LiFePo - charging, BMS and balancing

[defed]

i've asked something similar before, but thought i'd start fresh as it is a little different.

my 48v lifepo - charge settings are 57.6v (3.6v) bulk/absorb and 55.2v (3.45v) at float and recharge is 54v (3.375v).

i'm still a little confused on what is happening...tell me if i have this correct: the battery voltage drops below 54v...c/c goes to bulk, dumps the amps in. when a single cell goes above 3.6v, the bms shuts off the charge to the battery and attempts to balance by draining the high cells. when all cells are below 3.6v, the charging should resume IF it's still in absorb and looking for 3.6v. otherwise, it goes to float, so all cells will settle out to 3.45v.

i ran a discharge/recharge and after sitting for awhile, the cell voltages are from 3.45v-3.30v. the c/c is in float. do all of the cells have to go below 3.375v before it begins to charge again even though the battery voltage itself is only reading 53.9v?

every time i think i am beginning to understand what is happening, something happens that i don't think should happen. i also have read that it may take several charge/discharge cycles before it begins to really calibrate itself.

 

[BentleyJ]

Are you operating with closed loop communication between the BMS and inverter? If so, there is no mention of SoC values in your post.
Low cell is 3.30V while high cell is 3.45V. That is a 150mV delta indicating the cells are not balanced. Either the BMS is not balancing properly or the batteries have not had enough time in Float to balance. Most BMS only have mA up to max 2A balancing circuits, so it takes time depending on your battery capacity. Can you set it up to Float at the 55.2V setting continuously for 48hrs?

 

[defed]

Are you operating with closed loop communication between the BMS and inverter? If so, there is no mention of SoC values in your post.
Low cell is 3.30V while high cell is 3.45V. That is a 150mV delta indicating the cells are not balanced. Either the BMS is not balancing properly or the batteries have not had enough time in Float to balance. Most BMS only have mA up to max 2A balancing circuits, so it takes time depending on your battery capacity. Can you set it up to Float at the 55.2V setting continuously for 48hrs?

no communications.

currently, 1 battery (2 in parallel but that's another topic) is at 53.89v, 99.74% SOC (from BMS screen), and cell voltages from 3.44v to 3.35v. they seem to be getting closer, but all cell voltages seem to be dropping (though i haven't recorded them to know for sure). the battery itself is under the 54v recharge level, it is not recharging...is this because a couple cells are over the 3.375v threshold so the BMS is not letting it come on until all are below?

this morning, after full charge last night (bulk, absorb and float for probably 6 hrs) the overall voltage was below 54v so it went into bulk for a short time and been in float for 5 hrs now (probably 6 more until dark). i do not have a way to force it into float when there is no sun.

i have read that it is normal for them to take a several cycles to smooth out, i'm just trying to understand what it is doing....because the 1st time i charged it, and let it sit - charger completely off - the cells balanced out to within .002v and this time they didn't seem to.

 

[defed]

an extreme hypothetical example...but let's say one cell gets to 3.6v, and the rest are 3.2v. the BMS shuts off because 1 cell is over. at this point, is the BMS going to display 100% SOC? and once that one cell goes to 3.375v, should charging begin again? or does it wait until all the cells are the same? and is all of this irrelevant of overall battery voltage - (which i presume to be the sum of all the cells)? just because my battery says 53.89v (less than the recharge of 54v) it's not going to start because some cells are over 3.375v...

 

[JunglePower]

I've been resuscitating abused BB1002s for over two weeks now. I finally have three that for two days have maintained 13.8vdc resting individually. I found that if the charge diminishes before they are fully balanced, I just load each on for a minute at 10 amps, let them rest, one actually increased its voltage. Then another light charge @5adc seems to assist them in balancing.

 

[BentleyJ]

Sorry, I don't even know where to begin. What equipment do you have, model numbers?? Lets at least clear one thing up first.

The BMS is a safety device and should only disconnect the battery if voltage, current or temp goes above a safe value which should never happen unless something breaks. ALL of the control as to when charging starts, stops, voltages, current and transition from Bulk, Absorb and Float happen with the Inverter/Charger/Charge Controller.

EDIT: should mention that even with closed loop BMS communication, the above is still true it just that the BMS is able to pass data to the inverter but its still the logic and settings in the inverter that is the controlling device.

 

[DIYrich]

i've asked something similar before, but thought i'd start fresh as it is a little different.

my 48v lifepo - charge settings are 57.6v (3.6v) bulk/absorb and 55.2v (3.45v) at float and recharge is 54v (3.375v).

i'm still a little confused on what is happening...tell me if i have this correct: the battery voltage drops below 54v...c/c goes to bulk, dumps the amps in. when a single cell goes above 3.6v, the bms shuts off the charge to the battery and attempts to balance by draining the high cells. when all cells are below 3.6v, the charging should resume IF it's still in absorb and looking for 3.6v. otherwise, it goes to float, so all cells will settle out to 3.45v.

i ran a discharge/recharge and after sitting for awhile, the cell voltages are from 3.45v-3.30v. the c/c is in float. do all of the cells have to go below 3.375v before it begins to charge again even though the battery voltage itself is only reading 53.9v?

every time i think i am beginning to understand what is happening, something happens that i don't think should happen. i also have read that it may take several charge/discharge cycles before it begins to really calibrate itself.

I would set the recharge to 53.3v. After charging, my batteries settle to 53.5v. You charger may drop to float charge if the BMS cuts off charging. The charger would switch to float when it reaches 57.6v (not singlecell reaching 3.60v). I would set the cell protection (BMS stops charging) at 3.65v. If the cells are way out of balance, or charger doesn't stop.

What voltage is the BMS set to start balancing cells? 3.40 is a place to start. It would start balancing when a cell reaches 3.40, and continue from there.

 

[DIYrich]

charger completely off - the cells balanced out to within .002v and this time they didn't seem to.

Ignore numbers when charging is off. You want voltages when charging, not settled voltage. Also, when below full charge, they tend to balance. What you are interested in is how well they are balanced above 3.40v per cell. Maintain at least 54.5v until they are balanced to within .005v

 

[defed]

i apologize for being so dense...i read something, think i have a handle on it, see something different in reality, read some more, it conflicts (or maybe it doesn't but the terminology is just different). i guess what i need is lifepo charging theory for idiots.

batteries are sk48v100, 2 in parallel. c/c is xantrex xw-mppt60-150, currently i don't really have an inverter...working on that.

so i set the c/c to 57.6v (3.6v per cell) - supplier recommendation so i presume something in the battery wants to see that. it charges the batteries, one cell gets to 3.7v. nothing shuts off the charging to that battery to keep that cell from going up up up while others may be severely lagging? or does it keep charging until 57.6v is met regardless of what any one cell is 10v and the rest are 3.1v (extreme example) until the c/c sees 57.6 and stops?

 

[defed]

I would set the recharge to 53.3v. After charging, my batteries settle to 53.5v. You charger may drop to float charge if the BMS cuts off charging. The charger would switch to float when it reaches 57.6v (not singlecell reaching 3.60v). I would set the cell protection (BMS stops charging) at 3.65v. If the cells are way out of balance, or charger doesn't stop.

What voltage is the BMS set to start balancing cells? 3.40 is a place to start. It would start balancing when a cell reaches 3.40, and continue from there.

i don't know where to see where balancing starts. supplier said to flaot at 55.2v, so i fugure that has something to do w/ it. BMS status says OVP - which i think is over voltage protection...total battery voltage is 53.78v and cells are 3403mv-3339mv. the c/c says it's putting out .1A @ 54.9v.

and this is part of what is confusing me, the terminology. i thought the bms did everything, safety cutoff, balancing...battery MANAGEMENT. then i was told bms does nothing but protect the battery - a switch - to cut charging in emergencies. then you ask when the bms starts to balance...well i was told previously (and actually read) that the bms is simply protection, and nothing else. LOL yea, so needless to say i'm very confused.

 

[JunglePower]

i don't know where to see where balancing starts. supplier said to flaot at 55.2v, so i fugure that has something to do w/ it. BMS status says OVP - which i think is over voltage protection...total battery voltage is 53.78v and cells are 3403mv-3339mv. the c/c says it's putting out .1A @ 54.9v.

and this is part of what is confusing me, the terminology. i thought the bms did everything, safety cutoff, balancing...battery MANAGEMENT. then i was told bms does nothing but protect the battery - a switch - to cut charging in emergencies. then you ask when the bms starts to balance...well i was told previously (and actually read) that the bms is simply protection, and nothing else. LOL yea, so needless to say i'm very confused.

I agree, it can be confusing. I'm not dealing with a DIY Lifepo but BB10012s.
I depend on the BMS, not for HVD or LVD but for proper distribution of energy within during a charge cycle in the cell pack. I could be mistaken.

 

[defed]

I agree, it can be confusing. I'm not dealing with a DIY Lifepo but BB10012s.
I depend on the BMS, not for HVD or LVD but for proper distribution of energy within during a charge cycle in the cell pack. I could be mistaken.

mine's the sk48v100. the more i read and re-read the manual, i pick up a few things each time....easier to digest it one thing at a time as it comes up than the whole thing at once.

it says the BMS goes to over-voltage when it's balancing. i checked the batteries when i got home, so they've been sitting. one BMS still says OVP - and the cell voltages vary more but all are between 3390mv-3340mv (53.5v 99.7% SOC). the other one is also at 53.5v, 99.7% SOC but it's in 'idle' and the cell voltages are closer, like 3350mv-3340mv. i still don't know why i'm at 53.5v and not 55.2v.

from the manual:

Automatic Cell Balancer​

The BMS also features a cell balancer; when charging reaches nearly 100% full, the cell voltage rises rapidly. From the factory, the charge level within each cell differs slightly, this means that the entire pack can only charge until the fullest cell reaches its peak voltage. If it weren’t for the automatic cell balancer, natural drift among cells would cause major capacity loss.

When you charge the battery up for the first time, it is normal for the BMS to trigger OVP (over-voltage protection). This OVP is triggered because the BMS monitored the level of each cell and stops charging if the voltage is too high. When OVP occurs, the cell balancer is simultaneously working, bleeding energy from only the completely full cells, so that all 16 cells can charge as a group.

The initial balancing process is slow and typically takes about 10 to 15 charge cycles before perfect cell balancing has occured. Cells are considered perfectly balanced when the voltage deviation is within 30mv, as indicated on the cell voltages battery screen. Cell imbalances on brand new, fully charged batteries can reach as high as 250mv. This is normal and no cause for concern. The only time to worry is if there is a large difference at a state of charge between 95% ~ 5%.


this leads me to believe that the BMS has a set voltage (as mentioned by DIYrich) for high and low....and if any cell hits that, it shuts off the entire battery. because they want my bulk/absorb at 57.6, is it safe to say the high voltage cut off on the cells is 3.6 ish? and then the balancer takes over. is that how i get from 55.2v to 53.5v resting because as the balancer bleeds off the high voltage cells to equalize them w/ the low voltage ones so the entire battery voltage decreases?

 

[JunglePower]

mine's the sk48v100. the more i read and re-read the manual, i pick up a few things each time....easier to digest it one thing at a time as it comes up than the whole thing at once.

it says the BMS goes to over-voltage when it's balancing. i checked the batteries when i got home, so they've been sitting. one BMS still says OVP - and the cell voltages vary more but all are between 3390mv-3340mv (53.5v 99.7% SOC). the other one is also at 53.5v, 99.7% SOC but it's in 'idle' and the cell voltages are closer, like 3350mv-3340mv. i still don't know why i'm at 53.5v and not 55.2v.

from the manual:

Automatic Cell Balancer​

The BMS also features a cell balancer; when charging reaches nearly 100% full, the cell voltage rises rapidly. From the factory, the charge level within each cell differs slightly, this means that the entire pack can only charge until the fullest cell reaches its peak voltage. If it weren’t for the automatic cell balancer, natural drift among cells would cause major capacity loss.

When you charge the battery up for the first time, it is normal for the BMS to trigger OVP (over-voltage protection). This OVP is triggered because the BMS monitored the level of each cell and stops charging if the voltage is too high. When OVP occurs, the cell balancer is simultaneously working, bleeding energy from only the completely full cells, so that all 16 cells can charge as a group.

The initial balancing process is slow and typically takes about 10 to 15 charge cycles before perfect cell balancing has occured. Cells are considered perfectly balanced when the voltage deviation is within 30mv, as indicated on the cell voltages battery screen. Cell imbalances on brand new, fully charged batteries can reach as high as 250mv. This is normal and no cause for concern. The only time to worry is if there is a large difference at a state of charge between 95% ~ 5%.


this leads me to believe that the BMS has a set voltage (as mentioned by DIYrich) for high and low....and if any cell hits that, it shuts off the entire battery. because they want my bulk/absorb at 57.6, is it safe to say the high voltage cut off on the cells is 3.6 ish? and then the balancer takes over. is that how i get from 55.2v to 53.5v resting because as the balancer bleeds off the high voltage cells to equalize them w/ the low voltage ones so the entire battery voltage decreases?

From what I am experiencing, I do see a slight drop in overall voltage after removing the charge BUT, I had one that was reading 13.8 and after it sat overnight, its voltage was 14.1. This was no error, I checked it twice, the overall voltage went UP!
I've been discharging each at 10 amps for a minute then letting them sit, this seems to aid balancing. If the voltage drops too low, the charger goes back on.
I picture all the cells sitting like water glasses on a table and trying to fill each one with a bucket, some fill and overflow into the others until finally they are all full.

 

[defed]

From what I am experiencing, I do see a slight drop in overall voltage after removing the charge BUT, I had one that was reading 13.8 and after it sat overnight, its voltage was 14.1. This was no error, I checked it twice, the overall voltage went UP!
I've been discharging each at 10 amps for a minute then letting them sit, this seems to aid balancing. If the voltage drops too low, the charger goes back on.
I picture all the cells sitting like water glasses on a table and trying to fill each one with a bucket, some fill and overflow into the others until finally they are all full.

i agree about the water analogy - some fill faster/fuller than others. if you have an active balancer, it takes from the higher voltage cells and puts it into the lower cells. a passive balance just drains off from the higher cells.

because my voltage was down to 53.5v, the c/c did start charging them this morning, i imagine up to the 57.6v, then back to float where i presume the balancer is working. i think i've got my inverter repaired, i just need to finish wiring it up, then i can get some load on the batteries. the manual makes it sound like every time you discharge and charge, the balance should get closer - it says 10-15 cycles... i wonder if it balances when the battery is under load? i think it would have to, because who is going to shut down the inverter every day to let it work. maybe it balances better under load because it can drain the high cells faster? guess i'll see this wkend when i get it hooked up.

 

[Kenny_]

i've asked something similar before, but thought i'd start fresh as it is a little different.

my 48v lifepo - charge settings are 57.6v (3.6v) bulk/absorb and 55.2v (3.45v) at float and recharge is 54v (3.375v).

i'm still a little confused on what is happening...tell me if i have this correct: the battery voltage drops below 54v...c/c goes to bulk, dumps the amps in. when a single cell goes above 3.6v, the bms shuts off the charge to the battery and attempts to balance by draining the high cells. when all cells are below 3.6v, the charging should resume IF it's still in absorb and looking for 3.6v. otherwise, it goes to float, so all cells will settle out to 3.45v.

i ran a discharge/recharge and after sitting for awhile, the cell voltages are from 3.45v-3.30v. the c/c is in float. do all of the cells have to go below 3.375v before it begins to charge again even though the battery voltage itself is only reading 53.9v?

every time i think i am beginning to understand what is happening, something happens that i don't think should happen. i also have read that it may take several charge/discharge cycles before it begins to really calibrate itself.

Yes, this is about where I am as well. Trying to understand LiFePo batteries, charging, inverter and battery charging/use parameters. I have one seemingly rogue battery from Sungold that is acting vastly different from my other four and Sungold is telling me nothing is wrong. In any case I want to make sure I understand as much as possible about charging, discharging, and using these batteries (and possibly build my own DIY battery - separate thread. Also separate thread on my Sungold system, operation etc.)

Thanks for the above,
an old Engineer trying to learn new tricks

 

[Kenny_]

i apologize for being so dense...i read something, think i have a handle on it, see something different in reality, read some more, it conflicts (or maybe it doesn't but the terminology is just different). i guess what i need is lifepo charging theory for idiots.

Yes again. That's what I need as well. All these terms and different names for the same thing and different 'opinions' on what is best has got me totally confused. I finally understand that Float is Trickle Charge (I think) and Bulk Charge is "Charging" and Absorption is kind of 'Final Charge' before float ....
but then people start talking cell voltages and BMS and what the BMS is doing vs what the Inverter/(charge controller?) is doing and then Communication....are these BMS's really Talking to each other and to the inverter or are they just 'Protecting' and sending 'Error'/'Alert' messages.

arrrrgh...

please help... explain like to a neophyte! Which I am even though I hold an ancient EE degree....

 

[defed]

Yes again. That's what I need as well. All these terms and different names for the same thing and different 'opinions' on what is best has got me totally confused. I finally understand that Float is Trickle Charge (I think) and Bulk Charge is "Charging" and Absorption is kind of 'Final Charge' before float ....
but then people start talking cell voltages and BMS and what the BMS is doing vs what the Inverter/(charge controller?) is doing and then Communication....are these BMS's really Talking to each other and to the inverter or are they just 'Protecting' and sending 'Error'/'Alert' messages.

arrrrgh...

please help... explain like to a neophyte! Which I am even though I hold an ancient EE degree....

my bank had shut down at 49.6v...i was confused....what happened? there should be plenty of juice left! i scanned my cell voltages and i THOUGHT all of them were at 3.1v. battery shut down is at 48v (3v/cell). so there must have been 1 cell below the cutoff that i overlooked. i was fixated on the fact that i had nearly 50v and forgot about cell variations, where a single low cell will trigger the BMS protection (high and low)....in an extreme example, you could have 15 cells at 3.5v and one at 2v, the battery says it's at 54.5v, but it's still going to shut down to protect the low cell. i believe my batteries are still not balanced great at the top and bottom ends. in the middle, they are just about dead perfect, which is somewhat meaningless.

i'm still trying to figure out how to get them to balance, why they don't balance....everyone told me don't worry about it so much, just use it, it will take of itself, it knows what to do....and now i'm shutting down at nearly 50v because the balance is not that good.