question

Alistair Warburton avatar image
Alistair Warburton asked

Issues with smart shunt?

Hi folks,

I recently installed a 24/5000 multi on a none Victron LiFePo4 pack, 300AH by the spec.
6 Ultramax 100Ah at 12.8V Nominal in 3 strings.

I added a SmartShunt connected to the Cerbo Via VE.Direct.

The Smart shunt is set up as the Battery monitor, charging is only via the Multi, the Cerbo and my WiFi/Ethernet network kit is a DC load. Nothing, other than the shunt is connected to Bat -.
Currents and voltages look reasonable.

I have two issues. (And little idea what to ask)

1) The console seems to report erratic DC load as max as +50% and -100W. The network power requirement will be fluctuating but the power meter on the buck converter that supplies it never gets anywhere near 200W peaks I see on the console.
There is never a DC charge current that isn't coming from the Multi, so I am assuming DC load should never be negative, is that right?

2) My system has shut down a couple of times with the battery capacity showing a little over 50%
The shut down is due to the batteries internal BMS protection tripping, at 9.5V!! Although I could mitigate that with a low voltage disconnect setting/logic it isn't going to help with the capacity meter error. The multi is set to disconnect at 21V, which I am about to lift, but at 21V the DC load still pulls the battery down quickly when the inverter is off.

I have 90A Bulk to 28.8V Absorb and then 27.6 Float set in the Multi, 'Charged Voltage' was originally a little low, its now 27.4, but the tail current was set to 3%,so 3A, which the pack is always well under by the time the 1Hr Absorb time has passed.

Looking at the history, the deepest DOD was 88% which if it accurate suggests my batteries are not matching the spec. Possible I guess. However it doesn't explain the low voltages and internal BMS disconnects at above 50%, metered.

What I need to know is if I am missing something? Because if I am not then the only other explanation is that my batteries are only holding about 55% of the rated capacity which is an argument I ma not looking forward to having with the supplier.

I am unsure how to proceed here. It would be great if someone pointed out how silly I am being and which settings I have wrong. Failing that some a advice on the best way to run a capacity test would be good.

Currently I have the battery monitor in the Multi disabled, assuming the Smart Shunt was a better option. However I am questioning that decision and now wonder if I am missing some fundamental point. Looking at the documentation again I am not even sure now if 'Has DC' should be on or if it will work as I thought.

I have attached a Lash Hour screen cap from VRM showing the negative DC load reading and what the rest of the system was doing at the time.

Happy to look silly here folks but need some help.
Cheers,
Al

odd.png


MultiPlus Quattro Inverter ChargerSmartShunt
odd.png (80.0 KiB)
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10 Answers
ludo avatar image
ludo answered ·

Are your batteries suitable for series connections, maybe they'll need an external balancer ?

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Alistair Warburton avatar image Alistair Warburton commented ·

Good thought but I checked with the manufacturer before going that way... All good, apparently.

Its perhaps worth adding a mid point bus bar, which I don't have now, and setting up the second input on the SmartShunt though.

I installed the three strings with Identical cable lengths and a separate fuse on each to make checking and maintenance easier, hence the lack of a mid point bus bar.
When the fuse carrier, NH disconnect, is closed all three strings are parallel but a fuse could be removed if I had an issue with a string leaving the other two active.

I haven't done a great deal of manual checking but when I have looked I haven't noticed any significant asymmetry when charging or discharging. My DC clamp is only accurate to a few 10's of mA, at best, so by the time I move between strings its hard to say if the small differences are are actually differences or just fluctuations, with time, across the entire pack.

I do have a 4 channel scope which I guess I could look at the circa 1.2m lengths with. At 4kW I should be seeing circa 21mV/m on the battery cables, that may be enough to do a real time check but I suspect noise might be an issue..

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ludo avatar image
ludo answered ·

Try setting charged voltage to 28.7

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pwfarnell avatar image
pwfarnell answered ·

1. DC power

This question keeps coming up. The DC power is calculated from the other information available, it is not measured. The Shunt measures everything flowing into and out of the battery. The Multiplus as far as I know does not have a super accurate DC current measurement. Therefore, the Cerbo calculates the difference between the reading from the shunt, the estimate from the Multiplus, data from any other known sources if fitted such as an MPPT and this difference is the calculated DC load. An additional factor is that the time that readings are taken varies from device to device, so when you have fluctuating or changing loads, the calculated DC power can vary. For example if the Multiplus increases power draw from the batteries and the calculation of the DC power is still using an older lower value for the battery current from the shunt then the DC power is wrong.

2. SOC problems

Copy us a screenshot of your Smartshunt settings.

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Alistair Warburton avatar image
Alistair Warburton answered ·

I Set it to 27.4, with a tail current of 0.5%, yesterday, as per documentation, and then watched the charge terminate.
By the time the Multi dropped into float, 27.7, the battery current was below 1A, consistently, so I am pretty sure the battery was full and as you would expect showing 100%.

What's the thinking behind lifting that setting? I was under the impression that the battery percentage was supposed to be reset to 100 when the voltage is above Charged and the tail current requirement is met. My system had settled at 100% shortly before that but only perhaps 5 mins.

I am happy to change the setting but I would like to understand why, and what I am looking for.

I am wondering if the bulk and float voltages are a bit too low but I think I checked with Ultramax when setting up... Not that I made a note of the conversation.

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ludo avatar image
ludo answered ·

For me the battery is full when absorption is finished. So I set the "full" close to absorption voltage. Works fine for me.

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Alistair Warburton avatar image
Alistair Warburton answered ·

Seems logical and will not do any harm... I will give that a go thanks.

In other news...

I suspect my problem is partially to do with charge settings, they were a little shy for the battery, and also linked to temperature and load.
At C/2 discharge I am seeing a voltage drop at the terminals that is greater than I was seeing a few months ago. This caused me to set a very low disconnect voltage, which is why the internal BMS shut the battery down before the Multi switched off. Neither impact capacity but the capacity reduction associated with temperature, a thing apparently, isn't being compensated for in the shunt.

I have temperature monitoring but its done by the Multi. The shunt doesn't have that info as far as I know so couldn't trim the capacity even if it is supposed to.
Do you know it capacity trimming / derating is built in? Adding a temp sensor is hardly a big deal.

The second thing is that the volt trop associated with load looks like it is temperature dependant as in, colder battery lower terminal voltage but also bigger drop with a given load.
Is that plausible or am I grasping? Temp is 14C right now with a terminal voltage of 26.16 @ -8.3A

that would be about 2.77A per string
at -60.7A per string the terminal voltage fell to 25.92 within about 20 Secs. I ran that load for about 60 Secs, and then dropped back to the circa -8A base load and the voltage recovered to 26.12 within a couple of mins.

That doesn't seem excessive to me but if I am trying to set a disconnect at to capture say 15% SOC as an absolute minimum capacity the drifting voltage will mess that up.

BTW battery currently at 26.21V with a 200W draw. SOC reads 53% which is perhaps a few percent optimistic but not that far out at this stage. No charging since Last night.

I am beginning to think this isn't some horrible capacity issue but likely several smaller things conspiring to confuse both me and the shunt.

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Alistair Warburton avatar image Alistair Warburton commented ·
I have taken 10% off my bank capacity figure and increased the Charged value as suggested. To be honest I now either need to live with it and see what transpires or do a set of controlled discharge tests, which isn't easy as we are off grid and this is my primary system. I have been meaning to add a bypass to both Multi's, they are in series, to provide extra redundancy, so perhaps this is the time to do that.


Anyone have a suggestion as to where I can get a changeover switch at a good price, preferably one that will go into an MCB enclosure. ( I will need two)
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ludo avatar image
ludo answered ·

You have two multis in series ? Could you post a diagram of your wiring, I just can't imagine what you are trying to achieve.

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Alistair Warburton avatar image
Alistair Warburton answered ·

:-) Boxes are not my thing... I would love to say I think outsude them but the truth is I never lerned whre they were... Started with an easy solar / BYD 13.8 kW, absolutly standard install. Thn needed more battey capciy and higher load capacity, occasionally. Couldnt afford another 12K for second battery and another 5000/48/100, also wanted redundency which you dont get with 2 identical units in paralell. SO....

5000/48 feeds 5000/24, ACout to AC In. Solor feeds 48 pack, genertor feeds 5000/48, house connected to 5000/24.

The result is 8kW availability plus 3.8kW genny, with genny on and both units running in assist... Pretty cool.

Two transfer switches allow, will allow, the house to run from 48/24/genny or any combination of all three. Its just a chain. A Cerbo manages things handling what is on and what charge the 24 is taking.

When I present a high load, the AC limit on the 24 is used to split the site load between both invers. The ac limit on the 48 keeps the genny happy.

Would a couple of Quatro's and 3 times the battery capacity have been better... Technically, of course' but i am just some dood on the same wage as you so that wasn't happening, sadly.

Still fiddling with Node Red to hang all this stuff together. Technically now sorted but with manual control. Working on automated triggers now to replace my input.

BTW. Battery SOC from the shunt looking way more realistic after playing, as per previous posts/suggestions. Early days but beginning t think I may have a handle on it

Good to talk, thanks.

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Alistair Warburton avatar image
Alistair Warburton answered ·

What I have....Victron Systeml.pdf

To be fair the transfer switches are still in a box, wiring is as Pri. There is a CCGX on the 48 and a Cerbo on the 24. The Cerbo hosts Large OS with NodeRed handling the control.

Data from the 48, and some control, is via Modbus TCP, custom nodes are available in NodeRed on the Cerbo to manage the 24. Charge current control on the 24 is achieved by manipulating DVCC, which isn't an option on the 48n because the BYD BMS has control of that.


The DC-DC isn't in place, in fact I don't have one yet, still looking at options . My Cerbo and Ethernet/WiFi all run from the 24 DC. Adding resilience there, is/will be, a good thing.

Two multi's in parallel have to be identical and cant fail over if one of them has a problem.
Organised this way I have way more flexibility and auto failover right back to the genny.


victron-systeml.pdf (11.2 KiB)
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ludo avatar image
ludo answered ·

Ok, if this works for you it's fine for me. Glad I could help with your soc problem.

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