Low voltage lock issue

[Patrick Baghus]

Hi all,

So i have the following issue on my sailboat.

I'm at the drydock right now, not really using any power, and always hooked on gridd when i am working on board.

Somehow my 4s2p battery showed a little under 9 Volts with still 20% SOC on the SBMS0 and the following cell voltages when i came on the boat the other day:

1 : 3V

2: 0,9V

3 2,8V

4: 1.9V

so obviously low voltage lock (2.5V) kicked in. I lowered the following parameters to:

Under voltage: 0.8

Under voltage recovery: 0.85

Under voltage lock: 0.8

Low voltage chg: 0.8

Cell balance minimum: 0.8

sleep voltage: 0.8

My thoughts were that with this settings the SBMS0 would start charging with my victron multiplus (at a low amperage) again, but the low voltage lock flag stays up.

I have used the system for almost 4 years now as a liveaboard, without any real problems. (no solar power yet, so i stay connect to the grid, charging once a day) voltages mostly staying between 3.3V - 3.1V, but couple weeks a year we go sailing and it goes the way down to the default low voltage disconnect (it is always the 2nd cell that triggers lvd and hvd) 

I have not been using the system a lot lately. boat have been on the hard last couple of months and i have always unplugged gridpower when leaving the boat at the end of the day. My guess is the gridpower in the harbour has been down multiple times, so i have been using power tools on the inverter without noticing. probably the internal temperature of the sbms has been below 5degree C, so it would not start the charge cycle (i dont have an external temp. sens)

I knew the 2nd cell was the weak one, so i already lowered the capacity on the smsb from 560ah to 520ah, but this issue came as a total suprise to me, because i have not been using the system a lot lately. Maybe because it charged when temperatures were to low? i dont know for sure.

I know there are damaged cells that need replaced, and the entirety should be tested and balanced, but for now I want to charge the cells untill HVD kicks in, so i can at least use the battery again for small 12V loads and as extra with the inverter when gridpower is not sufficient for using power tools. I will test and make a new batterypack later.

So, my 2 questions are:

- Any idea what could have caused the 2nd and 4th cell to get this extremely low?

- how do i disable the low voltage lock and get the smbs0 to charge the battery again? (i do not have a manual charger unfortunately)

Thanks,

Patrick

[sailingharry]

I also have a sailboat.  I have fears of this situation (I'm an engineer, I like to dream up problems and then worry about them!).  There are two solutions to charging once SBMS0 locks out the battery.

On my boat, my contactors are the Egis contactor, very similar to the BlueSeas device.  They have a 3-position switch.  In addition to a "normal" operating mode, there is a mechanical "ON" and "OFF."  This allows me to override all controls and close the contactor (it also provides a very safe "OFF" mode).

If you don't have a contactor with that option, a simple jumper cable (car jumpers, a heavy alligator test-lead, or even a piece of #14 or #12 wire manually held across the contactor) will allow charging to occur.  I suspect that unless the low cells are badly damaged, even a 15A charge for 5 minutes will get volts up enough to allow the SMBS0 to kick in.  (I have an 8' chunk of #16 lamp cord with fairly large alligator clips on the ends as test jumpers -- much beefier than the normal small scale test leads, and very useful for tasks such as this)

Harry

[Dacian Todea (electrodacus)]

Patrick,

The way SBMS0 controls the charger is likely trough EXT IO4 (assuming you used that and not a different EXT IOx).

When charging is disabled the EXT IO4 is just open circuit while when it is enabled is close circuit and maybe 5mA or less will flow trough it.

All you need to do to manually enable charging for a few minutes for cell voltages to recover above 3V is to bypass the EXT IO4  so short the two wires EXT IO4+ and EXT IO- (probably easiest where they are connected at the charger) So disconnect the two EXT IO4 wires from your charger and connect a shorting wire to the charge to have it enabled. Then just manually monitor the charging and when cells get above 3V (should happen within minutes) you can reconnect the EXT IO4.

But also please use the default settings (after you manually force charged the battery as mentioned above) so go to Parameter settings select Battery type 1 (do so even if it is already selected as type 1 as this will actually restore the settings to default ones) set 4s and set capacity to 500Ah then push the Store Parameter button then power cycle the SBMS0 by removing the 12pin cell monitoring connector and reinserting.

Based on what you say that cell 3 gets to HVD I suspect that you have a bad connection at one of the terminals of cell 2 so high resistance connection.

[Patrick Baghus]

Thank you for the explanation.

So just to be sure of what you are saying:

I should disconnect the ext IO4 cables.

I shoud then shortcircuit the connection on the multiplus? (i think that is the Temp connection?) So I do not short the EXT IO4 on the SBMS0? After the cells reach 3V i reconnect EXT IO4 to the sbms0.

Great tip. i will check the connections for higher resistance, but i do suspect a bad cell. Here's why:

It is always cell 2 that triggers LVD and HVD. I made a rookie error when assembling the pack for the first time, and accidentely shorted this particular cell. When i found out that it was always this cell that triggered both LVD and HVD, I have marked each individual cell, dissassembled the pack, top balanced the cells, shuffled the cells and reassembled the pack a while ago. Again it is the same cell that triggers the LVD and HVD, making me believe it is a bad cell.

About the settings: I have always used the default settings. Just changed them when trying to reset the low voltage lock.

But when changing settings back to default, i should also remove and reinsert the 12pinn connector to powercycle the SBMS?

Op dinsdag 1 april 2025 om 19:19:13 UTC+2 schreef electr...@gmail.com:

[Dacian Todea (electrodacus)]

Yes that is correct you short the Temp connection if that is setup as enable charger.

A bad cell connection will look the same as a bad cell and a bad cell connection is much more likely. If you have a higher contact resistance at one of the cell terminals that will be in series with the cell internal resistance and so it will appear like a cell with high internal resistance. So in this case when pushing current in to the cell the measured cell voltage will include the voltage drop on the bad connection so cell voltage will be higher similarly while discharging the cell voltage will appear lower.

Cell 2 will be in the middle of a 4s pack so maybe you have a longer connection between cell 2 and 3 depending on how you setup your pack.

Yes Parameter settings will not take effect unless you power cycle the SBMS0 as this is a protection mechanism. Normally those parameter settings are only set once when installing the battery.

Those parameter settings are saved in an EEPROM inside the ISL94203 (specialized BMS IC) and during power up the ISL94203 will copy those parameters from EEPROM to a working RAM memory and only use the parameters it has in RAM. Thus when you push the Store Parameters button you write those to EEPROM and they will only become active after a power cycle when ISL94203 copy those to RAM.