What is my shunt actually for, if my BMS also has internal shunt?

[interestingfellow]

Please read all of it and answer the overall question, I'm not looking for specific advice, I'm trying to understand a concept.

I am looking at building and using 3 separate battery packs with Daly BMS
Why can't I just use the BMS as the shunt? Why would I need to add an extra shunt to the system or to each battery just to monitor that pack(s)? I feel like the BMS internal measurements will be as accurate or at leas accurate enough already.


To put this in context, I have been living off a 2s8p 560ah eve 280 pack with a daly 250 smart bms ran through a victron shunt to a multiplus all monitored by a CCGX/venuspi. In comparing the shunt to the bms measurements over 2 years, they have both matched up. Why would I need an extra external shunt then?

I am looking at adding 2 more 1p8s eve280 packs with daly smart bms as well as adding WNT boards to all of them.

 

[hwy17]

You don't. If your BMS SOC calculation is satisfactory and provides the necessary communications if any that you desire, then you don't need an additional shunt.

 

[interestingfellow]

You don't. If your BMS SOC calculation is satisfactory and provides the necessary communications if any that you desire, then you don't need an additional shunt.

And this is what I was thinking.
I feel like there are many parrots repeating what they have read who don't actually understand why and where you want a shunt.

 

[hwy17]

Well, I wouldn't take it personally. People get used to doing what they do, and when they see someone else having a problem that they don't they say why don't you just do it this way. There are a lot of people with problems.

If you understand what you are doing you can easily find yourself standing alone in your design and still know it is correct for you.

The Victron community in particular is infamous for its cult like design and encouragement of the Victron way. It's one of the reasons I choose to stay away. If you stray they will have no sympathy. Outside of Victron I don't really hear anyone advising master shunts anymore.

 

[Pappion]

You don't. If your BMS SOC calculation is satisfactory and provides the necessary communications if any that you desire, then you don't need an additional shunt.

That is a Big IF.
My BMS (Overkill Solar and Radio B) uses voltage to show State of Charge. You can set it under parameters, in 20% increments.
Current reading is not accurate or precise enough in both directions. Most likely it is based on the voltage drop across the power FETs, not a stable and accurate resistor shunt. FETs are subject to PVT variance: Process, Voltage, Temperature.

In another words don't expect a $100 BMS to have a $100 Shunt inside.

 

[Crowz]

A central shunt is still handy to know whats going on as far as amps being pulled or charged to the battery bank with something like solar assistant or an app.

 

[MiSh]

"I feel like the BMS internal measurements will be as accurate or at leas accurate enough already."

As long as you're confident and trust that what you're reading on your BMS is accurate, sounds like a plan. My SOK rack mount batteries' SOC is wildly inaccurate so I use Victron's shunt/app.

 

[Q-Dog]

The shunt should tell you state of charge of the entire bank of batteries, whereas each BMS will only tell you about the cells it is connected to. If you only have one battery with one BMS that you trust, you don't need the shunt.

 

[hwy17]

Yeah but it's also a question of how accurate you need that number to be and for what.

I notice that people who run off grid systems for years, decades, they don't usually know or care what their SOC is day to day.

 

[Pappion]

Yeah but it's also a question of how accurate you need that number to be and for what.

I notice that people who run off grid systems for years, decades, they don't usually know or care what their SOC is day to day.

I wish my Victron Smart Shunt app had the same type of daily history data as the Victron Smart Solar app. Maybe the wired system is better.

 

[interestingfellow]

I wish my Victron Smart Shunt app had the same type of daily history data as the Victron Smart Solar app. Maybe the wired system is better.

have you looked into a ccgx or venuspi? you can use the vrm portal to access that data from anywhere.

I"m actually looking at using daly paralell bms and wnt com boards to plug into my venuspi to be able to track and access even more data

I'm actually waiting for my chinese supplier to get me the quote for 16 more eve280's, 4 daly paralell bms, and 4 wnt boards.
The system will also be monitored by a bvm smart shunt anyway, I just didn't want to spend $800 on 3 more shunts that weren't actually *necessary*

 

[interestingfellow]

These have been really great answers and exactly the kind that I was looking for.

THANK YOU EVERYONE!

 

[RCinFLA]

SoC Columb counter will cumulate more errors over time, in reported SoC, the poorer the current measurement accuracy. Accuracy of current measurement over a wide dynamic range of current is critical for a good SoC indicator.

BMS's use cheap chip resistors, in parallel, distributed across their PCB. Stress on the chip resistors due to heavy interconnect cable and chip resistor heating can fracture the chip resistors' solder joints or even fracture a chip resistor. Resistance temperature coefficient on these chip resistors is not great and PCB copper interconnects and solder joints effects current measurement results.

A quality metal alloy shunt is built to provide near zero temperature coefficient and designed to take the heat generated from high pass current. They are individually calibrated for resistance by slight side grinding trim on the shunt plates. On the shunt pictured below, at 500 amps the shunt will dissipate 25 watts of heat and will get quite hot so don't confine its access to air cooling by installing it in a small, sealed box.

For parallel battery array setups, using BMS current measurement is a good secondary check on current sharing distribution across the parallel batteries.

JK BMS uses ten 1.0 milliohm chip resistors in parallel distributed across both sides of the PCB. Both pictures below are 0.1 milliohm shunts.

 

[interestingfellow]

Well, trigger pulled and we'll see how it all works out.

I have 16 more eve280's on the way
4 Daly 250 8s paralell BMS and
4 Daly WNT boards

I'll end up with 4 8s 280ah battery packs. I'm going to use the BMS to monitor each pack seperately and then the BVM to monitor the whole system.

 

[Steve_S]

If you have 4 Battery Packs in Parallel, the BMS' in each pack are just for THAT pack and not the whole. An external Shunt sees the Entire Bank of Batteries and therefore represents the collective SOC of the bank.

As an off-gridder (several yrs) with 6 Battery Packs in Parallel it is nice to see the individual pack status but the entire bank as a whole. Networkable/Linkable BMS' like Seplos, Pace & the new JK Inverter model can also provide more information about Pack & Bank State and if the proper (compatible) AIO/Inverter is used & programmed it can better manage the batteries which "must" be as accurate as possible.

 

[Pappion]

have you looked into a ccgx or venuspi? you can use the vrm portal to access that data from anywhere.

I"m actually looking at using daly paralell bms and wnt com boards to plug into my venuspi to be able to track and access even more data

I'm actually waiting for my chinese supplier to get me the quote for 16 more eve280's, 4 daly paralell bms, and 4 wnt boards.
The system will also be monitored by a bvm smart shunt anyway, I just didn't want to spend $800 on 3 more shunts that weren't actually *necessary*

links? I can find Victron CCGX.
No Idea what Venuspi is.
By BVM do you mean a Victron BMV product?

 

[Rocketman]

The Venus Pi - I think is the Victron GX software put onto a Raspberry Pi or is is the Venus GX device (a Victron product between the CCGX and Cerbo).

The Victron shunt will report its data to the GX (Cerbo), you can also use the DVCC to send the data to the charging devices (Multiplus and Mppt’s) so they can compensate for voltage drop.
For many people voltage drop is very little because they can put all the Victron stuff together. In my MotorHome that wasn’t an option, so I have a bit of voltage and current drop on each device. It is nice having it all compensate for.

Also that data packet can contain battery temperature, so the charging devices can be set so they will not charge if the batteries are under a certain temperature.

 

[interestingfellow]

links? I can find Victron CCGX.
No Idea what Venuspi is.
By BVM do you mean a Victron BMV product?

If you want to setup, manage, monitor, or otherwise access your victron equipment then a Victron Cerebo will let you do that with Victron Connect or Victron Remote Management as will the Victron CCGX (color control gx) which also has a built in LCD

Yes, I have a BMV712 (I think it's a 712. 750? I dunno) smart shunt

Rocketman is correct about venuspi. victron released the VenusOS from the CCGX and Cerebo which was ported to run on a raspberry pi. So for the cost of a rpi and an LCD, you can have a basic touch screen Victron Remote Connect capable device to monitor your Victron (and some other) equipment. go read some of those links and then feel free to ask questions, I'll be glad to help where I can