8 Cell 24V LiFePo4 - JKBMS working - but one thing confusing

[TheMadNucleus]

I just finished setting up this 24V system - It is currently charging and all looks good in the BMS Bluetooth app - except one thing I can't explain or figure out. On the cell voltage display which shows the state of balancing (Red, Blue and Green Highlights) when I turn off the charger those numbers are all very close and the red and blue numbers jump around based on cell state. When Charging, one cell always is red - it's the 8th and last cell in the series. The other cells the blue highlight jumps around the remaining cells. The 8th and last red highlighted cell is about .01 v less then all the others which are about .003 v difference.

If I disconnect the charger all the cells including the 8th are within .003v - All thoughts welcome.

TMN

 

[Steve_S]

It is not unusual, Red always indicates the lowest cell voltage, blue is always the highest cell voltage.
You'll notice that when Blue is blinking it is draining power to the capacitors and when it stops, the Red cell will blink as it receives that power from that capacitors. Balancing by default will start when there is more than 0.010mv difference (adjustable on settings screen) but the default is good and when the cell voltage is above the threshold voltage (also on settings screen).

Cell #8 is the last cell in the series and it is not unusual to be the lowest as it is the "out" cell.

 

[TheMadNucleus]

ok - Thanks Steve. The odd thing is that one cell is always the Red cell and always the one receiving the balance charge. Yes - that's the 8th.

And the cells are increasing in voltage as they undergo charge.

Appreciate the quick response.

 

[Steve_S]

The only other thing to do is to check that it is properly tightened and that the contact surfaces on the cell & busbars are clean, best to wipe all contact surfaces with Rubbing Alcohol.

 

[MisterSandals]

The odd thing is that one cell is always the Red cell and always the one receiving the balance charge. Yes - that's the 8th.

You could rearrange the cells to see if the behavior follow the cell or stays in the #8 position.
You should label your cells (i like A, B, C... because cell position is a number) beforehand and take good notes.

If the behavior stays in position 8, i would review all the crimps and connections carefully for #7 and #8.

EDIT: Instead of "rearrange the cells" i should have said swap the cell in #8 position with an average voltage cell, (not cell in #1 position). Change ONLY #8 with that one. Then retest.

 

[TheMadNucleus]

Thanks Steve and Captain Obvious Appreciate your input

Actually - after about an hour, it seems enough power got moved to the #8 Cell and then things looked much better. The Low level Cell (Red) started jumping around and the balancer did it's job up to my full charge limit (28.4 V)

I think all is good now.

 

[MisterSandals]

I think all is good now

Label your cells and take notes now so if it happens again, or something else happens, you can better figure it out.

 

[Steve_S]

I have extensively tested the Active Balancing systems and will suggest the following:
Set the Voltage Threshold for Balancing to start at 3.000V and leave the 0.010mv differential trigger (see settings page on BMS APP).
Active Balancing does NOT cost you power, it merely transfers it through the capacitors in the BMS/
It will ONLY Operate when above 3.000Vpc (Volts per cell) and only when the cells have greater than 0.010mv differential.
It does NOT affect your charge rate negatively and in several instances improves balanced charging and helps to keep cells in-line to control runner cells which will be more prominently seen after 3.350V and up.
Active Balancing will also correct imbalance resulting from deep discharges but only to the set voltage threshold.
- BEWARE OF WIVES TALES that say otherwise, often times this originates with Passive Balance (only burns off and wastes power from hi cells ! Some misquote information due to a lack of understanding and then they push the falsehoods along as they cannot admit they do not really understand what they are talking about....

LFP just like ALL other Chemistries has TWO Voltage Ranges and you MUST be aware of this fact.
The Allowable Voltage range is from 2.500-3.650 per cell. This is the Voltage that will not harm/damage the cells. Above or Below that range causes harm.
The Working Voltage range of 3.000-3.400 Vpc is where all the Amp Hours (per cell spec) originate from so if you have a 100AH Battery Pack the 100AH is delivered from 3.000-3.400, yes, that is within a 0.400 range. This is also the reason that LFP has a Nominal Voltage of 3.200Vpc _or_ 50% SOC. Most people will use 2.900-3.425 as their working range (if they have Grade A Cells) to get a wee bit extra. Typically A Cells will deliver the exact AH from the working range and if going beyond that and tested from 2.500-3.650 will show on average 10% higher Gross AH. 2.5-2.9V is fast and only represents about 5% of "gross Capacity" just like 3.425-3.650 which is the other 5% of Gross Capacity. It really is not good to dive into the Gross Rating and stick to the working rating.

If you charge cells to 3.425Vpc and stop charging, LFP will settle right around 3.400 +/- 0.005V provided that the cells have been saturated to the EndAmps.

EndAmps / Tailcurrent calculation:
100AH Battery X 0.05 = 5A, 280AH Battery X 0.05 = 14A

 

[TheMadNucleus]

Hi Steve - appreciate the input - Yes actually well aware of the settings and charge/power curve for LiFePo cells. In fact I have those settings set with the exception of a top cell charge of 3.5v not 3.4 - all else are the same as your post.

But last night I found another confusing aspect - maybe lack of my familiarity with the chemistry profile/behavior. The Charger (its a 24v 20 amp LiFePo4 charger) charged the cells up to 3.5 (Pack voltage of 28v). I have a voltage sense that shuts off the charger at that level (Full disconnect). The JK BMS balanced the cells from about .04 differential down to a ,004 differential. But within a few hours the Pack's total voltage dropped to 26.7 and cell voltage (Avg) dropped to about 3.33. I have noting hooked up to the pack that would discharge these cells and while the BMS will use some energy (loss in active balancing transfer and basic operation of the MCU) it clearly is negligible relative to the power levels involved. I understand settling but these numbers concern me - the drop is significant.

Thoughts?

 

[Steve_S]

Sounds like the cells were not saturated and so they settled lower and that's the trick really. When charging to "EndAmps" then switch to Float to top off the batteries with constant voltage variable current and that will continue to push power in as the amps taken decreases due to the IR.

If you take an LFP cell and charge it to 3.65 and even so far that it takes 0A charge and disconnect everything, you'll find that within a couple of hours it will have settled down quite a bit and will probably be close to 3.500 and likely even a tad lower. It is the chemistry and what the Working Voltage range is about.

I covered some of this in this resource PDF which you might want to download, there is voltage charts & tables + in there too.
Luyuan Tech Basic Lifepo4 Guide V1.0A

 

[TheMadNucleus]

Excellent - thanks Steve! Will read the document.

 

[MisterSandals]

I have a voltage sense that shuts off the charger at that level (Full disconnect).

This is probably not allowing your charger to apply the tail current.

 

[TheMadNucleus]

Hi MisterSandals - yes correct, however, I have that set at 27.9V which is a 1/10 of a volt below the OVP setting in the JK BMS. And that does disconnect the charger at that voltage - so at that point the pack with a VM reads 27.9 but then without anything discharging the pack it falls back down to about 26.7 volts. That seems like a huge drop for settling but I'm not sure since this is the first that I'm working with these cells.

That's my confusion.

 

[MisterSandals]

27.9V / 8 = 3.4875Vpc

You should have the BMS set to prevent going outside the cell/battery limits (2.5V and 3.65V, or a couple tenths inside to be conservative).

The BMS is NOT intended to control charging, it is to protect the battery.
The charge controller is designed to control the charging (it is called a charge controller for a good reason!).

so at that point the pack with a VM reads 27.9 but then without anything discharging the pack it falls back down to about 26.7 volts

This is normal because the 27.9V is just a surface charge, and if measured while a charge is being applied, is a blend of that voltage too.
If you can hold the battery at that 27.9V, it will absorb until full. How long depends on a lot of things but the current will slow to a couple amps when full.
Your charge controller will handle all this if given the time and leeway (not constrained by .1V by BMS).