Ok you win... Cell imbalance

[Quattrohead]

If you need 15 amps worth of cell balancing you have way bigger problems.

 

[Aussieguy]

original post has 4 x 304ah cells in parallel
larger 15amp balancer can be an option

 

[fafrd]

15amp balancer...... will get it done ?

Andy also had videos for:
you can leave balancer attached and externally switch it on/off

use a voltage sensing relay
at x volts turn on a balancer
at y volts turn off

don't know if the 15amp is a candidate for leaving it permanently attached?

Mine is only 5A but it’s been left attached 24/7 for 3 weeks now and counting (to good effect)…

 

[fafrd]

If you need 15 amps worth of cell balancing you have way bigger problems.

Take 280Ah cells, compute the %SOC mismatch you’d like your balancer to take care of daily, then calculate the delta voltage that translates to as far as one unbalanced cell versus all the others.

Now take your maximum balance current of 15A and multiply that by Vmismatch / 100mV to calculate actual balance currents involved when you are within your limits.

Last night, my lowest cell was 11mV below the other cells, so my 5A active balancer was only balancing with current below 550mA over the ~12 hours between sundown and sunrise. < 0.55A x 12h = < 6.6Ah which is less than 1.2% of the rated capacity of my cells…

I’m still in the midst of rebalancing after leaving my battery alone for over 6 months.

When I started 3 weeks ago, delta voltage to the lowest cell was 69mV so balance current to that cell was 3.45A…

As the balancer continues to make progress through daily overnight balance cycles (the active balancer does nothing when all cells are in the flats above 3.275V), delta V will continue to drop and actual balance currents with them.

Once I’m down under 0.5% SOC mismatch in the lower knee below 3.2V, Vmismatch will be under 3.6mV at which point my 5A active balancer will only be delivering balance current of 180mV.

The 5A or 15A balance current of an Active Balancer only becomes relevant when there is a significant imbalance to correct - when maintaining an already-balanced battery the balance currents are minuscule…

 

[740GLE]

I’d be shocked 100mv was high enough delta to cause a 5a or 15a balancer to max out, I thought the delta needed to be much higher to max out these balancers.

 

[fafrd]

I’d be shocked 100mv was high enough delta to cause a 5a or 15a balancer to max out, I thought the delta needed to be much higher to max out these balancers.

No, it’s 100mV delta v to all other cells to get the full 5A balance current (never looked at the 15A active balancer).

With 8S, all 7 other cells need to be 100mV above or below the pit of balance cell to get the full 5A balance current (so 714.3mA balance current from each of the balanced cells).

I believe it’s actually total correlated imbalance of 700mV so one of the nearly-balanced cells can be higher than 100mV from the unbalanced cell compensated for by another neatly-balanced cell a bit under 100mV from the unbalanced cell.

100mA is a huge imbalance. In the flats (3.3V) it corresponds to ~40% SOC, above 3.4V it corresponds it corresponds to ~9.5% SOC and under 3.2V it corresponds to ~11% SOC…

 

[fafrd]

Interesting article on bottom-balanced LiFePO4 batteries for those that are curious: https://cleversolarpower.com/bottom-balancing-lifepo4/

He recommends LVD of 2.65V / cell or 21.2VDC for an 8S battery.

My inverter has a minimum DC voltage of 21.0V, so accounting for I^2R losses in the cabling, I could conceivably get down close to that level if I bottom-balance well-enough.

2.65V corresponds to less than 2.5% SOC…

At night, I’m draining a pretty constant ~3% SOC per hour, so 2.5% SOC still gives me a ~50 minute buffer before BMS protection kicks-in and shuts down my inverter brutally.

Note that he also says BMS top-balancing should be disabled if using a bottom-balanced battery (which is pretty much the defacto result if you leave an Active Balancer attached always and never charge up over 3.375V/cell or 27.0V for an 8S battery) (or leave your BMS balancer active, for that matter).

If you only charge up to 90% / 3.350V / cell (26.8V for an 8S battery) your active balancer (or BMS top-balancer) will have meaningless balance currents everywhere but down in the lower knee under 3.2V / cell (25.6V for an 8S battery).

 

[740GLE]

I’ve always had the mindset a balancer adjusts small amounts over time, so if there is 100mv in the flat of the charge, there’s more to the story than an active or passive balancer can solve.

Thanks for the info, I’ve never tested mine but have seen 60mv max delta over all the cells with only about 30mv cell to cell. This was only observed at above 3.4v



No, it’s 100mV delta v to all other cells to get the full 5A balance current (never looked at the 15A active balancer).

With 8S, all 7 other cells need to be 100mV above or below the pit of balance cell to get the full 5A balance current (so 714.3mA balance current from each of the balanced cells).

I believe it’s actually total correlated imbalance of 700mV so one of the nearly-balanced cells can be higher than 100mV from the unbalanced cell compensated for by another neatly-balanced cell a bit under 100mV from the unbalanced cell.

100mA is a huge imbalance. In the flats (3.3V) it corresponds to ~40% SOC, above 3.4V it corresponds it corresponds to ~9.5% SOC and under 3.2V it corresponds to ~11% SOC…

See that’s where I was always under the mindset that balancing should only happen at the knees and balance frequently and balance often.

 

[fafrd]

I’ve always had the mindset a balancer adjusts small amounts over time, so if there is 100mv in the flat of the charge, there’s more to the story than an active or passive balancer can solve.

Thanks for the info, I’ve never tested mine but have seen 60mv max delta over all the cells with only about 30mv cell to cell. This was only observed at above 3.4v



No, it’s 100mV delta v to all other cells to get the full 5A balance current (never looked at the 15A active balancer).


See that’s where I was always under the mindset that balancing should only happen at the knees and balance frequently and balance often.

Absolutely. Active balancers don’t do much until you’re pretty far into the upper knee of lower knee.

If you set LVD above 20% SOC / 3.2V / cell, your active balancer will balance very slowly at the bottom so you are likely to end up with a top-balanced battery if you charge into the upper knee often.

If you program boost voltage below the upper knee, let’s say 90% SOC or 3.275V / cell, your active balancer will balance very slowly at the top so you are likely to end up with a bottom-balanced battery if you discharge into the lower knee often.

I’m actually in between these two scenarios: I never ever charge my 8S battery up to 27.0V (3.375/cell) and rarely even get up to 26.8V (3.5V/cell) and my LVD is currently set at 26.0V (3.25V/cell).

So my battery pretty much remains in the flats but hardly spends any time near the top while is gets discharged to LVD nightly where is spends ~12 hours balancing overnight.

So my battery is slowly becoming better and better bottom-balanced.

Last night, the lowest cell was 10.75mV below the average of the other 7 cells.

So for 12 hours that low cell was charged with ~5A x 10.75mA/100mA = 537.5mA or 6.45Ah over 12 hours.

That’s a little over 1% SOC so I’ll see if the remaining ~4.3% unbalance of that cell continues to close over the next week…