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Help understanding aberrant LFP cell voltages at close to full SOC

I do not mean to be rude or harsh, but there some a point where Nit Picking is pointless, especially when within the margin and trying to push beyond it. Honestly, you are NOT gaining anything by charging over 3.425Vpc / 27.4V.

As I already said;

I'm not trying to eek out every last watt of power, I just want a solid reliable system and a fairly detailed understanding of why it behaves as it does.

I do think that this inquiry in general still has both practical and educational value:
  1. If either of the prevalent hypotheses turn out to be true - bad connections or bad initial cell balancing - they could be important to resolve from a fire safety and a cell longevity perspective respectively.
  2. Most folks aren't going to have monitoring data like I have. Instead, they'll get tiny glimpses into similar behaviors with their multimeter and BMS. I think that it is useful to have some case studies where there is more detailed data that the system owner (me) and community have made sense of.

Brute statement.
IF you want the batteries to Settle to 3.400 when fully charged, do not exceed 3.510Vpc to full charge as LFP will ALWAYS DROP 1.0V average for settling within 1 hour. LFP Always Settles, no if's, and's or but's.

(I assume you mean 0.1V)

Past 3.450 your net gaining much of anything, other than quirks. At that level even with matched cells within 1.5milliohm difference will show their quirks and deviate. You can chase these issues for months and never get much further as your tickling the working edges. And you know what, even 30mv differential within a "working" pack is moot as that balances up after.

I think there must be a compromise between your statement and what I'm doing here. I take your point about diminishing returns beyond threshold X, but I'm also leery of just throwing my hands up, writing "here be dragons" on the map, and losing the opportunity to understand my system better. Particularly, where understanding my system better might involve uncovering real problems like bad connections/balance.

In pursuit of a solid reliable system, I'd like to be able to say "I've tested and understood how it works within spec up to threshold X so I'm going to operate up to 0.90X in normal usage."
 
I agree with your quest for understanding. Not all of the proposed diagnosis hang together with all the plots you have presented.
In particular you show periods where the voltages are tightly clustered during both discharge at -15 amps and charge at 20+ amps well into the knee of the curve. In order for this clustering to be true in both conditions then the differences in IR and contact resistance must be modest.
Is it possible that the SCC is just offsetting the inverter load at the top of the charge? If so, then the current thru the battery would be small. Under those conditions the cells could settle and/or the tiny balancing currents would be noticeable.

I’d still love to see a run with balancing off, and charge voltage down to about 3.45/cell. we need to see both voltages and currents at as many points in the chain as practicable.
 
In case I haven't emphasized it enough, I really appreciate the thought and insights everybody is sharing here! Thanks again!

In particular you show periods where the voltages are tightly clustered during both discharge at -15 amps and charge at 20+ amps well into the knee of the curve. In order for this clustering to be true in both conditions then the differences in IR and contact resistance must be modest.
Is it possible that the SCC is just offsetting the inverter load at the top of the charge? If so, then the current thru the battery would be small.

I assume you're referring to this graph;

1643386679621.png

That amperage data is coming from the Overkill BMS so it should only show actual charging/discharging of the battery, not current that passes directly from the SCC to loads in my system.
 
Hi @On_The_Road! I'd been meaning to come back and provide an update here...

What did you end up doing Symbioquine? Did you figure out the problem or just learn to live with it?

I've just been living with it. I think the conclusion that my cells aren't perfectly balanced is correct, but I'm not trying to eek every last watt out of my batteries so I just reduced my charging voltages such that I never trip my BMS' OVP and called it good.

I am curious what you are using to capture the data and graph it?

I have a Grafana/InfluxDB self-hosted with Docker on a machine on my local network.

Then I have some ESP8266 microcontrollers which push data into InfluxDB from my Epever SCC's. The hardware for that is roughly documented at the end of this post: https://symbioquine.net/2020-10-27-epever-esp-8266-controller/

The cell voltage data is captured via the bluetooth Overkill Solar (JBD) BMS modules using a Raspberry Pi. The code for that is based on the examples that can be found in this repository: https://github.com/tgalarneau/bms
 
Hi @On_The_Road! I'd been meaning to come back and provide an update here...



I've just been living with it. I think the conclusion that my cells aren't perfectly balanced is correct, but I'm not trying to eek every last watt out of my batteries so I just reduced my charging voltages such that I never trip my BMS' OVP and called it good.



I have a Grafana/InfluxDB self-hosted with Docker on a machine on my local network.

Then I have some ESP8266 microcontrollers which push data into InfluxDB from my Epever SCC's. The hardware for that is roughly documented at the end of this post: https://symbioquine.net/2020-10-27-epever-esp-8266-controller/

The cell voltage data is captured via the bluetooth Overkill Solar (JBD) BMS modules using a Raspberry Pi. The code for that is based on the examples that can be found in this repository: https://github.com/tgalarneau/bms

Thanks for the update and logging into.

What absorb voltage are you using to duck under the issue?
 
What absorb voltage are you using to duck under the issue?

I think I'm using 27.5 volts at the moment.

Here's a screenshot of the current behaviour. You can see there's still a pretty wide spread in cell voltages when the batteries approach full charge.

1666295642031.png
 
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