BSL Stubby and Multiplus-II: BMS bug(s) on 1.07 firmware

This is also what bothers me - there’s no clear indication of why the BMS decided to take such a drastic step as to completely disconnect itself. At the end of the day I’m just worried I have a faulty / damaged cell / cells.

This is a valid point - my logging is set to every 1 minute (which is the most frequent that it can be set to). A lot can happen in one minute. I don’t know if it just logs a data point every one minute, or if it averages out the data for the last minute and logs that. If it just takes a data point every one minute it’d be very easy to miss things like short current spikes, voltage drops, etc.

I have done some searching around and can’t find a way to let the Cerbo perform high frequency logging to a SD card or something like that. That would have been a very useful feature to debug issues like this.

@anrich Quick question from the side, At what volts are you charging the unit at?

It was configured according to the BSL manual. Does something look wrong?

On the MPPT the voltages are the same (I believe the Cerbo communicates the configuration in VE configure to it?), but the charge current is 70A.

I spotted something in the manual. it states 51.2v 16Series so it gives you 3.2v where some sells are advertise at 3.25v that gives you 52v. But we all know lifepo4 can store more then that.

I see you only got 0.20v between bulk/absorption and Float. I think the BMS is straggling to get the sells stable because there can be sell that keeps hitting the safety at at say 3.65v or 3.60v

You can do a test for a few days if you want to, Pull back on your bulk charge to 54.6v and say your float to 54 or 53.6v. You can drop on the Amps to do a test to. Say to 40Amps.

It al comes down to fine tuning the setup. I’m running a DIY setup and have found that this volts charging my units work really good and they about 0.002 and 0.005 cell difference.

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Again, from what I’ve seen, a cell reaches a pre-set voltage, the SOC is set accordingly.

I would speak to your supplier. Ask them to test the voltage of each cell, also check BMS settings.

OR, use Gman’s settings, as they have helped me in the past, that the BMS tries to fix the cells itself.

@anrich , there is another possibility. Something that’s actually happened to a customer with one of the other locally made batteries (I think it was a FreedomWon). The battery was accidentally programmed to think it’s capacity is larger than it really was.

The effect was that on discharge, when it got to around 40%… the SOC would suddenly drop to zero. And similarly, on recharge, it would get up to a about 60%, and then jump to 100%.

I’m not convinced that this is a incorrect capacity issue. It looks pretty much like it jumps to 98% (ish) when the highest cell reaches 3.4V, and to 100% shortly afterwards.

The BSLBATT battery has a charge voltage of 54.5V. It’s meant to be full at 3.4V per cell, although it can take way more than that. In testing it was pushed to over 58V before the BMS intervened…

It really looks like a BMS issue to me.

Thanks for the advice, much appreciated. I’ll do some more reading and consider lowering the charge voltages (and amperage) for a week or two to help the BMS along with balancing.

This makes intuitive sense if the BMS thinks there’s more capacity than there is. I haven’t seen this behaviour in my case, the battery seems to happily discharge to 30%.

So far everything is pointing to it being either a BMS issue, or severely unbalanced cells. I’m going to follow up with the supplier regarding the apparent BMS bug in firmware 1.07, hopefully they have some feedback.

Looking at your charts again… that is a possibility. You do see a high cell at 3.55V while the rest is probably around 3.35V. I’ve seen that many times with new batteries (the worst was with a BYD).

The best way to balance a battery like this is to cycle it numerous times between 80% and 100% SOC, preferably charging the battery as slowly as possible. You want it to spend as much as time possible in a condition where 1) one cell is higher than the rest, and 2) there is a small amount of current flowing through the battery.

I think the resetting to 100% SoC is normal. Many BMSes do that… mostly because in terms of support, for the manufacturer, it is better to jump to 100% than to field support questions asking why the battery is stuck at some SOC < 100%. You get less questions that way :slight_smile:

The battery once again gave a low voltage alarm while about 80% SoC, causing the inverter to restart. There is clearly something wrong. My best guess based on the feedback here and from the Victron Community forum is a serious BMS firmware bug or design flaw. It could also be severely unbalanced cells, or a faulty cell. BSL is apparently working on improving the firmware (they’re aware something is wrong).

I’m working with the installer to have the battery exchanged for something that actually works reliably.

I would not recommend buying a BSL stubby (8.2kWh) for the time being, at least not until they sort the BMS issues out.

Note: the installer mentioned that my installation was the first one they did with the stubby, and they only got access to them from the supplier a week before I bought the system. My gut tells me thr firmware is half-baked.

That would be my guess… except it is probably not reflected in the communicated metrics?

Nothing seems wrong in the communicated metrics. In fact, everything seems very normal, current draw on the battery was only 3.72A, battery voltage reported as 53.08V, minimum and maximum cell voltage 3.31V.

I went to the inverter immediately when it happened, and this time I caught it while the alarm was being raised. What is interesting is that the “Low battery” LED was illuminated on the Victron (hence the restart), but the battery itself wasn’t showing any issues. The SoC LEDs were all illuminated (above 80%), and the “Run” LED was flickering as normal, not a single illumination of the “Alarm” LED on the battery itself.

Also, if I look at the alarm notifications on the Cerbo, the voltages don’t seem low enough to warrant an alarm. I think these voltages are not actually communicated by the BMS as the “fault voltage”, but rather just a point in time voltage when the BMS reports the low voltage. And this doesn’t account for a single cell being faulty of course.

Screenshot from 2021-11-26 06-47-16

I was considering buying a RS232 to USB cable and using PBMS Tools to have a look at the pack voltages and alarm log on the battery itself (not communicated via CAN to the Victron), but I’d rather let the supplier sort this out (as they rightly should).


Is your dynamic cut-off correctly configured? Cause that could also cause that.

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The dynamic cut-off is configured as per the BSL manual. Both times the low voltage alarm triggered the system had very low current being draw (~500W).


The supplier indicated that a new firmware is currently being tested (which should take about a week). However, they still haven’t indicated that the 1.07 firmware can cause incorrect low voltage alarms.

OK that looks good. It’s just odd that you get a low voltage alarm at 52V. But it could also be because of a disconnecting BMS (which fits the other evidence better). By the time the alarm is raised visually, it will usually grab a sample of the voltage… and the voltage it shows on the screen may not be the one that triggered the alarm. It might have bounced back by then.