Understanding MPP Solar charge settings for LifePO4 batteries

[KenDan]

There are some 1C charge curves on LFP here:

Paralleling LFP cells at rest to "balance" them

Hypothesis: Paralleling cells at rest does almost nothing beneficial. While voltages may be equalized, their states of charge remain dramatically far apart. I decided to do an experiment. Take two CALB 40Ah LFP cells. Fully charge to 3.65V Fully discharge to 2.5V Charge ONE to 3.65V Let them...

diysolarforum.com

They hit 3.65 at about 25% SoC.

In reading that thread, your testing seems to indicate that the discharge curve of LifePO4 cells is so flat, that paralleling them - or any other means of equalizing cell voltages is not very effective. That begs the question - how to you equalize SOC accurately? The standard recommendation is simply to just "top balance". That is much easier said than done however.
I've tried to do that on individual cells - manually - by using a resistor to drag the higher voltage cells lower than the others when at a high SOC. This does not seem to work very well for me. Those same cells end up at a higher voltage once again after just one or two discharge/charge cycles.
The mAh that needs to be bled off from those high cells is quite small, but if left unchecked, it can result in those high cells rapidly going dangerously high if the entire battery pack is charged to an Average voltage of 3.65v/cell.
SO, what method works best to achieve an accurate balance in the SOC of all the cells?

 

[snoobler]

Top balancing means the cells are only equalized at 100% SoC (or other arbitrary peak the user desires). Below that, they diverge.

Parallel and CHARGING to 3.65 (or other peak) as detailed in the guide in the Resources tab.

The only way I could see your method somewhat working is with a very low rate of charge - maybe 0.005-0.010C with the ability to decrease it as needed. Once the top cell hits 3.65V, put a resistor on it sized to consume the charge current and continue charging ( R = 3.65V / I ). Repeat with each cell as they hit peak.

Once a pack is built imbalanced, the best way to balance is with the BMS. Find the total voltage that results in the highest cell hitting 3.65V. Enable balancing on charge only and only above 3.35V initially and 3.4V in the end. Eventually, the pack will balance, and you may need to adjust your total voltage up incrementally to get them all to within .05V @ 3.55-3.65, or whatever peak you select.

Balancing at any time besides charging and towards the higher voltage range may work to un-top-balance cells.

 

[KenDan]

Top balancing means the cells are only equalized at 100% SoC (or other arbitrary peak the user desires). Below that, they diverge.

Parallel and CHARGING to 3.65 (or other peak) as detailed in the guide in the Resources tab.

The only way I could see your method somewhat working is with a very low rate of charge - maybe 0.005-0.010C with the ability to decrease it as needed. Once the top cell hits 3.65V, put a resistor on it sized to consume the charge current and continue charging ( R = 3.65V / I ). Repeat with each cell as they hit peak.

Once a pack is built imbalanced, the best way to balance is with the BMS. Find the total voltage that results in the highest cell hitting 3.65V. Enable balancing on charge only and only above 3.35V initially and 3.4V in the end. Eventually, the pack will balance, and you may need to adjust your total voltage up incrementally to get them all to within .05V @ 3.55-3.65, or whatever peak you select.

Balancing at any time besides charging and towards the higher voltage range may work to un-top-balance cells.

I have my BMS set to only balance if cell voltage is above 3.35 volts. I tried setting it to 3.4 volts, but the voltage drops below 3.4 volts so quickly after charge termination that it doesn't have enough time to do anything.
Also - my BMS will only go into balance mode if charge/discharge current is very low (approx 2 amps or less). That makes sense to me because at high current levels, small differences in cell internal resistance will translate to greater voltage differences depending on the load.

This is like being trapped in a loop.
If I set the balancer to only balance at a higher voltage (3.4v or above) the balancer shuts off very quickly or so because even a small load pulls the cell voltages below 3.4v.
If I set the BMS to balance at a lower voltage - then it spends most of the time balancing at somewhat less than 100%.
You mentioned in your first sentence that below 100%, cell voltages will start to diverge from what they are at 100%. But, the voltage divergence is relative. This also means that cells balanced at 98% will diverge at 100%. Because cell voltages drop below 3.4v very quickly, all meaningful balancing action takes place below 3.4v.
The only thing I know to do at this point is to slightly increase the cell balance voltage threshold in my BMS. I could set it to 3.38 volts and see what happens. The cells rarely get to the required balancing threshold voltage now - it will be even less with the higher balancing voltage threshold.

 

[jasonhc73]

Do you have WatchPower?
The panel programs names slightly differ than WacthPower.

Program;
12, Battery cut-off voltage
26, Bulk Charging voltage (C.V. voltage) - Constant Voltage
27, Float charging voltage
29 (Low DC cut-off voltage), Battery cut-off
32 Bulk charging time (C.V stage), AUt, Not stable on WatchPower, set at the panel.



Study this chart and adjust your max V, you are wasting an enormous amount of energy trying to eke out that last top 1%.

LiFePO4 Battery Voltage

Sheet5 How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries <a href="https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/">https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/</a> These are big graphs, SCROOL RIGHT -----> for more info about HIGH V...

docs.google.com

 

[jasonhc73]

This is what I find as well, I can get to 98% soc @ 3.37v per cell but it will take 4-6 hours of absorbtion, absorbtion will start at around 90% soc. If I were to charge to 3.65v I would be able to get from 90% soc to 100% in no time, but I would start to get imbalances(40-80mv) and could possibly get a runner cell. Just not worth the risk..

I do not run a bms, the Lv5048 sets the high and low voltages.

Here are what my cells look like with an active balancer, not a BMS. These are very used and the internal resistance puts them in the junk category. (BYD packs).

This only happens after reaching the top end of the charge.

Even though I bulk charge at 58.4 (3.65 x 16), I have never been able to charge these BYD cells above 54.1V. One or two cells will start running away upwards, that's when I had a working BMS that would kick in and turn off the battery. Now I just let the "float" kick in in the "program 32, auto" setting.

 

[KenDan]

Here are what my cells look like with an active balancer, not a BMS. These are very used and the internal resistance puts them in the junk category. (BYD packs).
View attachment 27514
This only happens after reaching the top end of the charge.

Even though I bulk charge at 58.4 (3.65 x 16), I have never been able to charge these BYD cells above 54.1V. One or two cells will start running away upwards, that's when I had a working BMS that would kick in and turn off the battery. Now I just let the "float" kick in in the "program 32, auto" setting.

This sounds familiar. If I don't charge above 54v, the cells all stay nice and balanced - even under fairly heavy charge/discharge current. However, if I try to push it much higher the cell voltages start to scatter.
I set the bulk mode to "manual" today and set the timer to 180 minutes. Bulk voltage was set to 54.4v. I was getting a lot more charge current than I was getting before because of the increased voltage. I saw a peak charge current of 32 amps - ~1.7kW - during a break in the clouds. Not bad for a less than optimal sun angle on my 2kW PV array. But for some reason it didn't want to maintain that high charge current even when the sun stay out. It would spike to a high value and then drop way off. Not sure what is going on there.
I'm wondering what would happen if I connected my PV array to both PV inputs tied in parallel? Anything to be gained from that?
I have not been able to get WatchPower to run on my laptop - USB issues.

 

[jasonhc73]

This sounds familiar. If I don't charge above 54v, the cells all stay nice and balanced - even under fairly heavy charge/discharge current. However, if I try to push it much higher the cell voltages start to scatter.
I set the bulk mode to "manual" today and set the timer to 180 minutes. Bulk voltage was set to 54.4v. I was getting a lot more charge current than I was getting before because of the increased voltage. I saw a peak charge current of 32 amps - ~1.7kW - during a break in the clouds. Not bad for a less than optimal sun angle on my 2kW PV array. But for some reason it didn't want to maintain that high charge current even when the sun stay out. It would spike to a high value and then drop way off. Not sure what is going on there.
I'm wondering what would happen if I connected my PV array to both PV inputs tied in parallel? Anything to be gained from that?
I have not been able to get WatchPower to run on my laptop - USB issues.

My watch power has to be unplugged and plugged in again anytime I want to use it. It is an aweful program.

I don't think it is going to matter if you split your PV array on the inputs. 2kWp on the 4kW mppt is no where near at its limits. I think you would actually be less efficient by having two strings going. But not by anything that would be noticeable.

I have 2 strings going to one input on each of the pair of my inverters. Pretty much all the time they both show the exact same watts, but almost never the same volts. 1.8 kWp on each string. ~80V and ~27 amp strings

 

[Mtour]

The charging led on the lv-5048 dash will flash when in bulk and stay on when in float mode. Could the spike in charging current be following a load on the inverter in float mode..

 

[KenDan]

I was able to reduce the voltage difference between cells at high SOC (3.5v/cell) by putting the LV5048 in utility charging mode so that it would hold that voltage for an extended period of time. That allowed me to use a resistor and a seperate power supply to force the cells to the same voltage. I think that my cells are much closer to being at a true SOC balance now. I was unable to achieve this with the solar panels doing the charging alone.

By setting the bulk charge to manual and setting the bulk timer to 4 hours I am getting a LOT more charging current at near full SOC condition. I set the bulk charge voltage to 54.4, and the float to 54.2. This is working much better for me now with the shorter days. When the days get longer and the sun gets higher in the sky, I will look at reducing the bulk charging time setting. Thanks to everyone for the comments and suggestions.

 

[Capt Bill]

The issue I am having is that the float charge voltage setting causes the charge current to drop off long before full charge is reached. The battery can be at 80% SOC and it is drawing well below the current that is available from my panels and hence charging very slowly. I know that the power is available because if I increase the load on the inverter the PV power rises to meet the load requirements while the charge current (reported by the BMS) remains unchanged.

Just found this thread; I notice the same behavior with my MPP (brand name) LV2424s (I have 3 of those All In Ones now configured for 240vac split phase). I notice setting my Float voltage from 27.0 v up to 27.2v and then 27.3v increased my Solar powered Amps to my 24v battery set, when such amps were way low when compared to Solar Watts available from my panels if I loaded em (= Solar Watts not fully used). I am charging a 24v 280 Ah LiFePO4 set, and also notice from my grid charging cycle (running my house on batteries before my solar was wired in), at maybe 120 amps IN ... my 8 x 3.2v cell were had balance voltages until I got closer to full charge (as per my view of my Chargery BMS8T's individual battery voltage Graphic+ Info.), and then a lower amp charge (like maybe 60 A or even less) was much better when seek a full top up charge. ... So experimenting with everything to hopefully dial in my best options, and setting. ... Will be studying the rest of this tread next, while wondering what MPP charge setting I will settle for (currently set at Bulk Charge: 29.0v ; Float: 27.3V ; & 22.4v Cut Off, while noting maybe a 0.5V higher reading at my battery vs the MPP reading, and noting Will Prowse's info. about how setting bulk charge downward to 28.2v could increase battery cycle life to 5000+ cycle (maybe up for project 3000 cycles?). ... Yes, to getting the Amps into my Batteries when Solar can provide those amps and the battery can safely take em !!! I looking at this same arena of questioning. ... added note: No Options on my LV2424s for increasing the bulk charge time. Interesting to me if that is an option in MPP's 48 v based system.

 

[Capt Bill]

does not work well for lifepo4 batteries as they need different absorbtion times for different depth of discharge..

My work around is to set a high float voltage and change the absorbtion time depending on the time of year.

End amps would be the way to go but unfortunately the lv5048 are not capable of this..

Interesting: Wondering if my MPP LV2424 is a good way to fully charge my 24v 280Ah LiFePO4s after studying this thread a bit; with memory of my Chargery BMS page showing individual battery voltages when up, but up with more head room to go (like possibly not fully charged yet. I AM Still a newbie testing out what my equipment and choices can do. Mtour: Wondering what float voltage did you settle on? I do not have an absorption time setting in my MPP LV2424s, but wondering what kind of options you use in the LV5048s? ... I note my LV2424 does not have a LiFePO4 charging profile to select, and seems like (maybe All MPPs ?) require selecting "user" defined battery charge setting. ... In looking at amps going IN to my battery via myLV2424s while being charged by solar/ seems like I do not see any bulk voltage time (do not know what MPP set up on automatic) , but rather see my Solar powered Amps IN seems more related to my Float V Setting (maybe incombo w Bulk setting) . Plus seem like the Grid Power Charging cycles act differently in my MPP LV2424s, by inputting Fuller Amps which can be too much (IMO) near the top (as in; what I notice) causing noticalbe cell v imbalance near top of charge if not slowed down in amps in) ... Will read some more before letting go.

 

[Mtour]

As of now my voltages are 54v float and 54.4v bulk... I got to 100% charge the other day for the first time in a month, been sitting between 30-85% soc since late September. In the summer months I run 53.6V float and 54.2v bulk as I am at 100% soc daily by noon most days.. My bank of batteries is too large to get caught up in trying to charge to 3.65v per cell, I consider a cell at 3.45v to be at 100%soc.

 

[KenDan]

In comparing the manuals for the LV2424 and LV5048 it appears that quite a few of settings are different between the two.
The LV2424 seems to have no setting for bulk charging time like the LV5048 has (program setting 32). Perhaps it judges when to switch from bulk back down to float automatically. Or maybe it doesn't...? If it does handle the switch automatically, it would be useful to know how it decides when to switch and what needs to happen to get it from float and back into bulk mode once again when the charge level drops.

One feature that the LV2424 has that the LV5048 is missing - is a time of day clock. It appears you can program the time that you want it to start and stop charging every day, as well as being able to program when to turn the inverter AC output on and off. Unfortunately, there does not seem to be a timer setting to allow switching from bulk back to float.
I think you need to apply at least 27.2v to 27.3v to those 24v 280Ah LiFePO4s in order to get to full charge in a reasonable period of time. At 27 volts you are never going to get there. Any additional voltage drop before you get to the battery (drop across cables, shunts, or BMS) is going to slow you down also.

I'm sure some will suggest going even higher - perhaps as high at 29.2 volts (3.65v per cell). That would be OK if all the cells stayed at exactly the same voltage at that extremely high SOC - but it sounds like both of us have observed that perfect balance at such a high a voltage is very difficult to achieve without one or more cells going much higher. This can easily result in those cells causing the BMS to disconnect to protect them.

 

[Capt Bill]

I think you need to apply at last 27.2v to 27.3v to those 24v 280Ah LiFePO4s in order to get to full charge in a reasonable period of time. At 27 volts you are never going to get there. Any additional voltage drop before you get to the battery (drop across cables, shunts, or BMS) is going to slow you down also. ... I'm sure some will suggest going even higher - perhaps as high at 29.2 volts (3.65v per cell).

Thanks Dan for the dialog. I see you have experience with both those MPP 48v and 24v based All In Ones. ... I am having doubts my LV2424 has a bulk charging cycle from the MPPT Solar Charger side, and may test further to see if the AC Charger does ... My doubts about the Bulk Charging come from recently observing, after recently getting my 4000 watts of Solar Panel power wired in; ... that my solar charging amps in to my batteries slow down to maybe 20 Amps or less going into my 280 Ah battery bank ... way less that Solar Panel Watts availble as I get near what is set as float charge V; then doubles to 40 amps into battery when I bumped up the Float charge config from 27.0v ... to 27.1v or 27.2v. I currently have my float config set at 27.3v, while noticing a calibration difference of about + 0.5v at my battery as compare to the info on my LV2424. ... I am wondering if more expensive equipment has bulk charging cycles for LiFePO4s that first charge up to what you set for Bulk Charging, and then lean lean back to float, voltage. I am studying here and elsewhere to understand more about that. .... I still consider myself a newbie on this equipment and these LiFePO4s, so not cemented in my takes.

Meanwhile, I am ok with some quicks. I have another 280Ah worth of LiFePO4 on way. I think I will be testing the Grid Charger's behavior to compare with Solar Charger, to see difference, & if I think the Grid Charger appears to have a bulk charge mode. ... Re your "I'm sure some will suggest going even higher - perhaps as high at 29.2 volts (3.65v per cell)." Were you referring to possibly upping the "Float Voltage" config option there? I might play with upping my float value to see if I can get more quick amps into my battery when the Sun is Making the Watts, I would like to achieve a quicker full battery charge like that. ... I am also wondering about the possible drawbacks of keeping a constant higher float voltage on LiFePO4s, in terms of longevity of LiFePO4 service . ... I'd like to see my LiFePO4 cell reach 3.6v per cell on a solar charge/ ... and then later use Will P's recommendation of 28.2v absorb charge for getting 5000+ cycles longer life out of LiFePO4s. Not sure what Will's thoughts are on "Float charge config" for that same goal. ... I see some quirks; learning by my hands on, and still studying the details . Thanks

 

[KenDan]

last

Thanks Dan for the dialog. I see you have experience with both those MPP 48v and 24v based All In Ones. ... I am having doubts my LV2424 has a bulk charging cycle from the MPPT Solar Charger side, and may test further to see if the AC Charger does ... My doubts about the Bulk Charging come from recently observing, after recently getting my 4000 watts of Solar Panel power wired in; ... that my solar charging amps in to my batteries slow down to maybe 20 Amps or less going into my 280 Ah battery bank ... way less that Solar Panel Watts availble as I get near what is set as float charge V; then doubles to 40 amps into battery when I bumped up the Float charge config from 27.0v ... to 27.1v or 27.2v. I currently have my float config set at 27.3v, while noticing a calibration difference of about + 0.5v at my battery as compare to the info on my LV2424. ... I am wondering if more expensive equipment has bulk charging cycles for LiFePO4s that first charge up to what you set for Bulk Charging, and then lean lean back to float, voltage. I am studying here and elsewhere to understand more about that. .... I still consider myself a newbie on this equipment and these LiFePO4s, so not cemented in my takes.

Meanwhile, I am ok with some quicks. I have another 280Ah worth of LiFePO4 on way. I think I will be testing the Grid Charger's behavior to compare with Solar Charger, to see difference, & if I think the Grid Charger appears to have a bulk charge mode. ... Re your "I'm sure some will suggest going even higher - perhaps as high at 29.2 volts (3.65v per cell)." Were you referring to possibly upping the "Float Voltage" config option there? I might play with upping my float value to see if I can get more quick amps into my battery when the Sun is Making the Watts, I would like to achieve a quicker full battery charge like that. ... I am also wondering about the possible drawbacks of keeping a constant higher float voltage on LiFePO4s, in terms of longevity of LiFePO4 service . ... I'd like to see my LiFePO4 cell reach 3.6v per cell on a solar charge/ ... and then later use Will P's recommendation of 28.2v absorb charge for getting 5000+ cycles longer life out of LiFePO4s. Not sure what Will's thoughts are on "Float charge config" for that same goal. ... I see some quirks; learning by my hands on, and still studying the details . Thanks

I'm new to these MPPsolar units also. I don't actually have any experience with the LV2424 - I just looked up the manual and compared it to the LV5048 manual.
When I mentioned going as high as 29.2 volts, I wasn't thinking float or bulk specifically - just how high you could go and not exceed 3.65v per cell. Ideally that would be a bulk charge voltage - which would then back off to a lower float voltage after a set period of absorption at the bulk charge voltage. If your unit does not have a specific bulk absorption mode, then you have little choice except to bump up the float voltage if you want to get to full charge in a reasonable period of time.

 

[Capt Bill]

Here's from my Further Testing on My LV2424s , while wondering how to get a fuller battery charge on my 24v 280Ah LiFePO4 battery set via available solar watts, with my Absorption set at 29.1v, and Float set at 27.3V : .and with Sun Watt available for Higher A Charging. With my lv2424s indicating about 27.2v for battery, and my BMS showing 11A solar charge IN to battery; TESTing: When I increased my float setting from 27.3 to 27.6v ; I immediately got increase to 40A battery charge ( I liked that), which then reduced to about 23A about a minute later (wondered if a cloud floating by). I obtained similar results adjusting Float Voltage upwards to 28.0v, and then even tried on a 29.0v Float voltage setting to see what would happen. My RESULTS: I saw spurts of increased of Amp IN charge at each increase, but found out I was near my top individual cell voltages (not too far away); ... and as my individual cell voltages shot up , plus got uneven near top end; ... I got high enough for one cell to trigger a BMS stop at a 3.566v reading on one cell. I then adjusted my Float v setting back downward (maybe first to 28v , then 27.5v), leaving Bulk set to around 29.2v . or 29.0v. It then Seemed to me like my Solar was Charging at higher than my Float voltage setting (a short cycle, but contiunued to trigger my BMS alarm for one cell v too high). While am still wondering about exact automatic nature of MPP's Lv2424 MPPT solar charging profile; I think I have been near a full battery charge when my Lv2424 was showing 27.2 or 27 v for Battery Voltage. I am thinking that way... because it did not take very long (maybe 4 -5 minutes) at 40 A / ...varying to less Amps IN (I saw reducesd Amperage IN near top end) to see my Battery to get near a 3.65 top v for the highest individual cell (with the other were not far behind) .. So For Now: I am settling on Absorption Charge setting of 28.6v / & a 27.3v for my Float Voltage setting; and may check more on this aspect of my set up later. I like the idea of using the lower bandwidth of my LiFePO4s voltage to get a longer life 5000+ charge cycles out of them; and noticed Will P recommends a 28.2 v Absorption setting for that 5000+ cycle life idea (otherwise I have info aiming me at 29.0v Absorption/ 27.2 Float ) ... from what I have copied a few months ago/ printed out / and have posted near my batteries. I notice there is some variation of opinions on exact Battery settings; I also copied ranges recommeneded by others I have learned from, while ... Learning More All the Time :+)

 

[KenDan]

Here's from my Further Testing on My LV2424s , while wondering how to get a fuller battery charge on my 24v 280Ah LiFePO4 battery set via available solar watts, with my Absorption set at 29.1v, and Float set at 27.3V : .and with Sun Watt available for Higher A Charging. With my lv2424s indicating about 27.2v for battery, and my BMS showing 11A solar charge IN to battery; TESTing: When I increased my float setting from 27.3 to 27.6v ; I immediately got increase to 40A battery charge ( I liked that), which then reduced to about 23A about a minute later (wondered if a cloud floating by). I obtained similar results adjusting Float Voltage upwards to 28.0v, and then even tried on a 29.0v Float voltage setting to see what would happen. My RESULTS: I saw spurts of increased of Amp IN charge at each increase, but found out I was near my top individual cell voltages (not too far away); ... and as my individual cell voltages shot up , plus got uneven near top end; ... I got high enough for one cell to trigger a BMS stop at a 3.566v reading on one cell. I then adjusted my Float v setting back downward (maybe first to 28v , then 27.5v), leaving Bulk set to around 29.2v . or 29.0v. It then Seemed to me like my Solar was Charging at higher than my Float voltage setting (a short cycle, but contiunued to trigger my BMS alarm for one cell v too high). While am still wondering about exact automatic nature of MPP's Lv2424 MPPT solar charging profile; I think I have been near a full battery charge when my Lv2424 was showing 27.2 or 27 v for Battery Voltage. I am thinking that way... because it did not take very long (maybe 4 -5 minutes) at 40 A / ...varying to less Amps IN (I saw reducesd Amperage IN near top end) to see my Battery to get near a 3.65 top v for the highest individual cell (with the other were not far behind) .. So For Now: I am settling on Absorption Charge setting of 28.6v / & a 27.3v for my Float Voltage setting; and may check more on this aspect of my set up later. I like the idea of using the lower bandwidth of my LiFePO4s voltage to get a longer life 5000+ charge cycles out of them; and noticed Will P recommends a 28.2 v Absorption setting for that 5000+ cycle life idea (otherwise I have info aiming me at 29.0v Absorption/ 27.2 Float ) ... from what I have copied a few months ago/ printed out / and have posted near my batteries. I notice there is some variation of opinions on exact Battery settings; I also copied ranges recommeneded by others I have learned from, while ... Learning More All the Time :+)

The thing that will probably help the most to get to full charge quickly without having a few cells go higher and trip the BMS cell protection is to try to get a better top balance. In order do do this you need to hold a high state of charge (~3.5v/cell or higher) for an extended period of time so that the balancer has time to work. If the cell voltages drop to 3.4v/cell or less, voltage balancing does virtually nothing. If you can get the cell voltages equalized at their upper voltage limit, you can charge with a higher bulk (or float) voltage with less chance of getting higher voltage on individual cells that will trip the BMS into protection mode.
I was able to speed up the process by using a resistor to pull down high cells and a power supply to pull up lower cells. It all depends on how well the balancer in your BMS works.

 

[Capt Bill]

The thing that will probably help the most to get to full charge quickly without having a few cells go higher and trip the BMS cell protection is to try to get a better top balance. In order do do this you need to hold a high state of charge (~3.5v/cell or higher) for an extended period of time so that the balancer has time to work. If the cell voltages drop to 3.4v/cell or less, voltage balancing does virtually nothing. If you can get the cell voltages equalized at their upper voltage limit, you can charge with a higher bulk (or float) voltage with less chance of getting higher voltage on individual cells that will trip the BMS into protection mode.
I was able to speed up the process by using a resistor to pull down high cells and a power supply to pull up lower cells. It all depends on how well the balancer in your BMS works.

Hi Ken, I am still not fully aware how my lv2424s internal bulk charge cycle is set up to run and work, or if my 280Ah LiFePO4s need an extended period of bulk charge (that I may or may not be getting via solar charge?) to be fully charged, ... before my equipment throttle back to my float charge setting; ... yet am ok with what I have set up for now. Re: My BMS's cell balancing (in contrast to one point you made: I have the Chargery BMS8T, and within it's balance configuration, it has option to balance on charge (cycles); on discharge (cycles), and in storage mode. I was kind of surprised the default balance options were all "off" on my particular 1st hands on look see lesson (I had to turn em on). I currently have them set up to balance on "charge" and "storage" modes, ... and my cells seem to stay in good balance (via an easy visual Chargery on one of its' display pages for individual cell v levels). I like noticing my cells stay in good balance, with the exception of very near top end of a battery charge cycle (especilly at higher amp IN), or the low end of being discharged (an arena I do not need to go for either). ... So, I see your statement: "If the cell voltages drop to 3.4v/cell or less, voltage balancing does virtually nothing." as not matched up with what I believe my BMS is doing. I wonder which BMS you are talking about that acts that way? ... I think we agree it good to keep the cells in balance; and for me that seems to be related to setting up to use the widest middle of bandwidth of the LiFePO4 that does not produce the big cell variations. ... and I am still a newbie/ and now have set my LV2424 Bulk Charge to 28.6v / Float 27.3v /and Cut Off 22.6v ... thinking that is a good ball park (though I wonder about my cut off, cause it seems like my LV2424 gives a low battery fault sometimes above that battery voltage) ... I am still in learning by testing and dialing in mode. Good to compare notes :+)

 

[KenDan]

No balancer will do much when you are on the flat part of the discharge curve. You have to get the voltage on all cells up to 3.5v or higher in order for small differences in the state of charge of each cell to be seen in their voltage. Then the balancer can do something useful.

 

[Capt Bill]

No balancer will do much when you are on the flat part of the discharge curve. You have to get the voltage on all cells up to 3.5v or higher in order for small differences in the state of charge of each cell to be seen in their voltage. Then the balancer can do something useful.

Ken, I see results from my BMS, that give me a contrasting view to yours: Re: "have to get the voltage on all cells up to 3.5v or higher in order for small differences in the state of charge of each cell to be seen in their voltage." for how my Chargery BMS8T seems to handle cell balancing. I notice in the "Balance Parameter options, one can also select at what mv difference between cells (I think) before balancing proceeds (I selected 24mv know this BMS is not accurate in the smaller detail of mv measurements/ but good enough for me) ... and Start Voltage (selected 3.5v/ the higest value I can select) which appears to have my BMS balancing cell voltage appears of each cell below 3.5 volts. I say that, because I notice my cell being well balanced via my BMS visual info. , until my charging at higher amps (like today at 80 -90 amps on my 24v 280ah LiFePO4s) get to 3.5v, then I see one cell peaking up before the others triggering my BMS to cut charge at one cell reaching over 3.65v I set as my top limit. Not bid deal to me that w have a different opinion of when the balancing act happens, but glad to see mine working in the mid bandwidth range of my LiFePO4s that I plan on using the most. :+)