Hurricane Milton ongoing off-grid story

[keepsake]

I am starting my story about this storm Milton and operating off-grid for six days plus.

42 kw of LiFeP04 280ah cells. 3 banks of 17 cells each. XW6848 Pro and other grid-tie inverters

After six days my biggest hassle is the cell imbalance as I recharge the cells.

I have 2 amp JKBMS on each bank of cells. It cannot keep up with the balance needs when each bank is recharging at 35 ot 40 amps.
I prefer not to let any cell go above 3.4 volts.

On Sunday morning I did find 'the weakest link' in the batteries. Had a cell that was down to 2.5 volts and had to remove the discharge path from that bank to protect it integrity. We don't want any cells heating up or deforming and expanding. So that charge rate appears to be at most c/8 and slow enough to be safe. Fire is the number one enemy in this whole mix.

Daytime loads include:
Well Pump, 1 hp
Running a pool pump, 2 hp
hvac units, 2 kw each
1200 watt water heaters
Charging of the Lithium batteries

At times I see 12 kw coming in from the sun and charging at 4 to 5 kw, managing other loads accordingly.

 

[740GLE]

FYI If you don’t bring cells above 3.4v you won’t properly balance them. Also how long do you absorb once cells are close to 3.4v to allow properly balancing? 1h? 3hr?

Nice to know pool pump is a critical load. Nothing like maintaining creature comforts.

 

[BentleyJ]

The XW seems to have one of the widest DC voltage operating range. I've been considering an 18 cell battery. I guess that's pushing the upper limit but seems workable.

As far as the balancing issue, maybe consider a NEEY 10A balancer or something like that. JK BMS makes a 5A and a 10A balancer. I have one of each. Although the 2A is enough just for the daily top up charge.

EDIT: What 740GLE said. The cells need to reach at least 3.425V, 3.45V would be better.

 

[timselectric]

I prefer not to let any cell go above 3.4 volts.

This is why you are having balance issues.
Balancing should only take place in the upper knee (above 3.45v).
You're trying to balance in the flat part of the charge curve.
All that is happening is unbalancing.

 

[keepsake]

FYI If you don’t bring cells above 3.4v you won’t properly balance them. Also how long do you absorb once cells are close to 3.4v to allow properly balancing? 1h? 3hr?

Nice to know pool pump is a critical load. Nothing like maintaining creature comforts.

The pool chemistry will go to hell and create unusable water should that be needed as well.

 

[keepsake]

This is why you are having balance issues.
Balancing should only take place in the upper knee (above 3.45v).
You're trying to balance in the flat part of the charge curve.
All that is happening is unbalancing.

So what do you see an upper limit to be ?
3.65 ? Granted it is only one cell that hits the upper # I watch for. It could be this is a weak cell as well.
So would 30 amps into the 280ah cell, at 3.65 be generating any heating ?

 

[BentleyJ]

Yes, 3.65V is a good target value for maximum cell voltage. Charging to 3.5V per cell is ideal IMO. The literature indicates LFP electrolyte breakdown above 4.2V, have not personally experienced this issue. I've had 18650 cells up to 3.9V on the bench top by accident. No immediate damage observed. Not sure of any long term life shortening consequences.

30A into a 280Ah cell is just over 0.1C. If all the connections are good, there should be very little or no noticeable heat generated.

 

[SparkyJJO]

If your cells are getting down to 2.5V your UVP is set all wrong.
If you're not getting above 3.4V per cell you're probably unbalanced to begin with.
Your charge/discharge rates aren't so high to create problems unless you have totally junk cells and/or are not charging/balancing them properly.
Bad timing to figure this out, I get that. But set your balancing to start at the right point. You don't need to push to 3.65V every charge cycle but make sure you get to 3.45V per cell and HOLD for a little while to make sure they can balance out. Get your UVP where it belongs so you don't over discharge (you shouldn't have to manually kick off discharge on any packs to prevent it).

 

[740GLE]

The pool chemistry will go to hell and create unusable water should that be needed as well.

Sounds like a dire situation, least you’re in a position to keep it in proper order.

 

[740GLE]

So what do you see an upper limit to be ?
3.65 ? Granted it is only one cell that hits the upper # I watch for. It could be this is a weak cell as well.
So would 30 amps into the 280ah cell, at 3.65 be generating any heating ?

You don’t need the all the cells to hit 3.65 to know they are balanced.

More of keep the cell delta in check above 3.45. IMO The % capacity between 3.45 and 3.65 isn’t worth it, but making sure the cells are balanced when at 3.5v allows each cell is doing their part and not bringing down total pack capacity.

Aim for a delta of <0.010v at 3.5, and know your pack is doing what it can to help.

Care to share a screen shot of the JK app once close to fully charged? You mentioned it was a balance issue, but never gave hard # you are seeing.

 

[hwy17]

My system is charging or discharging 24/7 and only needs maybe 5-10 hours of balancing a month, with only 100mA of passive balancing current.

I charge to 3.437 maximum and cells only get balanced if they exceed 3.437.

 

[keepsake]

If your cells are getting down to 2.5V your UVP is set all wrong.

Only one cell did the 2.5 v -- so the active balancing is far behind on that cell -- aka the weakest one

 

[keepsake]

Sounds like a dire situation, least you’re in a position to keep it in proper order.

The energy for the pool is all excess, daytime wattage.
It's hard to manage 20 kw with a single XW6848 inverter
So much of the manual and automation is to switch in and out additional loads as sun permits.
Clouds are a nuisance of course.

 

[SparkyJJO]

Only one cell did the 2.5 v -- so the active balancing is far behind on that cell -- aka the weakest one

Well as stated the balancing if it isn't set right can actually make it worse. Make sure the balancing doesn't happen until above 3.4V ish (some say 3.45V I think).

Clouds are a nuisance of course.

Obligatory.

 

[keepsake]

I should point out, 5kw is on the mppt dc side of the XW and the other additional pv energy comes in as AC coupled on the output side. A Fronius IG5100 and two SMA 5k units

 

[keepsake]

Make sure the balancing doesn't happen until above 3.4V ish (some say 3.45V I think).

I don't think I have that control. The JKBMS decides to balance when my preset offset occurs. I have it set like at 0.02 volts
It works across the entire voltage range that I see.

 

[keepsake]

During non-hurricane times, maybe 2 weeks a year, whole system sits idle at 56.5 volts across 17 cells. We normally use the grid.

 

[timselectric]

So what do you see an upper limit to be ?
3.65 ? Granted it is only one cell that hits the upper # I watch for. It could be this is a weak cell as well.

3.65v would be the upper limit.
I float at 3.50v , so that I get a perfectly balanced battery.

So would 30 amps into the 280ah cell, at 3.65 be generating any heating ?

If you are already at 3.65v , there shouldn't be any amps going in.
Unless your settings are off.

 

[timselectric]

I don't think I have that control.

You do, in the settings.

The JKBMS decides to balance when my preset offset occurs. I have it set like at 0.02 volts

That's fine, but set it to start balance at 3.45v.

It works across the entire voltage range that I see.

This is throwing the balance out.

 

[keepsake]

3.65v would be the upper limit.
I float at 3.50v , so that I get a perfectly balanced battery.


If you are already at 3.65v , there shouldn't be any amps going in.
Unless your settings are off.

I find that manually, once I get to 3.39,3.40 and above, it wants to runaway to much higher voltages. With the AC coupling it doesn't regulate like we would all like. The XW does not work for me with the AC coupling feature enabled, the XW control of the frequency skewing sends the external inverters into shutdown way too easily.
I find at that top end, its one cell that's too high and the rest of the cells are like 0.2 volt lower

 

[timselectric]

I find that manually, once I get to 3.39,3.40 and above, it wants to runaway to much higher voltages. With the AC coupling it doesn't regulate like we would all like. The XW does not work for me with the AC coupling feature enabled, the XW control of the frequency skewing sends the external inverters into shutdown way too easily.
I find at that top end, its one cell that's too high and the rest of the cells are like 0.2 volt lower

That's because they have never been allowed to balance.
You may need to lower the amperage, at the top, for a few charge cycles. So that the balancing can get caught back up.

 

[740GLE]

I find that manually, once I get to 3.39,3.40 and above, it wants to runaway to much higher voltages. With the AC coupling it doesn't regulate like we would all like. The XW does not work for me with the AC coupling feature enabled, the XW control of the frequency skewing sends the external inverters into shutdown way too easily.
I find at that top end, its one cell that's too high and the rest of the cells are like 0.2 volt lower

What wants to run away? A single cell or entire pack?

What cell delta are you seeing when your first cell hits 3.45v?

 

[shvm]

Only the resting voltages are a true indication of battery balance.
A high IR runner cell will always be at higher voltage while charging than cells with higher SOC unless fullcharge is reached.

Here is something that you can try.
Turn off balancing altogether.
As soon as one of your cells peaks near end of charging, turn OFF BMS ChgMOS and then turn ON balancing.
Wait for them to equalize. Then let ChgMOS ON to pump some more charge into the cells and turn it OFF again.
Rinse and repeat until you get a balanced battery. It may take many hours.

Unfortunately, it is very hard to automate this step.

 

[timselectric]

Unfortunately, it is very hard to automate this step.

It will happen automatically, just by lowering the charging current.

 

[Quattrohead]

And this ladies and gents is why it is good to use to closed loop communications, so that the batteries can tell their charger how they are feeling.