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Chris Bray avatar image
Chris Bray asked

How to use BMV-712 Smart for internal SOC alerts/shutdowns etc rather than BYD B-Box reading?

I have a bank of 4 lithium batteries installed in a remote site, off-grid, 100% solar, no generator backup, about 200Ah capacity. SOC readings from BYD B-Box were hopelessly unreliable apparently because my typical background draw was 'too low' to be noticed by the BYD B-Box and was thus ignored, so SOC appeared to merrily sit at about 100-99% for days and days despite heavy cloud-cover (while system was in fact slowly discharging) until suddenly voltage reading on the battery started to drop (for lithium, when voltage drops you're almost dead), at which point the BYD B-Box would suddenly realise and over the course of just a few hours crash from like 99% down to 30% then 20% etc, causing panic at our end. So, to fix this, we bought and installed a dedicated current shunt 'BMV-712 Smart' and set this to be the battery level calculator... it gave much more realistic values, showing gradual, minimal decay each night, followed by solar recharge each morning, perfect. These correct/realistic values show up as SOC on the Colour control, and the online portal display etc. Great. But...


The problem is that it seems that the system's internal calculations that it uses for email alerts and red 'alerts' on the online portal etc are all somehow still using the old BYD B-Box reading, and worst, that the system's internal shut-off when it reaches almost empty, is also using this incorrect BYD B-Box value, causing the system to shut-off when there is in fact heaps of power left in the battery. Just today BMV-712 Smart showed 100% SOC (which I feel was perhaps accurate) while at the same time BYD B-Box reading was just 9% SOC, both showing a healthy 52V. A few hrs later when the sun went down, the system shut down because BYD B-Box reading thought it had run out of power!


How do I get the system to ignore the BYD B-Box readings and only use the BMV-712 Smart SOC readings? I have selected the BMV-712 Smart as the battery monitor but that only seems to use BMV-712 Smart for the 'displayed' SOC, while internally for alerts/shutdowns etc it is still using the BYD B-Box values!!?!


Can I just unplug the BYD cable from the batteries going to the Colour Control display? Or?


Thanks very much anyone who has any tips here! I'd really appreciate it,


Chris

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2 Answers
Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

The system hast to use the BYD readings for shutdowns.
If you disconnect the CAN.Bus cable the BYD BMU will shut down the batteries after a few minutes.

Maybe @Guy Stewart (Victron Community Manager) or @Daniël Boekel (Victron Energy Staff) can say something about what to do.

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Chris Bray avatar image Chris Bray commented ·

It seems a 'known issue' of BYD batteries (approved by Victron for use in their systems) is "Inaccurate State of Charge (SoC) Readings: https://www.victronenergy.com/live/battery_compatibility:byd_b-box

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Each BYD battery module has it’s own BMS. This BMS reports it’s battery information to the BMU which is connected to the GX device and reported to the user and the system.

Each BYD module BMS has a threshold current of 1A (~50W) before it begins to report. For multiple modules, this is additive. So 4 modules could supply a 180W load and report it as 0 W.

This leads to the state of charge reported by the BMU to not accurately match the actual state of charge of the battery. This limitation also applies to charge currents as well as loads.

The reported State of Charge will self correct when it reaches the calibration voltage point (55.2v ~ 100% full) or near 10% empty, and this can appear as sudden jumps in the VRM from very little load or charge. (IE TOTALLY UNRELIABLE EXCEPT WHEN TOTALLY FULL OR ALMOST EMPTY - GREAT)

This has no impact on actual usable capacity of battery, only on the how it is reported to the user and other systems (such as alarms or triggers). (GREAT SO 'no impact', except for alarms, triggers and reported SOC… great… thats exactly what is the source of all my problems)

Work-arounds

If precise state of charge is important, it is possible to install a Victron BMV shunt, and program the GX device to use it for the displayed State of Charge. (WHICH IS WHAT I DID, it just seems that there is no way to use this more accurate SOC for anything other than display purposes?!)

You should have alarms and auto-start generators programmed to operate on system voltage as well as state of charge. (THIS!!!! HOW DO I DO THIS!?!?! I feel my system is just basing it’s freak-out alerts and shutdowns etc on the SOC and ignoring voltage? How can we make sure all alarms and shutdowns are using voltage (So dumb that it can’t be set to use the BMV SOC though!?! But still, i’d be happy if the 'displayed value' showed me BMV, and that it only alarmed or shutdown if voltage got down to desperate 10% levels…?)

System voltage is always reported accurately, and action should be taken once batteries are below 50V for any length of time.

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But I worry that it also sounds like the batteries themselves have an inbuilt self-preservation shutdown state… which worries me, what is that based on, the hopeless BMS SOC or BMS voltage? If the batteries themselves use their own useless BMS SOC for internal self-preservation shutdown levels then I'm pretty screwed, right?


Thanks very much for anyones help!


Bit desperate - I have people staying at my lodge, with loads of charge in the battery, and the system has suddenly shut down because it thinks it's nearly empty... I've seen this coming for ages, and it's happened before, but I can't ignore it anymore...


Thanks

Chris


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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @Swell Lodge

52 volt is far from fully charged, so the BMV settings aren't correct. at no load, I'd say 52 volt is almost empty... 50 volt is 'running on fumes'


I'd try to get the BMV to read the correct SOC first.


Also I'd ask the installer to get things to work correctly


alarms can be configured per device, so you can choose what battery monitor you want to alert you.

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Chris Bray avatar image Chris Bray commented ·

Thanks very much for this insight re the BMV not being accurate either. I'll go through and triple check the BMV settings... and re-charge to 100% with generator, to force a BYD reset too... wish I knew what was going on. Thanks!

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