Ok you win... Cell imbalance

[ericfx1984]

Ok heres the deal

Over a year ago I built a huge TEMPORARY 4P4S battery

SHORT STORY

The battery is falling out of balance as we approach 100% SOC... 1 cell block will creep higher than the others (the highest I saw was about just over two tenths of a volt

These cells have been in service for about a year

These will be in this configuration only temporarily eventually they will be reconfigured to 16S but that is probably two or three months away

I am considering tearing this pack down and configuring them at 16p for a top balance... After all it has been over a year since the last top balance

In addition I'm considering adding an active balancer as I have been told that battery management system simply can't handle balancing such large cell blocks. (304 ah ×4 = 1216 ah)


LONG STORY

I used 16, Eve Cells, 304ah from Docan

Initially I did a 16P top balance to 3.65 volts

Are we configured to 4p4s and installed 200 amp battery management system with Bluetooth (JBD?)

I also installed it in parallel with another lifepo4 battery

I had no serious issues out of these cells slide from a very occasional misbalance on cell two... It wasn't a huge difference but it would creep up a little bit relative to the other cells this would all sort itself out after a few charge and discharge cycles

Every now and again the batteries would reach full charge and disconnect charge abruptly. This only happened on days when it was cloudy sometimes but the sun would come out from time to time... It also required that there was little to no load on the battery

What would essentially happen is after the charge disconnect happened It would cause the charge controllers to not react quick enough and feed too much voltage to the inverter causing an inverter shutdown

I solved this issue by lowering the charge controllers maximum charge voltage meaning that the battery would only charge to about 3.45 volts per cell (previously I was using 3.65 volts per cell)

 

[sunshine_eggo]

re-top balance OR active balancer. Not both.

 

[Quattrohead]

0.2 volts per cell is a large change when it comes to these batteries, do smaller incremental changes and see how it goes.

 

[ericfx1984]

0.2 volts per cell is a large change when it comes to these batteries, do smaller incremental changes and see how it goes.

Are you suggesting that most likely the issue was caused by me lowering the overall charge voltage?

 

[LydMekk]

Try 3.55V bulk and 3.45 float. Works for me. Charging with 3.65V is not necessary and might be detrimental.

 

[ericfx1984]

Try 3.55V bulk and 3.45 float. Works for me. Charging with 3.65V is not necessary and might be detrimental.

Yeah I stopped doing that... No need for 3.65v

 

[Quattrohead]

Are you suggesting that most likely the issue was caused by me lowering the overall charge voltage?

No just that you made a rather large change to a rather precision piece of equipment. These batteries do not react like lead acid at all. They are powerful and delicate all at the same time.

 

[fafrd]

Try 3.55V bulk and 3.45 float. Works for me. Charging with 3.65V is not necessary and might be detrimental.

The only problem with lowering voltages is that you are masking any underlying imbalance of more specifically, any creeping / increasing imbalance…

If your cells are perfectly matched and stay that way, you can get away with it because there is nothing to worry about.

My cells are not like that and wherever I set voltages, I’ll eventually get a runner.

My pack is 2x280 cells in an 8S string and I basically check it ~monthly and attach my 5A active balancer for a few days whenever I notice enough imbalance emerge to make use of it (full 5A into a cell only when 100mV or more of imbalance to all other cells exists, so only 10mA of imbalance would only result in 500mA of maximum balance current…).

 

[alferz]

Try an active balancing BMS such as JKBMS if you get an opportunity to switch it out. Their 2A active balancer made a huge difference with my slightly mismatched 248ah 8s pack.

 

[740GLE]

Making “4p cells” is a lot of connections, I’d wager once you make a 16s a worth while BMS with active balanced should keep things in check.

My 2p4s 230 set up with I’m guessing the same JBD 200a BMS is keeping cells within .050v with max charge voltage of 14v, “two cells” are both tracking the same voltage higher than the other two.

I flick on my 5a active balancer for 20-30 min and it brings them down to .020v.

 

[ericfx1984]

I flick on my 5a active balancer for 20-30 min and it brings them down to .020v.

Any problems with leaving the active balancer on and running all the time?

 

[fafrd]

Any problems with leaving the active balancer on and running all the time?

I’ve actually been doing that for the first time over the past week.

The concern is that if the cells don’t track well together through the entire charge / discharge curve, you can send charge from some cells to others at one point that will later need to come back (needless I^2R losses).

What I’m coming to realize is any impact from that effect is relatively minor.

I’m using a bottom-balanced battery and the inverter is shut down from 9pm to 6am or effectively 9am when incoming charge power begins.

Those 12 hours of balancing the near-empty battery that is into the lower knee dominates any charge transfer that happens during the day.

So I’m now thinking at most, I’ll disconnect the balancer once the battery is near-perfect (bottom) balanced and reconnect it whenever I see balance creep up to at least 50mV between the high and low cell.

 

[ericfx1984]

Oh my God active balancers are so crazy awesome

 

[sunshine_eggo]

Oh my God active balancers are so crazy awesome

Would only be impressed if you were showing all voltages above 3.40V.

Even imbalance batteries will show very similar voltages below 3.40V.

 

[ericfx1984]

Would only be impressed if you were showing all voltages above 3.40V.

Even imbalance batteries will show very similar voltages below 3.40V.

That's fair and mine do

 

[740GLE]

Any problems with leaving the active balancer on and running all the time?

This is a vacation spot that we spend 10 or so weekends a year, I shut everything off except the SCC when we’re not there. I just flick on the active balancer once I’m there incase something goes south.

I mean I could prob leave it on 24-7 as the battery stays at 99-100% after it get charged, but I have heard stories of the balancers letting out the smoke.

 

[740GLE]

Would only be impressed if you were showing all voltages above 3.40V.

Even imbalance batteries will show very similar voltages below 3.40V.

Agreed my .050v delta goes right down to .001mv soon as a few AH is drawn off the battery and cells are 3.3xxxv

 

[ericfx1984]

This is a vacation spot that we spend 10 or so weekends a year, I shut everything off except the SCC when we’re not there. I just flick on the active balancer once I’m there incase something goes south.

I mean I could prob leave it on 24-7 as the battery stays at 99-100% after it get charged, but I have heard stories of the balancers letting out the smoke.

So maybe just run the balancer for a few days to get everything back in sorts and then disconnect it for the time being?

Should I just disconnect it from the leads or should I add a switch on the positive lead?

 

[740GLE]

I’d need less than that.

Right now we arrive for the weekend, I notice .050v to .065v delta I flip the little switch I have soldered to the run tab and in 20 min that delta is cut in half.

If my delta keeps growing I’ll be more concerned but so far I think it’s manageable.

Also I don’t care about getting every AH out of the bank, so I’m not losing sleep over a little unbalance. As we add more load over the next few years I’ll be adding another bank and solar and will be upgrading the system to an 8s with most likely a JK.

 

[Alkaline]

Oh my God active balancers are so crazy awesome

Now you see the light