External Load switching Off and On

[Harold]

Hello Dacian and all.

When my battery is ending to charge, my external load (DC/AC inverter) is switching Off and On repeatedly.

EXTIO3 is set as type 4, (15%) and works as intended when battery is low SoC.

On the video provided, you can hear the fan of the inverter switching On and Off. Notice that when EXTIO3 is switching, the inverter needs a delay to react, so the command on EXTIO3 and the noise of the fan of the inverter are not synchronized.

Another video with cells' voltages

What is going on ? This is very ennoying.

Can you catch something with the video proveded.

[Dacian Todea (electrodacus)]

Harold,

Yes I can see what the problem is in the second video.   You have a bad connection at the cell monitoring wire connected between cell 4+ and 5- so the green sense wire.

You need to fix that connection and problem will go away. Is best if you solder those wires as crimping is not very reliable long term.

[Dacian Todea (electrodacus)]

Here is a screenshot from your video to better see the issue.

Cells 2, 4 and 6 are balanced and you can see cell 2 has very low voltage drop while cell 4 has a huge voltage drop due to bad connection while cell6 is not as good of a connection as at cell 2 but not bad enough to create problems.

That excessive voltage drop on the green wire will show up as subtracted from cell 4 and added to cell 5. This adding to cell 5 results in Over Voltage Lock limit being exceeded thus both charge and loads are disconnected since this is considered an error (not normal operation).

[Harold]

Hello Dacian and all.

Below are some picture after I tried fix supposed bad sense wire around cells 4 and 5. I think it's better now.

But inspecting the SBMS120 behavior I noticed some strange things.

1. I noticed cell 8 is often highest when charging, and always lowest when discharging. Thus, Does it mean of a potentially not so good connection fot power connectors on cell #8 or maybe higher internal resistance on that cell ?
2. I'm also surprised that balancing seems to happen on discharge despite the SBMS120 is configured :  "Balance on Discharge = 0". It seems to happen only on fairly high load .

1. 

[Dacian Todea (electrodacus)]

I can't say for sure if the problem with bad connection is fixed since cell balancing in your example happens for 7 cells at the same time so even if there is a bad connection it will not be seen since load on all cell sense cables is the same.

Regarding cell balancing on the SBMS120 and SBMS40 is based on ISL94203 current sensor and that can only measure the current flowing trough the Batt+ power connector and not the external current shunt that is only seen by the microcontroller.

As for cell 8 check the power connections but could also be that cell 8 has slightly higher internal resistance. Considering the 80A load vs 20A charge current it dose not seem like an extreme voltage drop.