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Old 20-09-2019, 21:00   #1
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Fuses on voltage sensing wires of battery monitor and/or BMS

Seems like you'd want the sensing wires to be connected as close to the battery as possible (like connected directly to the terminal). But then how do you provide circuit protection?

The BMV-712 I'm panning on installing comes with an inline fuse on the wire; not sure what kind, but its glass and ~100mA. So I doubt it has the proper 5000+ AIC rating. Should this be connected directly to the terminal, disregarding ABYC recommendations? Or should it be tied in after my 300 A Class-T main bank fuse?

The BMS I'd like to installed mentions nothing about installing circuit protection on the cell voltage sensing wires. These undoubtedly need to be installed directly on the terminals. Would fuses make the readings inaccurate?
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Old 20-09-2019, 21:51   #2
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

go find me any factory boat that has bilge pumps that are AIC fused rated... every boat has them connected to the battery or battery side of switch with AGC or ATO fuse...

battery monitors should be direct to battery for best results. ATO car fuses are better then glass fuses

this is one rule that is hard to meet.

I mean if you shorted the 20awg victron wire and the 100ma fuse shorted shut. do you really think it's going the blow a ~300a classt? I really doubt it. the wire will dissapear first.
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Old 20-09-2019, 22:12   #3
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

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ATO car fuses are better then glass fuses
I was having trouble finding ATO-ATC fuses that were less than 1 A. I'd rather use ATO-ATC but I'm not sure if it would be okay to have a larger fuse in this situation.

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I mean if you shorted the 20awg victron wire and the 100ma fuse shorted shut. do you really think it's going the blow a ~300a classt? I really doubt it. the wire will dissapear first.
Yeah, you're right. It's been said that a wire that small would fuse itself. And it makes sense. But I started second guessing that because the BMV comes with a fuse installed. I find myself doing a lot of second guessing... and first guessing.
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Old 20-09-2019, 22:36   #4
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

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go find me any factory boat that has bilge pumps that are AIC fused rated... every boat has them connected to the battery or battery side of switch with AGC or ATO fuse...

battery monitors should be direct to battery for best results. ATO car fuses are better then glass fuses

this is one rule that is hard to meet.

I mean if you shorted the 20awg victron wire and the 100ma fuse shorted shut. do you really think it's going the blow a ~300a classt? I really doubt it. the wire will dissapear first.
The wire may disappear first. The question is will it be laying on something that catches fire while in the process of disappearring.
Mine are fused.
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Old 21-09-2019, 04:27   #5
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

Won't CP interfere with sensor wires doing their job? Maybe a fusible link right at the post I suppose?
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Old 21-09-2019, 04:37   #6
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

Quote:
Originally Posted by expozen View Post
Seems like you'd want the sensing wires to be connected as close to the battery as possible (like connected directly to the terminal). But then how do you provide circuit protection?

The BMV-712 I'm panning on installing comes with an inline fuse on the wire; not sure what kind, but its glass and ~100mA. So I doubt it has the proper 5000+ AIC rating. Should this be connected directly to the terminal, disregarding ABYC recommendations? Or should it be tied in after my 300 A Class-T main bank fuse?

The BMS I'd like to installed mentions nothing about installing circuit protection on the cell voltage sensing wires. These undoubtedly need to be installed directly on the terminals. Would fuses make the readings inaccurate?

The purpose of the fuses is to protect the tiny wires. This is fire protection.
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Old 21-09-2019, 04:43   #7
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

Blue Sea makes an ATO fuse block with individual fuse connections. I use 2 amp fuses which is fine for #18 wire. Depending on the battery bank you are probably exceeding the interrupt capacity of the fuses, but with smaller wiring the available fault current decreases rapidly down stream.


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Old 21-09-2019, 04:50   #8
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

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Won't CP interfere with sensor wires doing their job? Maybe a fusible link right at the post I suppose?
CP: Cerabal Palsy? Wrong forum.

A properly installed low amp fuse isn't going to show any significant resistance that would affect the voltage reading.
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Old 21-09-2019, 05:03   #9
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

circuit protection

fusible link wire I think makes more sense, there really is no appreciable current flowing in normal use
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Old 21-09-2019, 06:03   #10
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

The sense wires for active shunts and battery monitors carry very little current.

My 500A shunt for my Pico is rated at 0.4mA.

Thus even a high resistance in the fuse protecting this sense wire will have very little voltage drop. At this low current the fuse and fuse holder could have a resistance of 25 Ohms before the voltage reading would be out by 0.01v.

The nominal resistance of a 2A blade fuse is around 0.05 Ohms. So even with the fuse holder the combined resistance is a couple of orders of magnitude lower than a level that will cause some practical effect, at least in a well designed battery monitor.

There are some battery monitors that have a higher consumption on the sense wires (sometimes the sense wire also supplies power to the display). This is not an ideal design, but the inaccuracies created are small. The resistance of the feed wire (the run can be very long if this is carried back to the display rather than an active shunt) is generally more of an issue.

So use a fuse.
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Old 21-09-2019, 06:27   #11
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

Thanks for clearing that up
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Old 21-09-2019, 06:43   #12
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

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The sense wires for active shunts and battery monitors carry very little current.
Just being called a voltage sensing line tells me there will be very little current going through it. In electronics, those wires are normally very small as compared to the normal voltage wires going to the load

Very similar to DC Power Supply sensing in electronics it seems

https://www.pickeringtest.com/en-gb/...s-power-supply
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Old 21-09-2019, 07:27   #13
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

You can also think of it like using a VOM to monitor your battery voltage. (other high voltage high current applications)

The VOM has very small leads (sensing wires) because of the high impedance of the meter.

Low current flow......
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Old 21-09-2019, 08:19   #14
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

https://marinehowto.com/alternators-voltage-sensing/

Really should be the standard wherever precision is important.

Some charge sources will, as higher SoC is reached, periodically pause charging to measure the "settling" at rest voltage to check more accurately

But of course that slows the charge cycle down
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Old 21-09-2019, 10:51   #15
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Re: Fuses on voltage sensing wires of battery monitor and/or BMS

I think the take away is: connect sensing wires directly to the battery terminals and fuse each (within 7) with ATO-ATC fuses, even though the AIC rating may be below the short circuit current of the battery. Thanks!
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