Attaching a fuse to a new set of batteries?

mlts22 wrote:
Thanks for the info. The purpose of the fuse is for use as catastrophic short protection, and one of the reasons I went with the current setup is because it is easy to install -- just bolt onto the positive battery terminal, call it done.
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Agree.

I have one of those Blue Sea MRBF fuse blocks, except it's the Dual version with room for 2 fuses, one for the inverter cable and one for everything else.

Very compact and easy to use, although I did have to cut out a hole in the lid of the battery box and siliconed the red fuse block cover into it.

Almot wrote:

MEXICOWANDERER wrote:
If you connect a LOUD alarm across both terminals of a fuse it will screm its fool head off announcing the Four Horsemen Of The Apocolypse are there.

Will it work?

Fuse blows open - big Amps flows through the alarm and it's screaming its head off for a half a second, and then goes up in smoke because it itself becomes a fuse.

Without the alarm - when fuse blows, my lights and everything else goes out, so I won't miss it, right? May take me a minute or two to figure out what happened, this is the only difference.

It will work fine because the resistance of the alarm is much much higher than that of the fuse, and so the high current will not flow through the alarm--it will be limited to whatever the alarm usually uses. It's no different than connecting the alarm directly to the battery. This is a straightforward application of Ohm's law.

I would probably put a small fuse in series with the alarm if I were doing this, but perhaps I'm just being paranoid. (I probably would not bother with the alarm, actually, as I'd be immediately pretty suspicious that something major was wrong when all the 12V power suddenly went out.)

If the battery cable goes direct to the fuse block in plain sight I would skip the big fuse.

DrewE wrote:
{alarm} will work fine because the resistance of the alarm is much much higher than that of the fuse, and so the high current will not flow through the alarm--it will be limited to whatever the alarm usually uses. It's no different than connecting the alarm directly to the battery.

If alarm is in parallel to the big fuse that is sitting on the Pos terminal - as it was suggested - then whatever high current flows through the big fuse and blows it, will flow through this alarm a moment later.

mlts22 wrote:
Due to various craziness in life, I let the batteries on my rig expire, so the only purpose they have is a core charge refund. This time around, I'm getting two 12 volt jars, using 36" 0 gauge cables with ring connectors to connect them in parallel, and fuse block with this fuse for it. The fuse will go on the positive side of the bank.

Other than going with 12 volt batteries, are there any downsides to fusing the battery bank with a 300 amp fuse? I can't think of any, and I'm doing this, mainly as catastrophic protection in case of a sh

Thanks for those two links you included! I've always wondered if something that simple and practical was available - of course I might have expected that the marine world would have fuse blocks and fuses like that.

One of those blocks and a 60 amp fuse will eliminate something I've been losing sleep over: I have two large 12V batteries in parallel and will use one of those fuse block setups in the negative paralleling cable that runs between the two batteries. This will protect from one battery every developing an internal short and the other battery dumping mega amps into the short for a possible nasty result.

A 60 amp fuse between the two batteries still permits the pair to deliver up 120 total amps into the RV's system. We don't use a super large inverter so only 120 amps from the pair should not ever be a limiting issue, but 120 total amps should still hopefully be plenty for starting of the Onan generator or emergency boost starting of the V10 chassis engine.

By the way .... this is an advantage of two 6V batteries in series - if an internal short develops in one of them the other won't mega-feed the short to create a hazardous situation.

Fuses and circuit breakers are intended to protect the wiring in a system.

Depending on which wire table, the application, and type of copper wire the recommended max current for a 1/0 copper wire is anywhere from 150 to 250 amps. You won't need anything close to that for a fuse in your application and no need for wire that size.

Go with slightly smaller wire as it's easier to work with.

Almot wrote:

DrewE wrote:
{alarm} will work fine because the resistance of the alarm is much much higher than that of the fuse, and so the high current will not flow through the alarm--it will be limited to whatever the alarm usually uses. It's no different than connecting the alarm directly to the battery.

If alarm is in parallel to the big fuse that is sitting on the Pos terminal - as it was suggested - then whatever high current flows through the big fuse and blows it, will flow through this alarm a moment later.

It won't be a high current that flows because the impedance of the alarm is much greater than the impedance of the fuse previously was. The alarm will limit the current. Electricity doesn't have "inertia" as such; it just follows ohm's law. (If the connected circuit has significant inductance, the sudden change in current will cause a voltage spike, which may cause its own problems. I would not expect a particularly large overall inductance in an RV's 12V system.)

This same basic setup is exactly how the little fuse blown lights on fancy 12V fuse panels work. The light (an LED and a current limiting resistor) is simply wired in parallel with the fuse, and when the fuse opens, the light illuminates due to the current path through the rest of the circuit. The suggestion here is exactly the same, except with an audible alarm rather than a visual one.

mlts22,

How do you feel concerning the issue I mentioned in my post earlier: For 12V batteries connected in parallel ... do you think it's necessary to also fuse in between them in case one battery should develope a short?

This has never happened to me and probably to no one else who uses two or more 12V batteries in parallel. It's just a safety issue that I recently thought about and I've already ordered the fuses from the links you provided.

A short would presumably be a single shorted cell, not a dead short across a single 12V battery. I don't suspect that a large capacity fuse would do much of anything--good, bad, or indifferent--in the case of a shorted cell; the current flowing between the two batteries would be well under the big fuse's rating. I haven't done any math to verify this suspicion, though, so may be wrong.

For a large battery bank, distributed fuses make sense from the point of view of protecting the interconnections etc. from short circuits. For a single pair of batteries, I think it's rather overkill unless the batteries are physically separated from each other by some not-insignificant distance and hence the interconnecting wires are more subject to various possible adventures.

The main purpose of the fuse is to protect the wiring (in the sense of preventing it from starting a fire due to getting overly hot). Using a fuse larger than the current carrying capacity of the wire is not a particularly wise idea.

KRumm wrote:
One of the downsides is that blown fuse needs to be replaced....

What about installing a circuit breaker instead?... You can also have it as a battery cutoff

XSCORPION (CB300L) 300 Amp Circuit Breaker with Manual Reset and LED Status Indicator

seriously? If he can blow a 300A fuse he's going to have a DEAD SHORT just south of the fuse. Let's worry about that than the trivial cost of the fuse.
And yes, it's way bigger than needed.