Recommended max cell differential 48V EVE 280 Battery Bank?

[apctjb]

Finally got my solar installed and able to charge at higher rates (~100A). As the day progressed the differential between my highest and lowest cells (cells 15 & 16) started to increase from about 20ma to max differential setpoint (100ma) I had set on my Chargery BMS, which triggered the relay and stopped charging.

So what are typically max cell voltage differential folks are seeing with EVE 280 cells at 100A rates?
Is there any harm in having higher cell voltage differentials? (as long as not exceeding voltage setpoints for any individual cell)

Hoping this is simply a busbar resistance issue. Will check torque on my busbar connections tomorrow.

 

[apctjb]

Related question. With a Chargery BMS, if the max cell voltage differential setting is exceeded (when charging) does this trigger the charging relay, the discharge relay or both?

 

[John Frum]

Tripping on cell differential is a nice feature.
100mv is a lot but it really depends where it is in the charge curve.
If its right at the top then your cells are out of balance.
If its in the middle or at the bottom the cells may converge, given a chance.
What was the approximate voltage of the cells when the bms tripped?
BTW the balancer should only be activated during charge and only above float voltage.

 

[Luthj]

After I got my cells in balance I was seeing about 12mv at 100A. With a 1-2% imbalance I would get 150mV at the top of the charge.

I had a high resistance bus bar connection early on, which was making 25mV at 100A.

 

[apctjb]

The voltage difference is occurring as the battery is reaching higher state of charge. Approx 3.4V/cell.

Interesting BTW. Curious to understand why the balancer should only be on during charge and only above float? As I have a large capacity bank 560ah I would have guessed it should be on at all times to give it a chance to balance (1.2A is not much on a 560AH bank)

 

[John Frum]

Interesting BTW. Curious to understand why the balancer should only be on during charge and only above float?

Because otherwise you run this risk of undoing the top balance.
Before you top balance the cells you might try charging with a very low current with balancing configured as suggested and see if they start to converge.

 

[John Frum]

Hmm you might check the busbars on the pack to see if a couple of them are getting warm.
If yes you need to redo the joinery.

 

[apctjb]

Because otherwise you run this risk of undoing the top balance.
Before you top balance the cells you might try charging with a very low current with balancing configured as suggested and see if they start to converge.

Thanks. Not intuitive to me why , but will give it a try.

 

[Luthj]

I would also double check the BMS voltage measurements with a good multimeter. There has been sporadic reports of the chargery having some measurement accuracy on a channel or two.

 

[John Frum]

Thanks. Not intuitive to me why , but will give it a try.

Top balancing is kind of misleading term.
What you want to achieve is to fill each cell full.
Think of it as synchronizing the cells at full charge.
If you leave balancing turned on all the time you will likely syncronize your cells to whatever your float level is.

 

[Steve_S]

Here are my BMS8T Settings as I run 24V but you can use this as a working template for settings that works reasonably well.

Setting	Default	My settings	NOTES
Over charge P Voltage	3.65	3.65
Over charge R Voltage	3.55	3.55
Over charge Current	50	86A / 150A	0.5C rate for 174AH / 280AH
Over Discharge P Voltage	3.00	2.65	CUTOFF Trigger
Over Discharge R Voltage	2.00	2.75	Release @ this V.
Over Discharge Current	300	175A / 250A	1.0C Rate for 174AH / 280AH
Low SOC cutoff	20%	0%	forces cutoff @ % (faulty)
High Temp cutoff	50C	70C
Diff of Batt Temp	10	15C
Diff of cell Voltage	30mv	200mv	Unmatched cells drift,
Temp Unit	C	C
Key Beeper	ON	ON
LCD Backlight	10	10min
Cut off Delay Time	10	10S
Current Calibration		-SET-
Temp Alarm	ON	ON
Cell Empty Voltage	2.50	2.50
Cell Full Voltage	4.20?	3.65
Default Setting		Enable
Balance Parameter		-SET- (OFF)*	Passive is ON charge only, start at 3.40V, 30mv diff.
Battery Capacity AH	1	174 | 280	Label Value of cells
Battery Power WH	1000	4554 | 7168	3.2V * 8 cells * 280AH = 7168 (24V Pack)
Low Temp cut off in Charge	2C	2C
Low Temp cut off in Discharge	-10	-10C

NB:
- I also have a QNBBM-8S Active Balancer on each battery pack.
- Also using the ISO Board between the BMS Main Module & DCC-300, and external power from the battery Main Terminals (Highly Recommended).
- BMS FirmWare Version 4.04

ISO Board Info: https://diysolarforum.com/threads/chargery-bms-dcc-solid-state-contactor-thread.11756/post-169742
Power Adapter: https://diysolarforum.com/threads/checking-chargery-voltage-measurements.14712/post-179788

Hope it helps, Good Luck
Steve

 

[apctjb]

Also using the ISO Board between the BMS Main Module & DCC-300, and external power from the battery Main Terminals (Highly Recommended).

Thanks Steve. Can you expound further on the recommendation to power the Chargery BMS from external power (battery terminal) ? Also why balancing is set to off. Doesn't that result in greater diff over time.

 

[apctjb]

Steve; just read the thread on connecting Chargery from external power. I have a 48V 560AH bank . 1) Can I power directly from the battery terminals or is a converter required? 2) What's the real world down side (other than voltage readings slightly off) if I keep it as is being powered by the voltage sense cables.

 

[Steve_S]

I have an Active Balancer on every pack which does it's thing but I have Passive ON during charge to keep the runners slightly more at bay.
I also turned it on for "Storage" on my two 175's because they have runners which force HVD and that puts the BMS into storage mode until the cell in question settles and that is a bit faster with passive also pulling power out of the Hi cells.

The Chargery using the internal power taken from the wire harness has caused issues with accuracy on reading the cells, some "flutter" if you will. I initially just made up a pig tail and used a barrel connecter and that helped but Jason sent me the premade ones with the Shottky diode, filters & switch and that improved it a little more. It is quite important to have stable cell readings rather than have them fluttering. Passive Balancing get's somewhat useless if the cells are fluttering too much as well. Also Note that Low SOC cutoff is set to 0% (OFF) it screws up very easily and trips LVD as soon as a cell dips below, even if it is just a momentary sag due to load start. The Cut off Delay Time is 10S (seconds) can be safely increased to 20S if you get deep sags from a heavy start. 20S is no issue as the cells will recover over that.

The SOC readings are still not great BUT it was much more important to ensure the voltage readings of the cells were stable. I don't worry much about what the BMS is reading as capacity because I monitor the bank voltage & SOC with my Midnight Classic SCC's WizBangJr & Shunt which is fairly accurate... Basically functions similar to a Victron Smart Shunt but works in conjunction with the Midnite SCC and it's software.

The BMS16T external power is 13-60V, 3A.
The ones I made used these Male Adapters below and 10g wire to the battery pack terminals.

COOLM 5 Pairs DC Connector Male and Female Power Jack Plug Adapter Connector Plug 2.1mm 5.5mm for LED Strip Light CCTV Camera : Amazon.ca: Electronics

COOLM 5 Pairs DC Connector Male and Female Power Jack Plug Adapter Connector Plug 2.1mm 5.5mm for LED Strip Light CCTV Camera : Amazon.ca: Electronics

www.amazon.ca

 

[SoundinSpirit]

Top balancing is kind of misleading term.
What you want to achieve is to fill each cell full.
Think of it as synchronizing the cells at full charge.
If you leave balancing turned on all the time you will likely syncronize your cells to whatever your float level is.

That is a really good point. I like the way you put that.