Fuses on voltage sensing wires of battery monitor and/or BMS

[northwestsailor]

You definitely want to fuse the BMV712. I shorted my battery briefly and the fuse took the hit but saved the wiring.

[CatNewBee]

Originally Posted by expozen

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 BMS sensing wires are an issue, it depends on the BMS.
They not only measure voltages - like the BMV, but also carry equalization currents of several amps (fuse sizes of 100mA is not sufficient, you may need up to 5A fuses) and are used to determine the cell resistances, so any additional resistance may jeopardize the results the BMS relies on. Inside of the BMS usually there are callibrated fuses for this reason, but external fuses are usually not intended to be used.

Main problem is chaffe and a short cut to another wire before they reach the BMS. Best to do a propper additional isolation and chaffe protection of the wires and routing to not create a short.

While voltage measurement is not sensitive to line resistances (there are some milliamps flowing through the wires and the connections, so even a high resistance across a connection or fuse will not change the reading), current readings would be affected.

[ayates]

To guard against shorts in the measurement wires, I always fuse them. For sensing at the battery I use inline ATC fuse holders, and for shunts I use an ATC surface mount fuse holder. Typically 2A fuses everywhere.


Allan.

[Frankly]

I have made this point before but may bear repeating.

In protecting a circuit against overloads the fuse can be located anywhere along the circuit just as long as it is upstream of the load. Protecting circuit against short circuits or ground faults the fuse needs to be as close to the current source as possible. In the latter case physical isolation/ additional insulation can help. If the current carrying conductor can not physically contact a second conductor/ surface (ground or otherwise) until it reaches the fuse, then it is essentially protected by geometry and spacing.

Codes typically don't recognize this last technique, but it can be a practical method of getting where you are going.


Frankly

[smac999]

All of you people are missing the real question. Which was how to fuse to meet aic rules. For example the above picture of atc fuses direct to battery. Which is how it is commonly done. does not meet the aic Requirements of abyc

[Frankly]

You raise a legit point, however ABYC is a Standard which falls more in the relm of recommendation. For example the NEC (Code) carries the weight of law in most jurisdictions. That is not to indicate that you thumb your nose at the ABYC, just to indicates there are rules and "there are rules".

As to the 1,000 amp interrupt rating of ATO/ ATC fuses that rating is at 32 VDC, I certainly don't know what the interrupt rating is at 13 VDC but is probably at least twice that specified. The interrupt rating is "safely interrupt" which means no damage, doesn't mean that subjected to 4,000 amps is going to blow out the side of your yacht (might start a fire, but probably just melt some plastic).

If you look at the link of the high amp set up on my Yacht, what is not visible is a 300 amp class T fuse in the main feed coming in from the bottom. Without getting into the weeds of series rating of two devices, I could argue my ATO fuses can handle fault conditions up to the rating of the class T (a large number) so I am ABYC complying . Do I think if I short the downstream side of the 2 amp feeding my Balmar 612 or Emeter is that 300 amp going to make a significant difference in the outcome.

Providing protection for all those small sense wires feeding all things that need a reading on the battery voltage may come up a little short in the ideal world, but sure beats most other reasonable options, and surely beats not providing any protection.


Let's talk about those tiny wires on those Hella fans most use.

Frankly

[expozen]

Originally Posted by CatNewBee

The BMS sensing wires are an issue, it depends on the BMS.

You've made a good sell on the REC ABMS and that's what I'm designing for. I'm really not sure about fusing before it. The largest wire that the harness will accept is 16g.... 2ft of 16g wire and a 25 A ATO fuse is 0.010552 ohm * 2.5 A = 0.026 V. Which means.... well, I don't really know. I suppose I'll just email and ask.

Originally Posted by CatNewBee

Main problem is chaffe and a short cut to another wire before they reach the BMS. Best to do a proper additional isolation and chaffe protection of the wires and routing to not create a short.

By isolation and chaffe protection do you mean something like taking the 5 voltage sensing wires and running them inside of some 3/8" Ancor Marine Grade Electrical Self-Extinguishing Split Loom? Or would there be a better solution?

Wait... shouldn't each of the BMS sensing wires actually be ioslated and protected also from each other? Because 4 of them can potentially be a positive and a ground?

[CatNewBee]

I have no additional fuses installed, yust put a fitting plasticc isolation tube on each before crimping the cable shoes on, then route them away from the battery, you can put them in a cable channel or zip-tie them together to organize the wiring to the BMS. I left them all to the original size and not cut them short. To make sure they do not add systematic errors to the measurement. Maybe a little too over-cautious.

I am no related in any way to REC, just a happy user.