Battery Charger fusing

njlarry

njlarry

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Sep 23, 2009
1,475
O'Day 34-At Last Rock Hall, Md
I have a basic question about fussing the 12 volt DC wires between the charger and the battery. Should they be near the battery or the charger. I believe I read that the ABYC says they should be within 7 inches of the battery or the electrical device.
Since the electrical device, the charger, is supplying the current and not the battery, it seems to me that just as the battery as the source of current needs the fuse in the supply line to protect the wire to the panel, the charger as the source of current should have the fuse close to it to protect the wire to the battery.
Also that is how they were installed in the boat prior. Certainly having the fuses near the charger would make them much more accesable.
???
The batteries are fused with 200 amp fuse links at the positive terminals.
 
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J

JohnShannon

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Jan 4, 2010
1,037
Farr 30 San Francisco
well either device can supply current. the charger is current limited if it is working properly. the battery is current limited as well, but the current is much much higher. Fuse at the battery for sure. As always if MS weighs in and says otherwise, ignore me
 
njlarry

njlarry

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Sep 23, 2009
1,475
O'Day 34-At Last Rock Hall, Md
What you discribe is true but would the charger allow current to flow backwards into it?
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
22,977
Catalina 34 224 Maple Bay, BC, Canada
njlarry said:
Since the electrical device, the charger, is supplying the current and not the battery, it seems to me that just as the battery as the source of current needs the fuse in the supply line to protect the wire to the panel, the charger as the source of current should have the fuse close to it to protect the wire to the battery.
Click to expand
Larry, the charger already has its own fuses.

The fuses at the battery end of the wires are to protect the wires in case of a dead short at the battery.

While charging the charger is the source. But, you must consider that when the charger is off, the batteries ARE the source, and that's what the fuse is there for.
 
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Johnb

Johnb

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Jan 22, 2008
1,456
Hunter 37-cutter Richmond CA
Fuses are to protect the wire from shorts. Even if your batteries are fused within a few inches of the battery what would happen of a short were to be applied to the charging wires at the point where they enter the battery charger? If the battery main fuses are of the order of 200 to 300 amps they may not protect the small wires that run to the charger, the result being melting of those smaller wires and in the worst case a fire. Note that in this same scenario the fuses in the charger do nothing to protect the wires.

The basic tenet of fusing is that it protects wires from whatever source can supply enough current to melt them and the fuses must be sized for the wire they are protecting. You need to be fused at the battery end of the charger wires and they need to be commensurate with the size of wire that is connected to them.
 
J

Jan11

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Apr 9, 2012
45
Ericson 35 Albany
Johnb said:
If the battery main fuses are of the order of 200 to 300 amps they may not protect the small wires that run to the charger, the result being melting of those smaller wires and in the worst case a fire.
Click to expand
That's why you use small fuses for small wires. You may notice there is more than one fuse or breaker on your main panel. Do the same for your battery leads.
 
Maine Sail

Maine Sail

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Feb 6, 1998
11,701
Canadian Sailcraft 36T Casco Bay, ME
njlarry said:
I have a basic question about fussing the 12 volt DC wires between the charger and the battery. Should they be near the battery or the charger. I believe I read that the ABYC says they should be within 7 inches of the battery or the electrical device.
Click to expand
The over current protection needs to be at the battery end. In a dead short the BATTERY is your source of current..! A 400Ah AGM battery bank can deliver 20,000A of short circuit current into a dead short. 20,000 AMPS!!!!!! The battery charger feeding it can only provide 100A..... Where do you suppose the fuse would best be placed if you were to accidentally short those wires?


njlarry said:
Since the electrical device, the charger, is supplying the current and not the battery, it seems to me that just as the battery as the source of current needs the fuse in the supply line to protect the wire to the panel, the charger as the source of current should have the fuse close to it to protect the wire to the battery.
Click to expand
A battery charger, solar system, alternator etc. is considered a "current-limited device". This means that it can not fail in a manner in which it can provide more current than it is rated for. If you are using properly sized wires, for the supplied current to minimize voltage drop, your charge source can not create the issue with over-current for the wires..



njlarry said:
The batteries are fused with 200 amp fuse links at the positive terminals.
Click to expand
Unless your chargers wires can handle 200A +, unlikely that you're using 2GA wire for the charger unless it is an inverter/charger, then you need to size the OCP for the chargers wire size to protect those wires from the battery bank.

ABYC Max Ampacity Table Non-Bundled Wire:
 
njlarry

njlarry

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Sep 23, 2009
1,475
O'Day 34-At Last Rock Hall, Md
Thank you all. I did not understand that the charger is a current limited device but do now thanks to MSs' detailed explanations.
I installed the fuses in line with the charger positive wires at the batteries. None of the local marine stores had 10g 30 amp fuse holders in stock. WM, Hamilton and Defender don't even have them in the catalog. I was able to find them with 10g wire and inclued 25 amp fuses at Pep Boys of all places but the wire is not tinned and felt warmer than the 10g guage charger wire while the charger was putting out the full 30 amps. I found 10g 30 amp inline fuse holders from Blue Sea on ebay that I will install.
 
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Ron20324

Ron20324

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Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
Larry, maybe a bit late, but there are the "maxi fuses". I mounted the holders to the battery cases.
 
Stu Jackson

Stu Jackson

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Feb 26, 2004
22,977
Catalina 34 224 Maple Bay, BC, Canada
njlarry

njlarry

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Sep 23, 2009
1,475
O'Day 34-At Last Rock Hall, Md
Thanks Stu, as aways your the man. The chart in the manufacturer instructions indicated the 10g wire. I will print out your chart and keep it aboard.
Ron, I like the maxi fuses but did not have a place to mount them with in 7 inches. Also could maybe use inline terminal fuses but I already use them for the positive battery feeds and the double fuse model might be unweldy in the battery box. The maxi fuse holders certainly would have been the neatest.
 
njlarry

njlarry

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Sep 23, 2009
1,475
O'Day 34-At Last Rock Hall, Md
Agree, that's what is already on the main cables but it gets a little tight for two or a double inside the box.
 
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