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Housing Victron shunt? And where to put in circuit


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Hi,

I want to add my new BMV702 between my Pylontech batteries and my fuses going to the inverter.  I have a Jean Muller disconnect for the fuses - firmly mounted on the wall.

Now Victron places the shunt right at the batteries on the negative side.

As you know, the Pylontech have nice premade cables with a snap-on connector at the battery side and ring terminals at the other.  I'm reluctant the cut those cables.  I'm not too sure what difference it would make to put the shunt at the batteries - can I just put it next to the fuses?  Would it make much difference?

Also - what have people used to house the shunt?  It needs something about 45mm deep - maybe 50 to have some extra space.  That's too deep to use 100x40 trunking that I have here, and too deep for a 4x4 box.

 

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Thanks,
Elbow

 

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13 hours ago, Elbow said:

can I just put it next to the fuses?  Would it make much difference? 

I'll tell you why you'd normally want it as close to the battery as possible, and then I will tell you why in your case it probably doesn't matter.

In many systems the BMV serves as a voltage measurement service. It is closest to the battery, so it has the best idea of the voltage. Other devices might be measuring higher or lower because of voltage drops on the cable. Now the BMV has this red fused cable going directly to the positive of the battery, and there is going to be almost no voltage drop on that cable, so on the positive side you are covered. On the negative side, however, it reads the voltage at the shunt, so you should put the shunt as close as possible to the battery. In most cases that is.

However... putting it slightly further down does not affect its capability to do its job (managing the SOC), and you already have another even better source of the voltage (the BMS itself, if you can read it that is). For this reason it is not going to matter if you put it on the other end of the cable.

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8 hours ago, DeepBass9 said:

... so I really don't think it matters much which end of the battery cable it is on.

I tend to agree. Even less so if you have voltage reading taken direct on the batteries that the inverter uses.

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45 minutes ago, The Terrible Triplett said:

I tend to agree. Even less so if you have voltage reading taken direct on the batteries that the inverter uses.

He doesn't have a Blue setup, so the BMV voltage isn't used for anything. In a Victron setup, perhaps with Shared Voltage Sense enabled, then it would make a difference.

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On 2019/01/29 at 5:26 PM, plonkster said:

He doesn't have a Blue setup, so the BMV voltage isn't used for anything. In a Victron setup, perhaps with Shared Voltage Sense enabled, then it would make a difference.

 

So I'm not even sure that I really need the BMV.  I get lots of data from the batteries directly.  So I could maybe lose the BMV and just show the info that Pi is reading the RS485 data from the batteries on a little display.

But the display is nice and provides more data that I can read and use to control things.

For sure the inverter's view of the SOC is completely wrong - so an upcoming project is to try to control the inverter behaviour over its USB port from the outside based on the info I get from the batteries and maybe from the BMV.

Perhaps at the end of the day I'll end up just pull the Replus and put in a Victron.Multigrid and get the approval of the group 😉

Thanks,
Elbow

 

 

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  • 1 year later...

Hi guys, apologies for resurrecting an old thread but I felt my question flowed nicely from here.

My question is also about the placement of the BMV, but slightly different...  I am planning to install a BMV-702 on my 12x 100ah12v (48v configuration) gel battery system.  I too have a inline fuse quick disconnect similar to @Elbow's picture above.  Both my positive and negative main cables (35mm2) from the battery bank pass through the fuse quick disconnect (100A fuse on each line) before entering my Axpert 5kva inverter.  From what I've gathered, the BMV's shunt should be close as possible to the batteries, i.e. on the battery side of the fuse quick disconnect, to get the most accurate readings from the batteries.  But to me it feels safer for it to be on the other side of the fuse quick disconnect (ie between fuse quick disconnect and the inverter), so that I can pull the switch if ever needed, and the fuses could provide protection to the BMV shunt as well. But will this affect the BMV's abilities to get accurate readings? 

I haven't been able to find any definitive info on this question through my searching. The Victron BMV-702 manual and all instructional material I found only says that the battery goes on one end of the shunt (the negative cable) and all load on the other end of the shunt. They do not, in fact, show any fuses/fuse placement on the main negative cable from the batteries at all.

Any advice on this would be greatly appreciated. I am amazed at & grateful for the amount of technical knowledge on this forum! 🙂

@plonkster your vast knowledge on Victron equipment would be massively helpful here! Thanks!

Jaco

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Edited by JAvandermerwe
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3 hours ago, JAvandermerwe said:

From what I've gathered, the BMV's shunt should be close as possible to the batteries, i.e. on the battery side of the fuse quick disconnect, to get the most accurate readings from the batteries.

Correct. On the negative side of the battery, as close as possible, on the battery side of the fuse (otherwise the voltage drop across the fuse affects the voltage reading).

The BMV has its own small red wire that feeds it power, and this is separately fused. This is the only protection the shunt and its electronics need.

Also, when using a steel case, you should ideally have the fuses as close as possible to the battery (the point of two fuses is to protect against multiple earth faults, eg should the battery cable short out against the steel case). Normally this means putting a fuse as close as possible to the positive pole of the battery. So shunt as close as possible to the negative, fuse as close as possible to the positive.

If not possible to put a fuse inside, then make sure a short to the metal case is physically unlikely.

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If you shunt is against the battery and the disconnect only later in the cable, it will still protect the shunt as well. Any disconnect or fuse will blow first. This will leave your circuit open and no current will flow even if your shunt is before that in the cable.

The only time this is not the case would be when there is a short before the disconnect, but then your cable is going to be a nice free firelighter anyway 🔥

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