Battery startup behaviour

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B0bD

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Jul 16, 2021, 4:46:13 PMJul 16
to electrodacus
Finally got everything working properly, and had a bit of sun so that the battery charged up.
During the charging, at a rate up to 90 amps, ( btw the battery is 3P8S, total 840AH), the cell voltages kept close together; the display showed the BMS balancing doing its thing.
Then, as 100% is approached, cell 8 sort of accelerates and reaches the 3.55V charger cutoff. Over the last 10 minutes before the charger is shut off, the dV rises from less then 50 mV to 200.  10 minutes after the charger is shut off, the dV is back under 25mV. 
There has only been two full charges so far, so I am not sure whether or not to be concerned about this.
A snapshot of the data is below, with the highest voltage cell highlighted.
Up to the few minutes shown below, the highest voltage cell is more or less evenly distributed among the 8, on a minute interval.


Capture.JPG

I have measured the resistance and the dV from each busbar to the battery terminal, and they are pretty consistent, and low.  Prior to assembly, the cell IR's measured consistent within 0.03 milliohm.

I have the charge controller set at 29 volts, and it is under control of the SBMSO, so no absorb or float.

Has anyone seen a similar characteristic?

Dacian Todea

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Jul 16, 2021, 4:59:44 PMJul 16
to electrodacus
Yes all that is normal.  The up to 200mV cell delta is in big part due to cell balancing current so the delta is not real just what the SBMS0 can read while cell balancing is active due to voltage drop on cell monitoring wires.
The closer a cell is to full 100% charge the faster the voltage will increase so even less than 0.1% delta between cells will make so that one cell gets first to 3.55V in your case cell 8 with cell 7 fairly close behind.

B0bD

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Jul 16, 2021, 5:29:06 PMJul 16
to electrodacus
Thanks - that is helpful.
I am impressed at how well the balancing holds the voltages, even on this initial charge, and without going through a preassembly balancing of any sort.
Sort of makes all the3 discussion about high current active balancing a bit moot, at least in a low C rate stationary setup.

B0bD

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Jul 26, 2021, 4:11:56 PMJul 26
to electrodacus
Some follow-up - Is the following behaviour typical or something to be concerned about?
There have been several charge cycles, with consistent outcomes.
The data is at 10 second intervals.
Up to 99%SOC cell to cell voltage is close - within 25mv or so. 
About 5 minutes before reaching 100%, the dV starts to increase, with cell 8 ramping up faster than the rest and stopping the charging.
After charging stops, cell voltages start to fall, with cell 8 falling faster.
dV reduces to 25mv after about 30 minutes, and voltages stay close until the next charge cycle.
Its hard to see in the chart, but the lowest cells reach 3.45 volts, and look like they have started into the knee.

It seems to me that this is all OK - that there will always be one cell somewhat ahead of the rest, but if anyone has charging cycle data from their battery for comparison I'd sure appreciate it.


Capture.JPG

Dave Festing

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Jul 26, 2021, 4:28:08 PMJul 26
to electrodacus
For the SBMS0 I see the same.  What I did observe was that my older SBMS60 didn't do this.  I would consistently get to 28.5V whereas now I get to 27.5V to 27.8V.  Apparently, it is normal behaviour and the system has been working well.

Dacian Todea

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Jul 27, 2021, 3:59:17 PMJul 27
to electrodacus
There is absolutely nothing abnormal in that graph. That is expected for any LiFePO4 battery. Cell 8 is just your cell with slightly lower capacity and there will always be a cell that is just slightly different.
If say you replace cell 8 with a higher capacity cell then it will be that orange cell that will be the first to fully charge and if you replace that it will be the green one and so on.
There will always be a cell to finish first by a few seconds.  

And Dave it is the same for any SBMS model you just did not noticed before or had different settings and did not notice because of that.


The important part is that as soon as cell are close to full you can see in the graph above 3.4 to 3.45V the voltage will increase super fast almost exponential since the cell can no longer accept any electrons. Since there are small difference between cells there will always be a cell to get first there even if is just a few second sooner it looks like a huge difference if you look at the voltage.
And of course it will also drop in voltage very fast after the charging is stopped as cell can not stay at that high voltage level.

There is nothing abnormal about your battery and your battery is perfectly balanced as perfect as it can be for a real battery and not an ideal theoretical one.

B0bD

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Jul 27, 2021, 4:07:20 PMJul 27
to electrodacus
Thanks for the comments - I can now quit obsessing over this and stop spending time gathering data.
I really appreciate all the helpful input from the forum as I took this plunge into the LFP arena.

Now I just need someone to take a huge lead acid battery off my hands.

Dave Festing

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Jul 28, 2021, 12:41:58 AMJul 28
to electrodacus
Default factory settings both units, except GPIO.  I have several hundred SBMS60 logs which show multiple charges on sunny days that get to 28.0 to 28.5V  Generally getting higher on days which had 3, 4 or 5 cycles.

With a default SBMS0 you only get one high charge but in the range 27.4 to 27.7.V and with noticeable jumping around.  I may have not noticed jumping around on the SBMS60, but for the days where you could get up to 28.5V of course there wouldn't be any.

One change was the battery sensing cable.  On the SBMS0 install I used the supplied cable compared with slightly shorter wire connections (100-300mm) for the SBMS60 install.

Unfortunately, when I changed to the SBMS0 I also moved the system inside so there could be subtle changes in where things are positioned and where things are grounded.

The new system has not missed a beat, so I am pleased with how things are working.

Dacian Todea

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Jul 28, 2021, 12:54:56 PMJul 28
to electrodacus
Dave,

The SBMS60 default may have been 3.6V per cell (do not remember) Also SBMS60 will have stopped charging in 1 second after any cell will have exceeded the set limit say 3.6V
The SBMS0 is set at 3.55V for 6 seconds consecutive (may be different if you changed that).  What you call jumping around has to do with cell balancing and both will have worked exactly the same if cell balancing was enabled.
I remember you had a hydro-generator and that may not have been properly controlled by the SBMS60 thus the possibility to get to 28.5V.
The much sorter cell balancing connection will have resulted in less voltage drop during cell balancing.
The graph with cell voltages shown above is perfectly typical of how SBMS0 and SBMS60 (assuming same settings) will have worked. Also the cells are perfectly fine and balanced.   
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