michael d
off-grid solar pilgrim
There will be no imbalance is just a voltage delta when the cells are at the full charge level. The voltage delta when the cells are around 100% SOC will not indicate an unbalance. | |||||||||
There will be less than 10mV delta as long as the battery is not fully charged so the last few seconds before the charging is stopped. | |||||||||
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Say it takes a cell 36 seconds to get two 3.4V to 3.55V while it is charged at 0.2C (that will be 20A for a 100Ah cell). | |||||||||
The SOC will increase by 0.2% in those 36 seconds (it is just an example it may take way less than 36 seconds but this is a round number as there are 3600 seconds in one hour). | |||||||||
Unless your cell balancing can dissipate 0.2C so 20A in this example with 100Ah battery you will not be able to correct that 0.2% SOC delta in a single charge cycle. | |||||||||
0.2C in this example means 200mAh as the delta between the cell at 3.4V and at 3.55V so it will require one full hour of 200mA of cell balancing to reduce the cell SOC by that 200mAh | |||||||||
Since cell balancing only has 36 seconds in this example it will require 100 charge cycles to reduce the energy in that cell by this amount. | |||||||||
If the delta is large enough between cells (more than 10mV) then cell balancing will start way before cells get to 3.4V and there will be a lot of time even multiple hours to correct the larger imbalance. | |||||||||
The last small imbalance that 0.2% or so will require more time as it is only visible in the last few seconds of the charge cycle and it is not very relevant since it is so small. | |||||||||
To correct those last 0.2% imbalance can be done in two ways. | |||||||||
One will be to have a huge cell balance current 20A in this example but in real life may need to be 60A for a 300Ah battery or more and it just not an economical option to have such large cell balancing currents. | |||||||||
The other option is to reduce the charge current to what the cell balancing can do say 200mA but then instead of 36 seconds of charging to get to full it will take one full hour and so there will be higher battery degradation as batteries like to spend as little time as possible charging. | |||||||||
Cells are not equal no matter the quality of the cells. They will have both different capacity and different internal impedance. | |||||||||
If they only had different capacity but exactly the same internal impedance then after the cells will be perfectly top balanced they will remain that way and all cells will get to 3.55V at the exact same time. | |||||||||
Of course cells will also have different internal impedance meaning each time you charge and discharge them they will get slight imbalance and that small imbalance is what is corrected by cell balancing. | |||||||||
The small 100 to 200mA cell balancing current is sufficient to keep a battery in balance (withing 0.2% or so) forever even for large battery capacity in the 1000 to 2000Ah | |||||||||
Say you have a cell with 0.5mOhm internal DC resistance and one with 0.6mOhm | |||||||||
Then during charge or discharge at 20A the 0.5mOhm cell will have a voltage drop of 0.01V * 20A = 0.2W so 0.2W will be wasted as heat instead of charging the cell. | |||||||||
The other cell that is 0.6mOhm will have an internal voltage drop of 0.012V * 20A = 0.24W so more of the charge energy will be wasted thus over say one hour of charge at this current the 0.5mOhm cell will have 0.04Wh more energy than the 0.6mOhm cell and that is how imbalance happens and it will happen both on charge and on discharge. |