SMBS0 going into battery overvoltage protection.

[Greg Norman]

 I have a 24V system controlled by Electrodacus SMBS0 and two DSSR20. First, dssr20 is connected to two panels in series, and the other is connected to two solar panels in series. The problem that I'm having is sometimes solar is so strong that it over shoots one or more cells, and the system shuts down for protection. The charging current is too high and the battery can't absorb it quickly enough. In other words, a battery overvoltage protection (OVP) event, where the SBMS0 stops charging to protect the battery. To restart the system, I must manually turn on the inverter and bleed the battery with some load. I think the issue is that this is a camper and I'm not using system at all times. I may be driving for eight hours or not using for some time. But when I do use it I need to charge batteries as quickly as I can. Can I have something wired up, i.e., heating, so that it turns on and bleeds the system down before it shuts down?  My sbms0 did not come with diversion.  

[Dacian Todea (electrodacus)]

What sort of PV panels do you have ? usually the panels will be in parallel not series but maybe you have small 32 or 36 cell panels.

What sort of flag stops charging ? If is just Over Voltage (OV) then that is normal operation as battery is full charging will need to stop.

But if it is Over Voltage Lock (OVLK) then there is an issue with either your settings if you did not used the default ones or with your wiring.

I will need a bit more details maybe a photo of the first monitoring screen when it happens.

If your battery is at least 100Ah I can not see how 40A charging will be to much.

[Greg Norman]

Sorry I should have proof read, correction I have 2 60 cell 235w panels wired in parallel connect to one DSSR20 and 2 30 cell 100w panels connected in series connect to another DSSR20. It is a Over Voltage Lock (OVLK). 

We went over this in past. I check all torques and wire conductivity. All good. I also checked settings as you instructed. Let me provide what settings I currently have, please let me know if this is what you need. And as always thanks for much for your help!

[Dacian Todea (electrodacus)]

Thanks for the details.

It seems that over voltage (OV) and over voltage lock (OVLK) are set

Since OV is set at 3.55V and OVLK is set at 3.75V that allows for up  to a 200mV and typical voltage drop due to cell balancing with the included cell balancing wires should not not normally be above 80 or 90mV thus it should not trigger OVLK but it is not the case in your installation for some reason.

Maybe you extended the cell balancing wires. Getting to 200mV when 80mV is expected is quite a bit of difference. Of course you can either decrease the OV to say 3.50V or increase the OVLK to 3.80V but I will just investigate the cell sense wires especially wire 9 and wire 10 in this case that are at cell 8 negative and positive terminals.

[Greg Norman]

Storming today so will need to wait until tomorrow to check sense wires.  Going to check torque and connectivity of wires. I did this in the past but doesnt hurt to recheck! Question, I've been looking into possibility of installing one of these:  8S 24V 25.6V 1.2A Li-ion Lipo Lifepo4 LFP Battery Active Equalizer BMS Balancer Inductive Balance Lithium Battery Energy Transfer Board (8S) from QINIZX. It cost about $15 on Amazon. What's your thought on helping improving balancing?

[Dacian Todea (electrodacus)]

Your battery is perfectly balanced. An external cell balancer will just be useless at best and interfere with SBMS0 internal cell balancer.

Your issue with OVLK is related to cell sense wires that have high resistance over 2x the expected resistance. Typical with included wires voltage drop due to cell balancing should be 80mV on both wires combined but it is more than double as it exceeds 200mV and that triggers the OVLK.

If you look at the SBMS0 while cell balancing is active you will be able to see if the problem is wire 9 or wire 10 as each wire should have about 40mV drop so say cell balancing is only activated on cell 8 then when enabled (3 seconds ON and 3 seconds OFF) during those 3 seconds while cell balancing is ON you will see cell 8 voltage decrease normally will be to decrease by 80mV and cell 7 increase by 40mV but since cell 8 decreases by about 200mV if you see cell 7 increase by say 150mV then you know is wire 9 that is the problem but if cell 7 only increases by around 40mV then you know that wire 10 is the problem (that will be the wire connected together with wire 11 and 12).

[Greg Norman]

I replaced the ring terminals on sense wires 9 and 10, torquing them according to cell specifications. I reset OV 3.55. Have not experienced OVLK since then. Cell 8 is the cell that consistently reaches OV before any other cell. I have attached videos of charging at 26.66V and another right as cell 8 triggers OV

[Dacian Todea (electrodacus)]

This seems normal operation. I see 3.647V peak witch means that while cell 8 is balanced voltage drops by around 3.647 - 3.550 = 97mV split on the two cell balance wires (wire 9 and 10) so about 48mV on each witch is within the expected range for the included cell balancing cable.

And yes cell 8 is the first to charge but that is not relevant as one of the cells will always be the first to fully charge and the others are just a few seconds away from that.

3.38V for the rest of the cells is 99.8 or 99.9% SOC.

[Dustin Dudley]

My system is a 24v (4p 8s) battery and I think I am seeing something similar (attached photo). For me, bank #8 seems to be the bank of cells that has been triggering CFET, EoC and OV (for the last few weeks anyway). But given the explanation above, I think it is operating as designed. Dacian, could you please take a look and let me know if you agree that the cells seem correctly balanced given the information in the photo? Please and thankyou!!

[Dacian Todea (electrodacus)]

Based on color coding is cell 1 in your case that is fully charged first.

Not sure if that log graph is at one second interval but it seems peak gets above 3.65V in some cases so not sure if you have the default 3.55V settings or you set it at 3.6V

In any case based on that many full charge cycles in a very short period of time you will likely want to use Max SOC  limit set at say 95% SOC (in DMPPT settings) that way the battery will not bee kept all day at the top of the charge cycles and it will stay within 93 to 95% during the day after the first full charge of the day.

[Greg Norman]

Thanks again so much for your expertese. I learn a lot from your explainations! I've been considering adding a solar dump load. I wish that I had ordered DSSR20 with diversion, but I didn't! I'm thinking about using Voltage Monitor Module, A30-U1 DC 0-99.9V Delay Relay and  DC to DC solid state relay plus DC load

[Dacian Todea (electrodacus)]

Currently since is summer I only use about 10% of available solar energy with 90% remaining unused.

My system was designed for winter when the day is shorter and there are more cloudy days and then I use all the available energy as I heat my house.

While it sounds strange it is cheaper to leave excess solar energy unused that to try to find uses just so you can say you used everything available.

[Greg Norman]

Make sense for your needs, be up north, and heating needs. My thoughts are that I do want/like hot water in van, and instead of turning on the inverter and turning on ac hot water heater to heat water, why not use excess solar to warm/heat up water

[Dacian Todea (electrodacus)]

If you heat the water during the day then energy for that comes directly from solar.  Yes maybe the inverter loss is about 10% but it is a much simple setup and you do not need a DC heating element and all the diversion components for not much energy.

Keep in mind that water heater is also a limited energy storage device. 

You only have about 600W of solar and depending on your location time of year and panel orientation you may have available up to 3kWh per day.

Your battery capacity is double of what your solar PV can produce in a sunny day so there is no issue using all the available solar energy if you want.

Not sure how large is your water tank but say is 20 liters and cold water is at 20C and your limit is set at 60C then a 40C delta is required to heat that water.

20 * 40 * 1.16 = 928Wh so say round that up to 1kWh not even 20% of your battery capacity and a third of what is available from solar PV in a sunny day.

There is nothing you save by not heating water trough inverter and you will still have unused solar energy even if you divert that to water heater. The diversion will also need to be stopped by the thermostat when water gets to set temperature.