Why do top balance or bottom balance
There are no two identical leaves in the world, and the same applies to LiFePO4 battery cells.
There are slight differences in capacity, voltage, internal resistance and self-discharge rate between LiFePO4 cells produced from the same batch. And this difference will be amplified as the battery is used for a longer period of time.
As we all know, the capacity of a battery pack follows the barrel principle. The lowest energy cell determines the capacity of the whole pack. Therefore, it is an essential function of BMS to maintain the consistency of the cells by using balancing algorithm.
Generally if your cells are new and their voltages are very close to each other, the balancing function of BMS can make them balance slowly during charging. So there is no need to do top balancing, you can just assemble them into a battery pack.
We should consider top balancing only when you have done an initial capacity test and find that it is far from the expected capacity (the rated capacity of the battery cells), or when these cells arrive with a large voltage difference between individuals.
The purpose of top balancing is to maximize the use of the battery cells. We balance all the cells to charge and discharge as their maximum safe capacity without damaging the cells.
How to do top balancing?
- Adjustable DC power supply
LiFePO4 cells have a voltage of 3.65V when fully charged, and all cells are charged to 3.65V so that they have the same voltage and capacity.
After balancing the cells, the pack will have the highest voltage and maximum capacity.
1, Connect all battery cells in parallel.
2, DC power supply set output voltage of 3.50V.
Due to the long charging time of the high capacity battery cells, we can’t keep watch over the batteries. For safety reasons, we highly recommend to set the initial charging voltage to 3.5V.
Lithium iron phosphate has a very narrow voltage window, so when the voltage rises to 3.5V, it means that it is already more than 90% full.
3, Connecting the battery cell to the DC power supply.
Note that in order to balance all cells and cell internal resistance.
It’s best to connect the positive terminal of the DC power supply to the positive terminal of the first cell. And connect the negative terminal of the DC power supply to the negative terminal of the last cell.
Be sure to take care not to reverse the wiring, as that will generate sparks and may eventually damage the battery cells.
4, Constantly test the voltage of the battery cell and the voltage at the DC power supply.
5, When the cell voltage rises to 3.5V, the DC power supply output voltage is fine-tuned to 3.65V.
For safety reasons, when the charging stage at 3.65V, we need to stay near the battery and observe the cell voltage and current uninterruptedly.
6, When the DC power supply shows that the current is 0 or close to 0, it means all the cells are fully charged. Disconnect from the DC power supply.
7, Keep the battery cells in parallel state for a while; let them do the final balance between themselves.
After the above steps these cells finally reach top balance.
Remove the parallel connection and measure the voltage of each cell to confirm that they are all at 3.65 V. There will be a few millivolts difference between cells, which is acceptable.
Finally you can assembly them into a battery pack with BMS.
- Be sure to connect the correct polarity when connecting to the DC power supply.
- Continuously observe the cell voltage and current during charging, if find any abnormal elevation, disconnect immediately.
- Do not leave the battery cell during the charging phase from 3.50V to 3.65V.
- Please turn off the system at night.
Top balancing is a long process and may last for several days or more than a week.
As an example:
To balance four 3.2V 280Ah cells.
Use a DC power supply with a maximum output of 10A.
The top balancing time will be more than (4 x 280)/10 = 112 hours. Even with a 24 hour uninterrupted charge, this would exceed 4.5 days.
If not necessary, not recommended to do top balance. Just let the BMS balance them during use.
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