Why do lifepo4 BMS cell balance programs require a charge current be present in order to work? I have an hour absorption at 14.4 v for my 12v battery but for most of the time the current is insufficient to allow balancing, which seems counter productive!
Why charge current is required for balancing.
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RandyP
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Which battery, which BMS ?
Why do you think LiFePO4 BMS cell balance programs require a charge current to work ?
As I understand it, there is passive balancing (running runner cell current thru a resistor to balance cell voltages)
and active balancing (Moving current from high v cells to low v cells and vice versa).
Cell top balancing works best when the cell voltage starts up the knee of the SOC/cell voltage curve, as I understand it.
Why do you think LiFePO4 BMS cell balance programs require a charge current to work ?
As I understand it, there is passive balancing (running runner cell current thru a resistor to balance cell voltages)
and active balancing (Moving current from high v cells to low v cells and vice versa).
Cell top balancing works best when the cell voltage starts up the knee of the SOC/cell voltage curve, as I understand it.
RCinFLA
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Strickly speaking, you don't need charging current to balance.
However, you should not balance until a cell is above 3.4v to get a definitive determination of which cells have greater state of charge.
Much above 3.4v cell voltage means you must have charge current to achieve that high of cell voltage.
So by default, it pretty much means you will be charging cell when balancing is going on.,
Balancing with an active balancer with higher balancing current can cause significant cell overpotential bump up due to cell current. Where there is little to no actual charging current the balance current will be the dominate source of cell overpotential voltage and balancing is only applied to selected cells. For five amps balancing and little to no charging current, this can bump up or slump a cell getting a balance push or pull to be plus or minus 15-25 mV of cell voltage shift.
When there is significant charge current, which will be the same for all series connected cells, the higher charge current will dominate the cells overpotential bump and the balancing bump or slump will be insignificant.
There is one condition that really needs balancing with no charge current. If you have a cell go overvoltage and BMS shuts off charging current, you want the balancing to continue to discharge the overvoltage cell to get the BMS out of its shutdown condition.
Many BMS's have a very confusing named function mode switch called 'charge balance'. It requires the BMS to detect charge current before balancing is allowed when this mode is activated.
This video talks about this function.
However, you should not balance until a cell is above 3.4v to get a definitive determination of which cells have greater state of charge.
Much above 3.4v cell voltage means you must have charge current to achieve that high of cell voltage.
So by default, it pretty much means you will be charging cell when balancing is going on.,
Balancing with an active balancer with higher balancing current can cause significant cell overpotential bump up due to cell current. Where there is little to no actual charging current the balance current will be the dominate source of cell overpotential voltage and balancing is only applied to selected cells. For five amps balancing and little to no charging current, this can bump up or slump a cell getting a balance push or pull to be plus or minus 15-25 mV of cell voltage shift.
When there is significant charge current, which will be the same for all series connected cells, the higher charge current will dominate the cells overpotential bump and the balancing bump or slump will be insignificant.
There is one condition that really needs balancing with no charge current. If you have a cell go overvoltage and BMS shuts off charging current, you want the balancing to continue to discharge the overvoltage cell to get the BMS out of its shutdown condition.
Daly BMS is one example.
Many BMS's have a very confusing named function mode switch called 'charge balance'. It requires the BMS to detect charge current before balancing is allowed when this mode is activated.
This video talks about this function.
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If the battery will be held at 14.40 volts for an hour it may as well balance at anything over 13.80 volts. Can the BMS settings be updated?
Are the cells even out of balance? Most would say 14.4 for an hour is more stress than needed. Once current goes to zero it may as well drop to float.
Are the cells even out of balance? Most would say 14.4 for an hour is more stress than needed. Once current goes to zero it may as well drop to float.
RandyP
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Could it be that all the cells in the battery are already well balanced (no need to balance any further) ?
Battery is SOK 100AH, and I'm not seeing any improvement of DV from 0.114 mv over four charge cycles and one cell gets close to 3.69 v and C MOS shutdown.
I have no control over the BMS balance settings. I am trying to ensure my system will not require regular physical maintenance before I install it in my RV.
I guess my question is, why would a manufacturer include this 1 amp charge requirement anyway? Wouldn't the cell voltage requirement of >3.4 v be good enough on it's own?
I have no control over the BMS balance settings. I am trying to ensure my system will not require regular physical maintenance before I install it in my RV.
I guess my question is, why would a manufacturer include this 1 amp charge requirement anyway? Wouldn't the cell voltage requirement of >3.4 v be good enough on it's own?
RCinFLA
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In my opinion, requiring charge current to be detected to enable balancing is a stupid function.
BMS shunt measurement of current dynamic range and accuracy is not really good enough to detect low current.
If your charger starts to taper bulk current as it approaches absorb voltage setting it may cause BMS to stop balancing just in the range of voltage where you need balancing to be active. During absorb level battery charge current taper off, it may cause BMS to prematurely stop balancing even though it is in prime-time balancing absorb voltage mode of charger.
I think the original intent of requiring charge current was to prevent the balancer from dragging all the cells down to 3.4v after charging stopped. I don't have a problem with this as cells are still pretty fully charged at 3.4v. If you don't like it then bump start of charge up a little. But realize the higher voltage you start balancing, the less runway until highest SoC cell reaches overvoltage before balance has been achieved.
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If there is a way to finish the charge at a slower rate the balancing would be more effective. Maybe a 50w solar panel or a half amp trickle charger to take it from 13.60 to 14.40 volts.
Reality is the BMS balance current is probably 100 to 200 mA and would take 5 to 10 hours to move the balance by one amp-hour. If a cell is off by 5 or 10 amp-hours this could take much longer than a few days.
And of course this is one of the reasons so many build their own battery and select a programmable BMS.
Another option would be to open the battery to add an active balancer to work alongside the BMS. Of course this probably voids the warranty.
Reality is the BMS balance current is probably 100 to 200 mA and would take 5 to 10 hours to move the balance by one amp-hour. If a cell is off by 5 or 10 amp-hours this could take much longer than a few days.
And of course this is one of the reasons so many build their own battery and select a programmable BMS.
Another option would be to open the battery to add an active balancer to work alongside the BMS. Of course this probably voids the warranty.
Pete@SolarBatteryBarn
New Member
My Daly BMSs balance at 36mA only while charging. They have worked perfectly since installed. They have no settings and I have no reason to change anything. I use them on several batteries over 40kWh each.
It sounds like you may have an issue with either your cells, connections, BMS, or settings?
Have you done a slow charge?
Once they are initially balanced, balancing should not need to do much.
It sounds like you may have an issue with either your cells, connections, BMS, or settings?
Have you done a slow charge?
Once they are initially balanced, balancing should not need to do much.
RCinFLA
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The most common issue with self-contained 12v LFP battery is users allows their battery to get significantly out of balance due to not bringing charge up to 14.2-14.4v often enough.
When balancing gets bad enough where they start complaining of loss of capacity or try a full charge only to find the battery stops working (BMS cell overvoltage cutoff), it can take a very long time of days to week at 14.2v absorb level to get the four cells into balance again.
Depending on charge current rate, it only takes 0.5% to less than 1% of cell SoC imbalance to start having overvoltage cell BMS cutouts. Variations in self discharge leakage rate between cells can get this much imbalance in 6-10 months.
When balancing gets bad enough where they start complaining of loss of capacity or try a full charge only to find the battery stops working (BMS cell overvoltage cutoff), it can take a very long time of days to week at 14.2v absorb level to get the four cells into balance again.
Depending on charge current rate, it only takes 0.5% to less than 1% of cell SoC imbalance to start having overvoltage cell BMS cutouts. Variations in self discharge leakage rate between cells can get this much imbalance in 6-10 months.
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12VoltInstalls
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An ignorant question I have.
Is it a safe assumption to say that all BMS’s have the ability to balance internal cells of a manufactured, cased battery (provided ’we’ give them the 14.2-14.4 conditions)??
RCinFLA
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With very high certainty, yes. Fixed 3.4v balance trigger BMS has a +/- tolerance based on their supervisor I.C.'s so an individual cell might be a little different than 3.4v.
There still needs to be some voltage drop across balance dump resistor to get much current so voltage must be greater than balance trigger voltage.
The greater the start of balance voltage, the less runway time there will be before the highest SoC cell hits overvoltage before balance has been achieved.
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Sleeper85
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@RCinFLA
Let's take the example of a 48V battery made up of unbalanced 280Ah cells.
Charging at 55.2V
Start Balance V. at 3.45V
OVP at 3.65V
From the moment a cell reaches 3.45V first with a high charging current.
How many amps should the charging current be reduced to prevent the cell from reaching OVP?
Is there a rule like Ah * 0.xC ?
Let's take the example of a 48V battery made up of unbalanced 280Ah cells.
Charging at 55.2V
Start Balance V. at 3.45V
OVP at 3.65V
From the moment a cell reaches 3.45V first with a high charging current.
How many amps should the charging current be reduced to prevent the cell from reaching OVP?
Is there a rule like Ah * 0.xC ?
RCinFLA
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Charging current reduction algorithm depends on given BMS, adjustment resolution of charging device and how it responds to request for charging current reduction by BMS.
Generally, the closer a cell gets to overvoltage limit the more charge current reduction requested. Some BMS's with comm just tell charging device to stop charging to give BMS time to bleed down runner then re-enable charging.
BMS with comm to charging device and only 50-200 mA of resistor dump balancing current is limited.
The lower the ratio of charge current to balance current the better the chance to avoid an overvoltage charge shutdown.
High charging current and low BMS bleed current does not give much runway to overvoltage charging shutdown.
You should always try to avoid getting a BMS charging shutdown from cell overvoltage. Many BMS's don't manage the discharge override well when under a charge shutdown condition. Also a charging shutdown may cause charger to abort charging early.
When discharge current demand exceeds a few amps, the BMS must override the charging block and reenable the charging path MOSFET to avoid too much MOSFET heating due to its diode drop during discharge when charging is blocked.
If you happen to hover in this discharge current range, the BMS is making decision to re-disable and re-enable this charging MOSFET block by current level and current direction measurement. The turn off and turn on time of the pass MOSFET's in a BMS is relatively slow which can result in a short 'squirt' of charging current to battery with an already overvoltage cell during the re-enable/re-disable charge blocking MOSFET. If done enough times due to discharge current variation, it can overcharge an overvoltage cell and damage it.
This video shows this happening on a JK BMS. Sorry you have to search for title of video on Youtube.
Watch the 'charging' ON/OFF indicator in video bouncing on and off due to discharge current level.
" Jk BMS failed to protect battery "
Generally, the closer a cell gets to overvoltage limit the more charge current reduction requested. Some BMS's with comm just tell charging device to stop charging to give BMS time to bleed down runner then re-enable charging.
BMS with comm to charging device and only 50-200 mA of resistor dump balancing current is limited.
The lower the ratio of charge current to balance current the better the chance to avoid an overvoltage charge shutdown.
High charging current and low BMS bleed current does not give much runway to overvoltage charging shutdown.
You should always try to avoid getting a BMS charging shutdown from cell overvoltage. Many BMS's don't manage the discharge override well when under a charge shutdown condition. Also a charging shutdown may cause charger to abort charging early.
When discharge current demand exceeds a few amps, the BMS must override the charging block and reenable the charging path MOSFET to avoid too much MOSFET heating due to its diode drop during discharge when charging is blocked.
If you happen to hover in this discharge current range, the BMS is making decision to re-disable and re-enable this charging MOSFET block by current level and current direction measurement. The turn off and turn on time of the pass MOSFET's in a BMS is relatively slow which can result in a short 'squirt' of charging current to battery with an already overvoltage cell during the re-enable/re-disable charge blocking MOSFET. If done enough times due to discharge current variation, it can overcharge an overvoltage cell and damage it.
This video shows this happening on a JK BMS. Sorry you have to search for title of video on Youtube.
Watch the 'charging' ON/OFF indicator in video bouncing on and off due to discharge current level.
" Jk BMS failed to protect battery "
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RCinFLA
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LFP positive cathode is the most rugged cathode of all lithium-ion batteries. It has no problems with full charge due to the iron providing vertical lattice support which other chemistry lithium ion cathodes do not have. At full charge most of the lithium has been extracted from cathode leaving the cathode support lattice weak due to lack of vertical support.
3.65v is a set limit that is pretty conservative based on high cell temp and high charging current. At less than 0.2C charging and 25 degs C you could go to 4.0v maximum absorb voltage with insignificant degradation to LFP cell.
(not that I am recommending you regularly use 4.0v absorb voltage).
Electrolyte decomposition goes up exponentially above 4.3v cell voltage.
3.65v is a set limit that is pretty conservative based on high cell temp and high charging current. At less than 0.2C charging and 25 degs C you could go to 4.0v maximum absorb voltage with insignificant degradation to LFP cell.
(not that I am recommending you regularly use 4.0v absorb voltage).
Electrolyte decomposition goes up exponentially above 4.3v cell voltage.
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