Cell differences during solar charging

[pdxbill]

I just completed my diy lifepo battery install in my small rv. It consists of a 12 V 280 ah 4 cell battery with JBD 120amp BMS. It's being charged by a victron SCC(configured for lifepo charging). I'm sharing some photos of the cell reading on the first day that it reached full charge after assembly. Prior to assembly, I top balanced the cells.

I installed the battery this week and I noticed that 3 or 4 cells had maxed out at just above 3.7XX volts and the BMS was shutting down on the first day it reached capacity(I had drawn down about 40 ah testing it out). Everything seems to work fine.

In trying to understand what is happening, I think that those 3 are "closer to full" and that's why they jump and the BMS protects kicks in. I noticed that when there is a draw on the battery, the cells differential closes, and the next morning they were all within 0.019 volts of each other.

My questions: Is this harmful to the battery? Is there a setting on my BMS that should be adjusted to prevent this? This will not be a high usage battery in my RV, and will often sit at or near full except when in use. If anyone has suggestions, I'd love to hear them.

If it's a minor issue and can be left alone that would be great to know as well.

 

[snoobler]

Your question:

Yes. Repeated and frequent trips above 3.65V are not healthy for the cells; however, the small incursion above are unlikely to cause measurable damage. Repeated and frequent BMS trips are not good for the BMS itself. Your equipment would operate within the BMS limits, and the BMS should only trip when something goes out of whack.

Make 1000% certain that all connections are tight. Bolts/nuts, terminal crimps - etc.

Confirm the BMS values are the same as a separate voltmeter.

If there is not a connection issue, and there is correlation with a separate meter, then the imbalance is legit, and it looks like cell 3 is lagging just a tiny bit. They are likely a fraction of a % out of balance with each other.

If you let the cells sit after top balancing, they can easily go out of balance.

Ensure BMS balancing is set for only during charge and only above 3.4V. Full time balancing can eventually undo a top balance.

You can get your batteries 95% charged at absorption as low as 13.6V, but it's slow. 13.8 is a little faster and will get you to 98%+. If you are trying to charge to 3.65Vx4 = 14.6, you're going to trip the BMS even with a very well balanced battery. Pull it back to at least 14.4. The aforementioned 13.8V absorption and a 13.6V float will serve to prolong cell life as the filling at the top is at lower current and is less stressful to the cells.

Recommend you lower your absorption voltage by 0.1V until it stops tripping. Assess if the performance is adequate. If so, leave it there and enjoy.

Alternatively, you can simply hit the single cell with your 10A supply until it hits 3.65V, and that should get you damn close. Will likely only take a few minutes.

 

[pdxbill]

Your question:

Yes. Repeated and frequent trips above 3.65V are not healthy to the cells; however, the small incursion above are unlikely to cause measurable damage. Repeated and frequent BMS trips are not good for the BMS itself. Your equipment would operate within the BMS limits, and the BMS should only trip when something goes out of whack.

Make 1000% certain that all connections are tight. Bolts/nuts, terminal crimps - etc.

Confirm the BMS values are the same as a separate voltmeter.

If there is not a connection issue, and there is correlation with a separate meter, then the imbalance is legit, and it looks like cell 3 is lagging just a tiny bit. They are likely a fraction of a % out of balance with each other.

If you let the cells sit after top balancing, they can easily go out of balance.

Ensure BMS balancing is set for only during charge and only above 3.4V. Full time balancing can eventually undo a top balance.

You can get your batteries 95% charged at absorption as low as 13.6V, but it's slow. 13.8 is a little faster and will get you to 98%+. If you are trying to charge to 3.65Vx4 = 14.6, you're going to trip the BMS even with a very well balanced battery. Pull it back to at least 14.4. The aforementioned 13.8V absorption and a 13.6V float will serve to prolong cell life as the filling at the top is at lower current and is less stressful to the cells.

Recommend you lower your absorption voltage by 0.1V until it stops tripping. Assess if the performance is adequate. If so, leave it there and enjoy.

Alternatively, you can simply hit the single cell with your 10A supply until it hits 3.65V, and that should get you damn close. Will likely only take a few minutes.

Snoobler, thank you for the detailed reply, as a newcomer this is extremely helpful advice.

 

[snoobler]

I just noticed you configured the SCC for LFP charging. That puts absorption at 14.2 and float at 13.5. Those are acceptable, but if you need to tweak them, you'll have to go to user defined.

 

[mikefitz]

This will not be a high usage battery in my RV, and will often sit at or near full

This may not be the the best for long service life. Some research papers suggest that having the cells at maximum state of charge continually will accelerate ageing. Having solar charging to full each day perhaps with a long absorption time and high float voltage may no be ideal when the RV is not in use.

Mike

 

[Wibla]

Just touching on this:

This will not be a high usage battery in my RV, and will often sit at or near full except when in use. If anyone has suggestions, I'd love to hear them.

1. Put in the effort to top-balance the battery properly, this will extend the life of your battery.
2. Set the charge controller to 13.5V absorb and 13.35V float if you're going to let it sit for a while, you'll still be sitting at >80% SOC, but you're not prematurely aging the batteries by leaving them at 100% SOC and at high cell voltages for hours every day.

 

[snoobler]

This may not be the the best for long service life. Some research papers suggest that having the cells at maximum state of charge continually will accelerate ageing. Having solar charging to full each day perhaps with a long absorption time and high float voltage may no be ideal when the RV is not in use.

Mike

Good catch.

I completely missed the "often sit" bit.

Storing LFP at elevated SoC can damage them, particularly at higher temperatures.

Any time it's going into storage for more than a couple weeks, I would run it down to 70-80% or lower and isolate the battery and disconnect the BMS sensing wire. It will retain it's charge for quite some time.