you decide to fuse it (again, best if you have a picture of the whole system before you decide that) you need to find out what the alternator is, and pick a fuse that will protect the output wire itself. i.e. if the output wire can only carry 50 amps and you have a 65 amp alternator...you need to run a new output cable as well as put in a 65+ amp fuse.

30 volt? Voltage rating isn't really critical, but there's no need for more than a nominal 12-volt DC rating. Probably the old fuses were just "what I could find at the store".
 

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GreenasGrass
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Keep it simple. 15.3 volts at the alternator post is about right. Your voltmeter may also read a little high. Lead acid batteries are often charged at about 14.8 volts until they are almost fully charged.

Your fuse is sized for a smaller alternator output.....it is far too small. This problems seems to be the simplest imaginable. You may want to consider a larger wire to carry the additional amperage that this alternator produces. Have you noticed if the current wire gets warm while charging the battery?

Get your battery charged over 12.5 volts asap. Starter batteries suffer permanent damage when kept in a depleted state of charge. 12.2 volts is 50% charged. 12.8 volts is fully charged....at approx. 70F.
 

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Keep in mind that the Fuse is technically protecting the cabling not the devices....

So first, you need to figure out how large of a wire you need for the 50amp alternator for the distance it runs between the Alternator and the Battery.

For example, for 30 amps, 3% voltage drop, 6 feet of length you'd need ~12AWG wire or bigger. At 12 feet, it would need to be ~10AWG. And you should have a Class-T fuse or similar rated for 150%, or about 45Amps or larger. The idea here is to protect the wire from burning up (and creating a fire hazard) if the current through the wire increased well over the expected load (ie: from a short somewhere, including inside a battery or inside the alternator). The fuse blows and the current stops, preventing the wire from burning up.

Now, since your alternator is newer and now 50Amps rated, you recalculate the wire size for this new setup.. 3% voltage drop, 6feet @ 50amps is going to be ~10AWG while 12 feet will require ~6AWG. And then you need to bring the Class-T fuse up to 150% of 50Amps so 75Amp rated fuse. The voltage of the fuse matters very little other than it should be rated for a higher voltage than your circuit will use.

In any case, a 30Amp Fuse is too small for a 50Amp alternator.. But make sure you size the wire appropriately first. Since the fuse protects the wire, if you just increase the fuse size while the wire is too small, you risk a fire from the wire burning up before the fuse blows.

Note: these are minimum wire sizes based on two different online sources I checked. You may want to use larger wire than the minimum spec.
 

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Now, since your alternator is newer and now 50Amps rated, you recalculate the wire size for this new setup.. 3% voltage drop, 6feet @ 50amps is going to be ~10AWG while 12 feet will require ~6AWG. And then you need to bring the Class-T fuse up to 150% of 50Amps so 75Amp rated fuse. The voltage of the fuse matters very little other than it should be rated for a higher voltage than your circuit will use.
Figuring wire gauge the measurement is there and back. For a 6' run it is therefore 12' and a 12' run becomes 24'. My calculation is 6 gauge for a 6' one way run and 4 gauge for a 12' one way run. This is 3% drop. For alternator output 3% drop is quite a bit though - 14.4 at the alt is only 13.97 volts at the battery. I always try to minimize the drop as much as reasonably possible. For a 6' one way run I would use 4 gauge minimum and 2 gauge minimum for a 12' one way run.

The fuse does not have to be a class T. An ANL or better yet a MRBF on the battery post are good choices.
 
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