How many hours to top balance cells?

[AlexanderKristiansen]

I have 8 x 220Ah LiFePo4 cells and I use the Overkill BMS.

I can charge the pack to 28.0V before one of the cells peak at my 3,63 limit and the charging stops.

Attached are 2 screenshots from the xiaoxiang-app.

1. Are these cells balanced?

2. How long many hours will it take to top balance these cells with the Long Wei power supply (30V 10A)?

 

[Steve_S]

Top Balancing cells is done at 3.60-3.65 volts per cell.
The 8 cells would be setup in Parallel, charged at 3.6X volts till the amps drawn falls to below 2.0A
Once all the cells have reached that level of charge & saturation, they will naturally settle to 3.550-3.500 within an hour & half or so.
Then you can install the Cells in Series and you have your battery pack ready to go.
The cells as shown above are NOT in Balance - typically "in-balance" means there is less than 30mv between all of them at worst.

 

[Supervstech]

I have 8 x 220Ah LiFePo4 cells and I use the Overkill BMS.

I can charge the pack to 28.0V before one of the cells peak at my 3,63 limit and the charging stops.

Attached are 2 screenshots from the xiaoxiang-app.

1. Are these cells balanced?

2. How long many hours will it take to top balance these cells with the Long Wei power supply (30V 10A)?

View attachment 45634

In order to balance the pack as you have it arranged, you are going to have to discharge and recharge multiple times over and over... the balance current on the BMS is very small...
as Steve said, parallel the cells, and charge to 3.65 until charge current drops to zero.
Since you have them all close, it shouldn’t take long on the few stragglers.
I would set the 5 lowest ones in parallel and charge until it reaches 3.52 then add in the 6th lowest one, and charge until they stop drawing amps... then tie them all together in series.

 

[Diysolar123]

as the others have pointed out, you have over 200mv of difference in your cells, no they are not balanced at all.

while all BMS's seem to say they "balance", I have never seen them actually do anything... and that is measuring currents on balance leads down to the 10ma range. Active balancers (the good ones) would work but do not even bother unless you are willing to spend $100+; the "Cheap ones" are rather pointless.
You can hookup passive balancers like an isdt batgo, they will balance your cells to within millivolts, they are slow but work. You can also "charge balance" your cells with something like a isdt Q8 max; this also does a good job at hands off balancing without having to reconfigure your pack, it just uses the balance leads. I have used both of the isdt devices and they work quite nicely, hands off, just plug in and let it work which will get your batteries within a few millivolts of each other. FYI, make sure to disconnect your BMS of course as the balance leads will go to the isdt device.

or you can do the "reconfigure" way, you have done the first step which is the serial charge until the first cell hits the limit. Then you tear it down, let it rest for an hour, and wire in parallel and charge again. Keep in mind that your charging current in parallel is divided across however many cells need the current. It can take a long time to get from 3.4v to 3.6.

supervstech suggestion about grouping your cells is the best way. start with the lowest cells that are within a few millivolts of each other, let them charge up and as you reach the point where they equal to the next lowest voltage battery...you add that one to the parallel set. You do NOT want to parallel batteries that have widely different voltage. This is why I like the isdt active balance charger way...its attach and forget hehe

 

[AlexanderKristiansen]

In order to balance the pack as you have it arranged, you are going to have to discharge and recharge multiple times over and over... the balance current on the BMS is very small...
as Steve said, parallel the cells, and charge to 3.65 until charge current drops to zero.
Since you have them all close, it shouldn’t take long on the few stragglers.
I would set the 5 lowest ones in parallel and charge until it reaches 3.52 then add in the 6th lowest one, and charge until they stop drawing amps... then tie them all together in series.

Thanks a lot, @Supervstech !

I read somewhere that it's important to either connect the leads to the cells before or after you turn the power supply on. Dont remember which way is the right way. Do you know which way is correct?

And since I am at my off-grid cabin I need to power the power supply with my generator. How many hours do you think it will take? 5-10-15-20?

 

[John Frum]

1. Are these cells balanced?

Those cells have a delta of >250mv so they are quite un-balanced.

 

[Supervstech]

I am pretty sure you want to set the values on the power supply before connecting the load.

 

[time2roll]

How many hours do you think it will take? 5-10-15-20?

Overkill BMS is just a few milliamps to maintain balance. I think you meant DAYS not HOURS.

At this point consider an active balancer that moves 1.2+ amps
Or manually bleed off power with maybe 1 to 3 ohm resistor from the high cell.
Or using a power supply that can be set to 3.65 to charge the low cell.

Ebay 1.2 amp balancer $20

 

[Supervstech]

Overkill BMS is just a few milliamps to maintain balance. I think you meant DAYS not HOURS.

At this point consider an active balancer that moves 1.2+ amps
Or manually bleed off power with maybe 1 to 3 ohm resistor from the high cell.
Or using a power supply that can be set to 3.65 to charge the low cell.

Ebay 1.2 amp balancer $20

Low CELLS, plural... 6 in total are out of balance still...

 

[John Frum]

Low CELLS, plural... 6 in total are out of balance still...

I would use a capacity tester to drop those 2 high cells down.
Then configure the bms...
app->function_setting->balance_enable=true
app->function_setting->charge_balance=false
and let it sit for a day.
See how close it gets.

Don't forget to set
app->function_setting->charge_balance=true
for normal operation.

 

[AlexanderKristiansen]

Overkill BMS is just a few milliamps to maintain balance. I think you meant DAYS not HOURS.

At this point consider an active balancer that moves 1.2+ amps
Or manually bleed off power with maybe 1 to 3 ohm resistor from the high cell.
Or using a power supply that can be set to 3.65 to charge the low cell.

Ebay 1.2 amp balancer $20

@time2roll I'm not referring to balancing with the Overkill BMS, but with the Long Wei power supply (30V 10A)

 

[Supervstech]

@time2roll I'm not referring to balancing with the Overkill BMS, but with the Long Wei power supply (30V 10A)

You cannot balance a series pack reliably in any measurable timeframe.
The cells will need to be manually drained down, or all put in parallel, and charged at 3.65V until full... and yes... days to charge...

 

[AlexanderKristiansen]

You cannot balance a series pack reliably in any measurable timeframe.
The cells will need to be manually drained down, or all put in parallel, and charged at 3.65V until full... and yes... days to charge...

@Supervstech sorry, I'm a bit confused, earlier in the thread you wrote "Since you have them all close, it shouldn’t take long on the few stragglers."??

 

[Supervstech]

Correct... compared to charging them ALL from 50% ish, just tackling the stragglers paralleled with a 3.65V power supply shouldn’t take long, but most of your low cells are still in the 3.2V range, and that could be a long time charging with 10A...

 

[time2roll]

@time2roll I'm not referring to balancing with the Overkill BMS, but with the Long Wei power supply (30V 10A)

If the cell is 10% low then a couple hours at 10 amps. May as well just get started.
Set the charger at 3.65 to boost them up individually. Or are you going to break it all down and parallel them all at once?

 

[AlexanderKristiansen]

If the cell is 10% low then a couple hours at 10 amps. May as well just get started.
Set the charger at 3.65 to boost them up individually. Or are you going to break it all down and parallel them all at once?

I think I will do as @Supervstech suggested, charge the lowest cells first.

But the best thing would be to bring all the cells closer in voltage. Is there a way I can discharge the cells with the highest voltage? Then I could put them back in series, add the BMS and them charge them with my generator. That way the voltage of the cell would be more equal and the parallel charging would go quicker. Any suggestions?

 

[Gazoo]

@Supervstech sorry, I'm a bit confused, earlier in the thread you wrote "Since you have them all close, it shouldn’t take long on the few stragglers."??

It will not take long. Your lowest cell is 3.395 volts. You don't even need to disconnect the pack. You can charge each cell individually to 3.65 volts.
Turn balancing off on the BMS and disconnect all loads from the BMS.

Or you can connect the cells in parallel and charge to 3.65 volts. I would bring the low cells up to match the higher cells before doing that.

Finally you can put a load on the cells so they will be between the knees. Then disassemble the pack and charge in parallel.

Always make sure you set the voltage on the power supply before connecting to the cells.

 

[Ampster]

An alternative would be to set your CV (Absorb) set point to 27.5 volts and not worry about the 5% capacity you are losing. That is 3.44 volts per cell. My CV set point is 3.4 volts per cell on a 2P16S pack.

 

[AlexanderKristiansen]

It will not take long. Your lowest cell is 3.395 volts. You don't even need to disconnect the pack. You can charge each cell individually to 3.65 volts.
Turn balancing off on the BMS and disconnect all loads from the BMS.

Or you can connect the cells in parallel and charge to 3.65 volts. I would bring the low cells up to match the higher cells before doing that.

Finally you can put a load on the cells so they will be between the knees. Then disassemble the pack and charge in parallel.

Always make sure you set the voltage on the power supply before connecting to the cells.

@Gazoo can I leave them in their current position, disconnect the wires and charge the cells one by one to 3,6V?

 

[John Frum]

@Gazoo can I leave them in their current position, disconnect the wires and charge the cells one by one to 3,6V?

You don't even need to disconnect the wires.
I'll let someone else explain that today.