question

baxter avatar image
baxter asked

VE.Bus low battery Warning -> false alarm??

Dear all,

I have the following system:


3phase ESS with MP2 5000 Firmware 494

1 MPPT RS450/100 Firmware 1.09

REC BMS with 16s4p LFP

Fronius Symo 6 on ACout

Cerbo 2.92

DVCC active with STS and SVS

The system is running flawless since may this year.

Today I got a Ve.Bus Alarm Low Battery Warning and was wondering why. Voltage was 53V (according to BMS), cell voltages fine, SoC was 70% (min SoC in ESS is 65%).

Alarm Timestamp is 14:58, and now see the difference between BMS, MPPT and VE.Bus concerning voltage readings:

BMS (53,02V):

1674398948360.png


MPPT (52,39V)

1674399064745.png


VE.Bus (50,39V)

1674399119020.png


The Warning was cleared after 30seconds, all three devices are showing again the same voltage reading like it was before.


Does any one have an idea about the root cause?


Thanks i advance

Br

Mike


low battery warning
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4 Answers
kevgermany avatar image
kevgermany answered ·

Looks like something was turned on with a high startup current, causing the voltage drop.

Inductive loads (most things with electric motors, microwave ovens etc) usually have a very high inrush/startup current, which can cause this. And in this case enough to trip the alarm.

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baxter avatar image baxter commented ·

Thats right, there was a high load at that point, but inverter power was limited in ESS to 1000W and the battery current was not more than 17A, which is kindergarten for my battery (920Ah, 4 times 16s in parallel, so not more than 5A per pack)

But that hight load does not explain the difference in voltage readings between the devices, and I had higher peaks in the past without this warning.



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nickdb avatar image nickdb ♦♦ baxter commented ·

Alexandra has already pointed out the common causes for differences under load.

The best resolution is to resolve that difference.

You can also consider using the vsense inputs on the inverter to provide a better reference, or consider enabling svs in dvcc for the bms. These are just workarounds to the problem which most likely is wiring or something connected in between (isolators etc). If you have a thermal camera it can help find hotspots or just use your trusty meter.

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baxter avatar image baxter nickdb ♦♦ commented ·
As mentioned, SVS is enabled. Thermal camera is in use, everything fine with the connections
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nickdb avatar image nickdb ♦♦ baxter commented ·
Ok. So if you load the system and measure all the bits with a meter, do your readings agree with what is reported?
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baxter avatar image baxter nickdb ♦♦ commented ·

Yes, just verified the connections again with thermal and voltage meter having some loade (around 5000W). Connections are cold and voltage reading between the multis is equal (no voltage difference between the multis) and just 0.01V less as measured at the busbar (direct at the Lynx) and by the BMS . Between busbar and Multi there is a NH00 fuse, which will cause this slight voltage drop. So based on that, the wiring looks fine.

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nickdb avatar image nickdb ♦♦ baxter commented ·
I once had an unexpained voltage loss but I could measure it a meter. It seemed to be lousy terminals or something internal to the battery connection, wiring was fine. It went away when I changed battery. One other was a bad fuse holder, but that eventually just melted.
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baxter avatar image baxter nickdb ♦♦ commented ·
Thx I will keep an eye on it. It occured just once in 8month. If it happens again I have to dive deeper.
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nickdb avatar image nickdb ♦♦ baxter commented ·
Sometimes weird stuff just happens and isn’t seen again..
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Alexandra avatar image
Alexandra answered ·

@Baxter

If the cables(too small)/resistance (too high) between the batteries and components.

Loose connections cause voltage drop, that could be the explanation.

The drop is highest to the inverter as it is the main curret draw.

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baxter avatar image
baxter answered ·

I will check again the connections at the Multiplus (cables are fine), did that just a few days ago with my thermal but you never know.


What I am wondering, the high current was produced by the dishwasher, which has two high current periods during the cleaning programme, marked as A and B


1674402207880.png


During A, inverter power limit was active (1000W) which can be seen by the grid. During A the alarm was triggered (not even in the current peak). During B no alarm at all, even voltage readings between all devices AND inverter power limit was not active?

So if it a loose connection (which I will check anyway) it should be seen every time, right? But could the ESS limit inverter power cause a wrong voltage reading or some other side effects with peaking loads?


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nickdb avatar image nickdb ♦♦ commented ·
No. Having used inverter limits a lot, all it does is pull from grid, it won’t cause disagreements in voltage readings.
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Alexandra avatar image Alexandra ♦ nickdb ♦♦ commented ·
Other possible cause is the bms disconnecting and reconnecting, but it measures its own voltage before the disconnect, so will measure fine on itself, but the rest of the system notices.
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baxter avatar image baxter Alexandra ♦ commented ·

Nothing in the BMS Logs. If the BMS disconnects I would see 0W which is not the case

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baxter avatar image
baxter answered ·

UPDATE

This night the alarm raised again, but under complete different circumstances.

The system was in ESS #1 Low SoC, so the battery was idle, no charging or discharging going on.

Without any reason, the Ve.Bus Voltage is dropping down an recovering after a few seconds. VOltage measured by BMS is stable.


1674545449215.png



1674545495047.png


Based on that, for me the issue is not hardware (cabling and so on) related but a software bug.


@mvader (Victron Energy) Is this known? Maybe you have any advice?


Thanks in advance!

Br

Mike


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1674545495047.png (19.4 KiB)
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nickdb avatar image nickdb ♦♦ commented ·
There aren't any issues like this that we have been informed about, it does tend to be installation related, and unfortunately like all DIY batteries you aren't able to escalate for any support, you would need to direct your queries to REC. There are plenty of people using that BMS so there may be an existing topic hiding away in the depths of the forum which might help you or with a fellow-user who you could tag.

Complex issues are tough to triage online and this is the point we would usually encourage the member to try the official channel for support, something in your case which isn't an option.

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baxter avatar image baxter nickdb ♦♦ commented ·

You are assuming that is a REC issue, which is in my opinion not. Why should the voltage drop for just a few seconds in an idle system? The Ve.Bus is measuring the correct value just 20seconds later. For me, that is independ of the BMS or battery. How would the BMS could cause such a voltage drop?

I will connect know a meter to the Lynx for longterm recording of the voltage. So we have an independent measurment device. Let`s see ...

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Alexandra avatar image Alexandra ♦ baxter commented ·
@Baxter

I have seen systems that have this issue and have seen the cause.

The bms reads its voltage usually before the relay or disconnect system. Sometimes a battery unlatches or disconnects (cause not always known) so it will see the voltage as good, but the rest of the system has its voltage disconnected.

It is less noticeable during the day if solar is producing and voltage voltage is passing over the ve bus from production.

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baxter avatar image baxter Alexandra ♦ commented ·
ooh I see your point, very good approach. I will investigate that further, thx a lot!
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