Dual Duty For Battery Fuse? -- OK or Not?

[rardiH36]

I've been reading the "Lack of Fusing" thread -- also other fusing threads often started by Mainsail. Always good stuff.

First my situation and then the question:

Situation: My 110-120v shore power 3 battery x 10 amp each smart charger came with pre-fitted in-line fuses located within just a few inches from each of the +12V battery attach terminals. Obviously, I installed the charger to the batteries as per the instructions = the in-line fuses are at the batteries' plus terminals. I also have a 30w solar panel that feeds into a Morningstar clone type "Duo" (dual) battery charge controller. Currently the + connections from the solar controller to the two batteries are not fused. (I know, my bad).

Question: Can I tap my solar + wires into the pre-fused side of my 110V charger leads so that my solar side is fused from a major short event? Since the solar panel is very low watt/amps and since I very seldom use my 110V charger, seems to me that I wouldn't t exceed the fuse rating even if solar plus shore are working at the same time?

This isn't a money thing. Just convenience and keeping the complexity to a minimum.

Thanks

 

[Johnb]

There is a fundamental question to ask - if you shorted the wires from the battery to the solar panel controller together, would the fuse protect them from the heavy current that would flow out of the battery?

To answer that you need to know the size of the wires and determine if the fuses protecting the charger (from the battery) are small enough to not fry those wires.

After that you would need to know if the solar and charger together would supply enough current to blow the fuses, which is most likely not, but you do need to know the size of the fuses to determine that.

More information needed....

 

[Bill Roosa]

These fuses do not protect the device they protect the wires. Which is why you place them as close to the battery as possible. If you put the fuse on the other end of the wire and it shorts you get massive current and burned wiring.
With that said the real risk is if the solar power wire shorts between the battery and the controller will the fuse protect that wire also?
So it is again a wire size vs the fuse that protects it issue.

Does the total amps the fuse could possibly see in the worst case of normal usage (solar in full sun and charger going at it to recharge a deeply discharged bank) less than the fuse rating?
AND
Is the fuse sufficiently small enough to protect the smaller (probably the solar) wire?

 

[Ron20324]

With a 10-amp/per charger, i'd guess your fuses are at least 15, probably 20 amps? I certainly hope you used at least 14 wire on the solar controller output? I don't see why you can't wire to the pre-fused side of your charger's fuses. I wonder how the third battery is charged?

I wanted my controller at the nav station, so I ended up wiring the contoller output to the 2 heavy (#8/6?) from the batteries to the breaker panel. Those 8/6s are heavier than anything i would have installed to the batteries. This way, the solar charges any 1/2/3 of my 3 batteries that are switched onto the +12 buss.

 

[rardiH36]

Thanks folks. All useful. I've got 16 gauge wire from the solar controller to the batteries. Distance is about 1.5 feet (x's 2). Since the panel is only 30 w = about 2 amps at full 18v output, 16g seems adequate. I don't remember for sure, but my recollection when installing my 10A x 3 120v shore charger, it's wire gauge to each battery also was 16. I will need to double check.

As to the dual duty use for the shore charger fuses, I'll need to open the in-line fuse fitting to see the rating. Then calculate to verify that it isn't too high to protect the solar charger wires.

Ron, my third battery is a small AGM which is my reserve emergency start battery. Has enough capacity to turn my engine over. I've installed an "A/B" switch on the solar controller output to my #2 house deep cycle. Every couple of weeks, I divert the solar to the AGM for a few days to keep it topped up. My recollection is that AGM's have very slow self discharge rates. So I hope that this charge regime is adequate. I periodically start my engine with the AGM just to be sure the back-up works. Two years and so far so good.

 

[Stu Jackson]

rardi, 16 ga sounds awfully small for a solar panel. Our 11w Unisolar trickle has probably 12ga that came with it. I'd seriously consider larger wire.

 

[rardiH36]

rardi, 16 ga sounds awfully small for a solar panel. Our 11w Unisolar trickle has probably 12ga that came with it. I'd seriously consider larger wire.

Hi Stu:

Your comment caused me to break out the amperage/gauge/length table and the formulas in my copy of Don Casey's "Sailboat Electrics Simplified" to check. Using the formula (if I'm doing it right), for a 3' round trip distance, 16g will handle 20 amps. The table allows up to 5 amps for a 10' round trip 16g wire length (although the footnote also states that "18g has adequate capacity but 18g is considered too fragile for boat use.") Considering that only 2 amps or so can be generated by the solar panel, 16g would seem to give plenty of headroom. But if I ever go to a bigger panel, then larger wires are a must. My charge controller can do up to 10 amps.

 

[LloydB]

No. I would not use a solar panel as a fuse between a charger that can blow a 10 amp fuse to the battery it is charging. --but that's just me

 

[redhead78]

I have 2 batteries an I think a faily common setup wf a guest 1, 2, or both positions. My leads come off the battery to neg and pos I guess u would call them 6 position blocks, and they feed appliance from there. Would I put the fuses on the pos block?? what are they called so I can see an application in a catalog... Red

 

[Dantodd]

I'd use a second fuse. It makes troubleshooting a popping fuse easier. It also lets you use one charger if the other is faulty without having to rewire or disconnect one.