How to top balance correctly with cells that i dont trust.

[Bitbin]

So ive got 8 100ah (supposedly) cells from great power which came from a 48v 200ah battery that had 1 bad cell. I dont know if these are grade b or c. And i dont have a way to capacity test each. They are all made in either 2022 and 2023. As you guys can see in the screenshot they are all right now fully charged between 3.45 and 3.55 approx.

Im not getting full capacity but around 75-80 ah because when the lowest cell reaches 2.5 the highest is still 3.15 and the others between 3 and 3.1. What should i do step by step to top balance them knowing they may have diffrent capacities.(they are the same sorts of cells)

If i where to parrallel them now while they are full wont the weaker cell make the other ones also weaker? Or can you top balance cells in parrallel even if the cells arent the same capacity. I would love to see some detailed info about this. Ive been searching here and on google but im not finding my exact situation it feels.

Thanks

 

[Bitbin]

I currently dont have a power supply but im gonna buy one in these couple of days. Locally I can only find 3 brands

Korad kd3005d
Longwei 30v 5amp
Kuaiqu 30v 5amp

Which one do you guys recommend. They are all around 60$ but the korad felt the heaviest by far.

 

[hwy17]

Yes you can top balance cells of different capacity together.

 

[Bitbin]

Yes you can top balance cells of different capacity together.

So i i were to connect for example 4 100ah cells with 4 80 ah cells in parrallel and charge them to 3.65v they shuld all reach their own maximum capacity without sucking the charge of the higher capacity batteries?

 

[hwy17]

So i i were to connect for example 4 100ah cells with 4 80 ah cells in parrallel and charge them to 3.65v they shuld all reach their own maximum capacity without sucking the charge of the higher capacity batteries?

Correct.

 

[Bitbin]

Correct.

But then a problem arises when i go back to connecting them in series to form a 8s battery. I dont know the actual capacity of each cell and will be left with the weakest cell being my real capacity right?

 

[740GLE]

Get your 8S BMS that balances a 2A and let it do all the hard work for you.

I enjoy my 8s JK with 2a balancing.

 

[hwy17]

But then a problem arises when i go back to connecting them in series to form a 8s battery. I dont know the actual capacity of each cell and will be left with the weakest cell being my real capacity right?

Also correct.

 

[time2roll]

Get a BMS with active balancer and accept the reduced capacity. Use the battery hard and start saving for some new quality cells.

 

[Ampster]

If they are top balanced, the loss of capacity will show up as they discharge and the weak cell reaches cutoff. Preferably you may want to set minimum pack discharge on your inverter above the point that the weak cell triggers the BMS to shut down the pack. That is preferable than having the BMS disconnect each time.

 

[Bitbin]

can i just connect an independent 2a active balancer that on all the time?

Get a BMS with active balancer and accept the reduced capacity. Use the battery hard and start saving for some new quality cells.

 

[time2roll]

can i just connect an independent 2a active balancer that on all the time?

Yes that works.

 

[Bitbin]

Yes that works.

Just one question. Is active balancing more important for new cells that match 100% or for my situation where cells differ?

 

[time2roll]

Just one question. Is active balancing more important for new cells that match 100% or for my situation where cells differ?

Same importance. Less hassle. Especially less hassle with B cells that are more likely to get out of balance during normal use.

 

[sunshine_eggo]

Just one question. Is active balancing more important for new cells that match 100% or for my situation where cells differ?

I would actually say LESS importance because new matching cells are likely to need less balancing.

From what I'm reading, one could conclude that an active balancer fixes all problems. Yes. They're great, but they aren't a magic fix-all. Some try to use them to claim additional capacity during discharge. Yes. That can work, but if you exceed the balance current, it's a losing battle.

Active balancers work best when applied the same way passive balancers work - only at or above 3.40-3.45V.

 

[Ampster]

Especially less hassle with B cells that are more likely to get out of balance during normal use.

I agree. I had a pack of B cells in a 3P16S configuration and also needed the 2 Amp active balancing. In my case I had three 280 Ahr cells in parallel so that was a lot of capacity to balance with anything less than 2 Amps. If one of those groups was only 1 percent out of balance it would still take 8 Ahrs to bring it up or down to match other cells.

 

[Bitbin]

I would actually say LESS importance because new matching cells are likely to need less balancing.

From what I'm reading, one could conclude that an active balancer fixes all problems. Yes. They're great, but they aren't a magic fix-all. Some try to use them to claim additional capacity during discharge. Yes. That can work, but if you exceed the balance current, it's a losing battle.

Active balancers work best when applied the same way passive balancers work - only at or above 3.40-3.45V.

So i just did another discharge cycle and this time cells number 4 and 8 where the first to emtpy. As you can see cells number 1 and 2 are still over 2.92. How much diffrence is that approx in terms of AH in a 100ah battery?

 

[Ampster]

As you can see cells number 1 and 2 are still over 2.92. How much diffrence is that approx in terms of AH in a 100ah battery?

I did not see the new numbeers. I is hard to say how much difference in Ahrs there is between batteries of different voltages, especially if several of them have reduced capacity. The simplest answer is by counting Coulombs (Ahrs) out of the pack when it is full until that one cell hits bottom and the difference between that number and healthy cell of 100 Ahs is how much that pack capacity is reduced. A more complicated process would be to Cycle each cell through a charge and discharge cycle and measure its capacity. Voltage in the middle is not a good measure of State of Charge except near 3 volts and near 3.5 volts. In most cells ninety percent of the capacity lies between resting voltage of 3.3 volts per cell and 3 volts. There is no capacity lost when a cell settles from full charge to resting voltage of 3.3 volts.

 

[time2roll]

So i just did another discharge cycle and this time cells number 4 and 8 where the first to emtpy. As you can see cells number 1 and 2 are still over 2.92. How much diffrence is that approx in terms of AH in a 100ah battery?

3.00 Vpc is about 10% charged. Best to stop discharge when the lowest is about 3.100 volt as it represents 80% discharged.
Make a note of the battery voltage when the lowest hits this level to give an idea when to stop and avoid the hard cut protection.

 

[Bitbin]

I did not see the new numbeers. I is hard to say how much difference in Ahrs there is between batteries of different voltages, especially if several of them have reduced capacity. The simplest answer is by counting Coulombs (Ahrs) out of the pack when it is full until that one cell hits bottom and the difference between that number and healthy cell of 100 Ahs is how much that pack capacity is reduced. A more complicated process would be to Cycle each cell through a charge and discharge cycle and measure its capacity. Voltage in the middle is not a good measure of State of Charge except near 3 volts and near 3.5 volts. In most cells ninety percent of the capacity lies between resting voltage of 3.3 volts per cell and 3 volts. There is no capacity lost when a cell settles from full charge to resting voltage of 3.3 volts.

 

[Bitbin]

Sorry forgot to attach picture

 

[Ampster]

I agree with the earlier poster. Set your minimum discharge voltage higher so that no cell goes below 3.1 volts.

 

[sunshine_eggo]

IMHO, that's a pretty decent spread. I would not be concerned by a 2.654-2.964 range. Both are at very low states of charge.

 

[Bitbin]

IMHO, that's a pretty decent spread. I would not be concerned by a 2.654-2.964 range. Both are at very low states of charge.

Is this a good active balancer? Says its a 5ah balancer. But the thing is it gonna be on all the time. Wont it affect capacity?