Geomz avatar image

Functional purpose of voltage sense wires?

Can someone shed some light on the intended functional purpose of the Voltage Sense wires on a Multi/Quattro?

Are you using it? If so, how/why?

The manual says loosely that it's "to make up for voltage losses across the wires".

With the volt sense wires connected or not connected, under minimal load or loads in excess of 100A, I've never seen any change in the reported voltage by the system.

This was true even before my installation of a BMV (with the lithium batts) and still so now, since SOC and voltage readings (I assume) are now coming exclusively from the BMV.

The voltage reported (by the BMV) is within .01 V of the voltage reported by the BMS on the batteries with minimal load, and within the expected 2-3% difference under heavy loads.

I'm using 2/0 (70ish mm cross section) wires and runs are around 10-13 ft to the batteries. The BMV is 5-7 ft to the shunt. 24V lithium system.

The lead-acid system prior was considerably sloppier, although still using 2/0 cable, and I still recall no differences.

BMV Battery MonitorvoltageVE.Bus
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They're important in my system, because there is definitely voltage sag under load.

Where are you looking when you describe seeing "reported voltage"? Victron systems report different voltages in different places, and it can also change depending on what options you elected in the software.

The quickest way to determine if you have any sag (and I suspect you probably do, at 100A with 12ft of 2/0 and presumably at least one fuse and contactor) is to take your multimeter and probe the batteries and then the DC input terminals of the inverter directly. Do so while the system is under a continuous, heavy load, and let us know what you find.

Thanks @ben.

I'm comparing the voltages as reported by the BMS (I figure it's the closest point to the batteries and spot on with a multimeter) with the voltage reported by the BMV and by the inverter. For the inverter, I'm looking at the DC voltage for the inverter itself in CCGX, under its own entry there.

I do have some sag, it's the expected 2-3% under heavy load.

But nowhere is that reflected/changed on my displayed numbers (BMV, CCGX, alert triggers, etc) any differently with or without the V sense cables.

Perhaps it's more of an issue with lead as there's heavy voltage sag with them under load and not as much with lithium? But, that shouldn't really be an issue, because that sag is internal to the batteries themselves, and not as much because of the wiring/fuses/etc.

The sense cables are to eliminate the drop due to interconnect, which of course varies a good bit depending on your circuit.

This drop is actually more important when using voltage-based charging strategies on a lithium pack, since lithium batteries exhibit such small voltage differences to begin with.

When the sense cables aren't hooked up, the inverter will report the voltage it measures on the main power cables. As soon as the sense wires are connected, it will instead show the new voltage. (This is the inverter input voltage reported under that module in the CCGX UI.)

You should be able to see it change, unless you have superconducting power cables or something.

I'm just trying to help you figure out why you don't see it, first. Whether it matters or not is a separate question and will depend on the rest of your system design and your personal preference.

Can you just put your meter directly on your DC input terminals on your inverter, and then put your meter on that end of your Vsense cables, and see what it says for each?

I'll try to do that sometime this week and see if there's any difference. I ran similar testing when I installed the system. But I'll try it again with more meticulous/controlled testing procedures. I guess in the long term it doesn't really matter, as I have already run the cables and attached them. I just saw no difference and wondered if I'd just wasted a bunch of time for no reason :D

And LOL, re: superconducting cables.

I got a set of Victron's new superconductors.

They're hush hush, and keeping the nitrogen cooled conduit plasma flow stable is a bitch, but boy it's worth it ;)

If your BMS has full charge authority and you trust it, then whether the inverter has the very best voltage data is not so important... it won't be doing much other than possibly triggering a LV shutdown or something.

Alternatively, if your BMV has good voltage data, you can now turn on Shared Voltage Sense, and your CCGX will "overwrite" the inverter's internal voltage reading with the one from the BMV via software. In a roundabout way, this may achieve the equivalent of a Vsense pair.

But if neither of those are true, Vsense can be very useful or even critical.

A quick calculation with values predicts a 0.25V drop across the length of the cable. Thus 12V at the battery charger will be 11.75V at the battery. With the voltage sense wires, the charger will raise its output voltage to 12.25V so as to get 12V at the battery. (Using a 12V target voltage as an example)

2 Answers
BenL avatar image

This is 4-wire sensing aka Kelvin sensing, a basic electrical measurement technique.

Whether it makes a practical difference in your setup is dependant on cable size and current load, as others have noted. Unless you've massively over-sized the cable it would be a worthwhile thing and provide improved regulation.

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Paul B avatar image

Keep in mind that the volatge sense also increases the charge voltage by the volt drop so that the batteries get the corrected volatge for charging

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