Fuses Required for Battery Monitor Wires?

[rardiH36]

A new-in-box vintage ProMariner analog battery monitor recently came into my possession. Photo attached.

All it really does is show the voltage for each of up to four batteries on the meter as the selector switch is rotated. Just a crude measurement of battery state at best. :Liar: Nonetheless I want to install it on my plastic classic boat.

I will be "monitoring" three 12V batteries.

Question: Do I need to install <1amp fuses for each of the three wires that will lead into the monitor?

Two of the +12V wires will only be about one foot in length between my 1/2/Both switch and where the monitor will be mounted. The third wire will be about eight feet in length from the emergency reserve battery which is normally isolated from the 12V system by it's own dedicated high-amp switch.

Considering that the ProMariner monitor will draw only milli-amps, voltage drop over wire distance shouldn't be much of an issue. So I am thinking only say 18-22 gauge wire will be sufficient. The single negative wire with thick insulation can be robust in comparison - say 14-16 gauge. Other than right behind the battery monitor, each positive wire will be well separated from any negative contact possibility.

The instruction sheet that came with the monitor doesn't indicate installation of any fuses at all.

 

[Stu Jackson]

I will be "monitoring" three 12V batteries.

Are two of these three in one bank? If so, separate readings won't be different. Just use two.

I feel that a minimum of 12 ga. on my boat is as small a wire I would ever want to deal with, electronics notwithstanding. If you're measuring voltage, bigger is better. I think there was a discussion about this sometime back: "What's the minimum gauge wire you would use on your boat?" or something like that.

Fuses can't hurt, but appear to be unnecessary, since there is no load on/from the meter.

 

[Davidasailor26]

If you're measuring voltage, bigger is better.

I think that depends on how you're measuring the voltage. In a dedicated circuit like this one, the only current on the wire should be the meter itself. Let's assume that the meter draws 20 mA (very conservative; I found a Blue Sea analog meter that draws 1mA), and the resistance of 22 gauge wire is .053 ohms/ meter. Let's also assume the round trip wire run is 1 meter. The total voltage drop would be just (0.02A * .053 ohms) = .001 Volts. I'm guessing that one-thousandth of a volt accuracy is probably close enough to not be worth the weight and stiffness of 12 gauge (which, for the record, would be 0.1mV accuracy).

I do agree that a fuse can't hurt. If anything ever shorted out inside the meter it would prevent an overload on the measuring wired. As long as the fuse has an internal resistance less than 0.1 ohms or so it should have no significant affect on the readings.

 

[JohnShannon]

It only draws milliamps if it isn't broken. Fuses are for when things aren't working as they should.

 

[Ron20324]

Fuses are for when things aren't working as they should.

+1 . If it's connected to a battery, it gets a fuse :naughty:

 

[Ken13559]

Always good to have fuse for each + wire leading out of battery. Its to break the circuit in case of shorts on wire after the fuse and not for protecting any load connected to it.
Imagine the wire chafed on something sharp and touching ground/- part, the small mA draws by the meter can becomes >>Amp till wire burns.

 

[Johnb]

Fuses are necessary.

Proper practice is to install protection (fuse or breaker) anywhere that a wire is connected to a source that is not itself protected by a fuse or breaker that will interrupt a current greater than the connected wire can handle.

 

[Scott T-Bird]

Xantrex LinkPro installation manual shows fuses for each battery connection. I would install them even if the manual is silent regarding this issue.

 

[mz4wheeler]

I beg to differ. An analog 8-16VDC volt meter will draw about 1 milliamp (full scale). Worst case, you'll fry the meter, which in itself is almost a fuse, and internally about 12,000 ohms.

Never heard of a 2 milliamp fuse.

Wouldn't worry about it.

 

[Stu Jackson]

Xantrex LinkPro installation manual shows fuses for each battery connection. I would install them even if the manual is silent regarding this issue.

Except he's only installing a voltmeter. If he tied it directly to a battery, maybe, like I said before. If he ties it into the wires at the back of his battery switch, not so much.

 

[rardiH36]

Just to confirm that I (the OP) am following the responses.

As usual, some different takes ... All look very valid. Thanks all.

The amp draw to the meter really is negligible. But even with say a 22g wire (which if shorted might "self fuse" by default when it probably would melt apart pretty quick on it's own), I accept the comments that a short along it's length needs to be protected against. We don't ever want fire temp heat or sparks on a boat.

So I will fuse. But will go the budget route. A 4-pack of 0.50 slow-blow amp glass fuses are available at my local Radio Shack for only a few $'s. Unlike the old days, "Wonder of Wonders" that today's Radio Shack has much of anything for the DIY'er!. Here is the link.

http://comingsoon.radioshack.com/500ma-250v-5x20mm-slow-blow-glass-fuse-4-pack/2701061.html

Rather than bother with proper fuse holders, I will just do a quick solder of the wires to each fuse's metal ends. Then cover with heat shrink tubing that is in my garage inventory to provide the insulation. Further, wrap with red electrical tape for good measure. And then label "0.5 amp fuse inside". Not pretty, but will accomplish the task.

 

[FastOlson]

+1 . If it's connected to a battery, it gets a fuse :naughty:

I would "up vote" this answer if possible.
Any wire starting from the battery gets a fuse. The ABYC likes this idea and so does your insurance company.
It's just a Good Idea.

Loren

 

[Maine Sail]

This definitely requires a fuse, under current US safety standards.. This fuse is not for the protection of the device, I've yet to see any fuse that could protect an electronic device, but more so to protect the wiring from turning your boat into a smoldering pile of rubble should that wire chafe and short. Eventually the wire would fuse itself but the question is would this happen before or after your boat is already on fire....

Even the shortest wire should get over current protection. This wire eventually fused itself, luckily before the AC/DC panel caught fire and burned the boat... But, but, but the wire is only 8" long do I really need a fuse???? ......

 

[Jim 5757]

Maine Sail is correct as always! The fuse has nothing to do with the meter, but EVERYTHING to do with the wire going TO the meter.
FWIW, I like the new-style automotive spade type fuses. You can get insulated right-angle spade connectors most anywhere, and the resistance of a 2-amp fuse with 2 spade connectors is negligible. The problem with soldering the glass fuse is that the fuse link inside is also soldered to the metal end caps and they tend to pop apart when heated (don't ask how I know that!).
While on the subject of wire size, voltage losse, etc..., understand that this 'expanded-scale' voltmeter is nowhere near as accurate as the digital LCD meters available today. We got a nice one on eBay last year for only $6.00. It's accuracy is better than 0.5%, resolution down to 0.01-volt and current draw is less than 20mA . With a simple 3-position switch we can monitor all our batteries.

 

[JamesG161]

I beg to differ. An analog 8-16VDC volt meter will draw about 1 milliamp (full scale). Worst case, you'll fry the meter, which in itself is almost a fuse, and internally about 12,000 ohms.

Never heard of a 2 milliamp fuse.

Wouldn't worry about it.

Right!!


Fuses essentially protect WIRE from higher amp load than designed. What is the amp load of the meter? If is more than a few milliamps, trash it for a LED type.

Jim...

 

[Ron20324]

... volt meter will draw about 1 milliamp (full scale). Wouldn't worry about it.[/quote...

You are overlooking what is the amp draw of a screwdriver dropped across the + & - wires/ terminals.

 

[mz4wheeler]

Right!!


Fuses essentially protect WIRE from higher amp load than designed. What is the amp load of the meter? If is more than a few milliamps, trash it for a LED type.

Jim...

Correct. A wire that is 22 AWG is rated for 1.84 AMPS. The rule of thumb is take the maximum load of the wire and multiply it by 1.5 to determine the fuse size:

1.84 X 1.5 = 2.76 AMPS (fuse size)

Which is absurd for a wire that is drawing a maximum of 1 milliamp. Worse, since you are trying to monitor a critical voltage, it is imperative that you have minimum resistance loss to get an accurate reading.

Adding unnecessary fuses inline, and the potential for corrosion would just hinder the voltmeter from accurately measuring the voltage.

A an ex-electronics technician, I have never seen a fuse in-line with a meter. Ever.

 

[dlochner]

Worse, since you are trying to monitor a critical voltage, it is imperative that you have minimum resistance loss to get an accurate reading.

Adding unnecessary fuses inline, and the potential for corrosion would just hinder the voltmeter from accurately measuring the voltage.

A an ex-electronics technician, I have never seen a fuse in-line with a meter. Ever.

I'm not an electronics technician, so let me ask a couple of questions.

If you want to get minimum resistance, why not use a larger wire? 22GA seems very small. I think ABYC standards call for a minimum of 16 gauge wire. Wouldn't using 16 or even 14 gauge offset any loss associated with a fuse?

If fusing a gauge is really unnecessary, why does the Balmar Smart gauge ship with a 3 amp ATC fuse for each lead and require 14 gauge wiring?

(edit: screwed up the quote, reformatted it for easier reading)

 

[walt]

Ideally if you want to measure voltage, you want zero current in the sense wire. Zero current = zero voltage drop from wire parasitic impedance.

Someone said that meter uses 1 ma.. I don’t know if that is accurate but it is probably close and I would not be surprised if the current is 1/10 that.

Look at the meter.. not even remotely an accurate readout. You could use just about any gauge wire you wanted (even very small) and it just would not matter.

Also, keep in mind that energy = power * time and the energy required to burn up a wire goes up as the gauge of the wire goes down. More energy is better at starting fires. Very small wires will vaporize with little energy if you short them.. less important to fuse. Large diameter wires with a 12 volt battery capable of a lot of current - very good at starting a fire - very important to fuse.

The Balmar gauge likely also measures battery impedance and the wire between the battery and the gauge will be part of that measurement. That gauge may have some clever method for calibrating out the wire impedance but it does more than just measure voltage.. so needs a lower gauge wire (like 14 gauge).

A lot of the battery monitors have a 12 volt wire that powers the device and a seperate 12 volt wire to measure the battery voltage. The measurement wire will have almost no current flowing in it. Both of those wires will be fused. .

 

[JamesG161]

You are overlooking what is the amp draw of a screwdriver dropped across the + & - wires/ terminals.

You are right I did overlook it and a crow bar and chewing gum foil.

My screwdriver would bounce off. It takes time to overheat a wire. Remove the short quickly or hide the terminals from kids behind a panel.

Jim...