14.2, 14.4, 14.6 - seems like there is no agreement on charging voltage

[John Frum]

My JBD balances at milliamperes. And not very darn much, half an amp or something. Balancing isn’t very impressive, or I’m just impatient. Lol

I suspect you misunderstand me.
The current accounting in some BMSs needs to register a charge(which requires ~3 amps) in order to turn on balancing.
I have a JBD and the default configuration is to only balance during charge and from memory the current accounting works as described in the previous sentence.

 

[KITROBASKIN]

I guess I'll see what I can work out with SOK.

Please report back. Thinking I have a similar situation here but really not sure about the details brought up in this thread so skillfully by participating diysolar members.

 

[KITROBASKIN]

I let the pair of 12V 100Ah SOK (in parallel) get plenty of time to float yesterday. Right after the Victron charge controller went back to bulk at the end of the day, I recorded these images. I have all the charge controller settings as CurrentConnected recommends (14.6V) and one Bluetooth display insists there is a load on the battery even though the inverter is totally off. I made a video showing that battery completely disconnected from anything yet the Bluetooth still showed a couple amps load.

 

[archjeb]

IMHO a 2A or 4A charge won’t make much difference- whatever charge voltage you set, they will taper current as the battery voltage reaches that setting. Some chargers have a duration, some sense tail current - a level that is decidedly full charge and terminate.

Bleeding one cell at high knee is a quick way to bring it down to the level of the other cells. I have done this several times on my diy battery, and is very well balanced up to about 3.55Vpc.

You may need to repeat the process again at an interval. A severely degraded cell that doesn’t match the others is a warranty claim on a bought battery.

I use a 12V auto headlight bulb to bleed individual cells - done near full charge while charging to assist bms balancing. Only takes a minute or two to drop a cell down and let others catch up before the whole pack reaches the full voltage charge setting.
I don’t bounce off the bms over volt settings.

For dropping a cell down by, lets say, 100mV, how long do you put your lamp on the cell? Just so I have a ball park idea? And you do this after you tripped the BMS on overcharge correct? Or is this during charge?

 

[archjeb]

Please report back. Thinking I have a similar situation here but really not sure about the details brought up in this thread so skillfully by participating diysolar members.

So SOK is telling me not to worry that it will balance itself out. Direct quote:
"""
I noticed this, this Voltage difference in range, you don't need to worry about this.
If you checked the capacity, it over 100Ah already.
Also, you could keep using, after using a while, this Voltage different will drop down.
"""
I'm getting full rated capacity on discharge (I did confirm that) with a resistive load of 45amps.
Once I draw down the battery a little, they do balance out. Its just on full charge where I run into this balance issue. Notice the screen shot below, they are all pretty close. This is after putting a 45Amp load for about 5minutes on the battery pack.




At this point, since I'm a LiFePo4 rookie, I have no conviction
Am I over thinking all this and should just install these batteries and go fishing

 

[time2roll]

go fishing

 

[KITROBASKIN]

Well said about going fishing. My numbers normalize after use of the batteries, but it is good to learn more about these things, right?

Thing is, Battleborn does not give the option of closely monitoring each cell in their battery packs. They may have more than one runaway cell in their sealed, not bluetoothed units.

 

[KITROBASKIN]

This thread is very valuable because of the discussion of where to set the absorption voltage. How long should absorption be? Victron default was what, an hour? CurrentConnected says 15 minutes?

 

[time2roll]

This thread is very valuable because of the discussion of where to set the absorption voltage. How long should absorption be? Victron default was what, an hour? CurrentConnected says 15 minutes?

I believe both could be correct. Generally more voltage, less time. For light daily cycling... charging at the lower end and less time will still get over 95% and extend the life of the battery some. For occasional use, like fishing, perfectly OK to charge on the high side and get out on the water faster with 100% charge.

 

[sunshine_eggo]

Max charging voltage is 4.2V per cell.


I’m sure someone will get all bent out of shape about that one sentence. It is the true max safe charging voltage just like most lithium ion chemistries but common sense will tell you that past 3.35V you start to see the voltage climb rapidly and if your charging current is a fixed value then 2+2= there’s not much capacity up there. You decide for yourself how high you want to go. Most of the research and real world experience has shown us that past 3.50V you’re seeing maybe the last one or two percent of capacity. This also depends on how your charger cuts the current off. Most people follow the 0.05C rule.

Also, keep in mind I don’t really use BMS’s unless it’s a pack I know I’m not going to monitor so I’m free to charge up to 3.65 on a balance charger if I want to so long as my charge current is low enough for the balance current to stop any runners. I couldn’t fathom having an off the shelf battery with no access to the balance leads with how slow most of these BMS’s balance and worst of all the non Bluetooth BMS’s that have balance start set to 3.60V

Do whatever makes you happy. Maybe take notice that these BMS’s aren’t making you very happy.

Please cite a LFP cell specification that permits this.

I know what you're saying, but it's old information, and it shouldn't be perpetuated.

 

[hwse]

So SOK is telling me not to worry that it will balance itself out. Direct quote:
"""
I noticed this, this Voltage difference in range, you don't need to worry about this.
If you checked the capacity, it over 100Ah already.
Also, you could keep using, after using a while, this Voltage different will drop down.
"""
I'm getting full rated capacity on discharge (I did confirm that) with a resistive load of 45amps.
Once I draw down the battery a little, they do balance out. Its just on full charge where I run into this balance issue. Notice the screen shot below, they are all pretty close. This is after putting a 45Amp load for about 5minutes on the battery pack.

View attachment 130810


At this point, since I'm a LiFePo4 rookie, I have no conviction
Am I over thinking all this and should just install these batteries and go fishing

Almost all LFP packs will show very little delta voltage between 3.0 and and 3.3v which is where 90% of the Ah are given. The imbalance is only at the knees top and bottom. I had a 300mv delta on my 480Ah pack and I have been working for the past couple of weeks to get that balanced and am down to about 140mv now. It is a slow proocess but doable.

 

[Browneye]

For dropping a cell down by, lets say, 100mV, how long do you put your lamp on the cell? Just so I have a ball park idea? And you do this after you tripped the BMS on overcharge correct? Or is this during charge?

A couple of minutes. You have to do it while charging at near full - over 3.45Vpc.
You should not be tripping bms. The idea is to get them all over 3.55 without one running up.

 

[sunshine_eggo]

I have found it INFINITELY easier to charge vs. discharge. If you have an imbalanced pack, simply hit each low cell with the 10A/30V power supply set to 3.65V/10A. The deviations are often only an amp-hour or two, and 15 minutes per cell is enough to achieve balance with zero guesswork.

Alternatively, if you must discharge, do it in 1Ah increments, i.e., if you're bleeding 4A off, do it 15 minutes at a time. Once you over-shoot, you know that you're within an amp-hour.

 

[archjeb]

Rookie here,so excuse me if this is a dumb question.

If I charge to cutoff and one cell is at 3.65v and the rest are at the 3.4v range why can’t i just crack the battery case and put a load on that cell that is high and bring the charge down a little…put the cover back on…charge again and see where I am at to see if I have to discharge some more? Baby steps kind of approach/a few minutes of discharge so I don’t over discharge?

Or am I over simplifying the process?

Also, if I remove the battery cover, do I need to disconnect cell and BMS leads or can I just clip my load over the cell I need to discharge? I was hoping the latter so less messing around with stuff in there ?

 

[sunshine_eggo]

Rookie here,so excuse me if this is a dumb question.

If I charge to cutoff and one cell is at 3.65v and the rest are at the 3.4v range why can’t i just crack the battery case and put a load on that cell that is high and bring the charge down a little…put the cover back on…charge again and see where I am at to see if I have to discharge some more? Baby steps kind of approach/a few minutes of discharge so I don’t over discharge?

That's literally what's being discussed.

Or am I over simplifying the process?

A little. It's iterative and annoying. IMHO, better to do it in a controlled fashion with a set Ah removal, so you know how far you're off when you overshoot.

Also, if I remove the battery cover, do I need to disconnect cell and BMS leads or can I just clip my load over the cell I need to discharge? I was hoping the latter so less messing around with stuff in there ?

Yes. Can do the same with a charger - hook only to the individual cells to add.

 

[hwse]

Rookie here,so excuse me if this is a dumb question.

If I charge to cutoff and one cell is at 3.65v and the rest are at the 3.4v range why can’t i just crack the battery case and put a load on that cell that is high and bring the charge down a little…put the cover back on…charge again and see where I am at to see if I have to discharge some more? Baby steps kind of approach/a few minutes of discharge so I don’t over discharge?

Or am I over simplifying the process?

Also, if I remove the battery cover, do I need to disconnect cell and BMS leads or can I just clip my load over the cell I need to discharge? I was hoping the latter so less messing around with stuff in there ?

If you have a pack that can be opened up without damaging the case that is a great way to go. My pack is in a case that is sealed to an IP67 rating and the only way to open it up is to pry or cut open the lid which would make it nearly impossible to reseal.

 

[archjeb]

Excellent. So the SOK is fully servicable and I can remove the cover and get to the cells. I just wasn't clear if I had to physically disconnect the BMS or not. Thanks for the insight. I'll give it a shot and see about draining the one cell a little at a time to see if I can help it along on the balancing. Its only this one battery that I'm worried about; the other 2 batteries are close enough.

Thank you for the great dialog and helpful info on this thread.

 

[time2roll]

Can bleed the cell while in motion. Identify the high cell with a voltmeter and attach directly while charging. Take the cell 25 mV below the others and remove until it becomes 25 mV high again. Continue as needed.

 

[Pierre]

Can bleed the cell while in motion. Identify the high cell with a voltmeter and attach directly while charging. Take the cell 25 mV below the others and remove until it becomes 25 mV high again. Continue as needed.

I had to do this dance many times !

 

[Browneye]

I had not tried charging individual cells, it was easier to take one or two down.
Time2roll gave me the same solid advice, my pack is happily balanced now.

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