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102 Posts
I fail to see a reason for all the antagonism. Perhaps it can be explained. The BMS that I'm developing happens to incorporate charge only active balancing but that is a bonus that comes with a very effective isolation method, using tiny low cost high frequency transformers. These transformers allow extremely low parasitic losses when powered down. By using active "rectification" on the secondary (cell) side very accurate measurement of cell voltage can be done on the primary side of the isolation.
You take a very strong stand against active balancing yet you acknowledge a 3% variation in cell amp hour capacity. Without active balancing the capacity of the pack (amp hours) is that of the weakest cell and no more. With active balancing that is 100% efficient and "up to the task at hand" the pack capacity is the arithmetic average of the cell capacities. Without active balancing a single weak cell WILL reduce the pack to it's capacity (amp hours). In addition, with "conventional BMS technology, either top balancing or bottom balancing, the depth of discharge will be greater on the weak cells shortening their life relative to the stronger cells. Here again active balancing forces the depth of discharge to be the same for all cells lengthening the life of weaker cells. The "holy grail" of battery management is full time active balancing that is "up to the task" and can transfer charge fast enough to keep up with the needs! Most active balancing technologies are far from "up to the task" especially with high charge and discharge rates and very "flat" capacity curves. With LIFEPO4 most all the charge transfer has to take place in the nearly full and nearly empty ends of the cycle between 3.3 volts and 3.0 volts very little balancing can be done.
You take a very strong stand against active balancing yet you acknowledge a 3% variation in cell amp hour capacity. Without active balancing the capacity of the pack (amp hours) is that of the weakest cell and no more. With active balancing that is 100% efficient and "up to the task at hand" the pack capacity is the arithmetic average of the cell capacities. Without active balancing a single weak cell WILL reduce the pack to it's capacity (amp hours). In addition, with "conventional BMS technology, either top balancing or bottom balancing, the depth of discharge will be greater on the weak cells shortening their life relative to the stronger cells. Here again active balancing forces the depth of discharge to be the same for all cells lengthening the life of weaker cells. The "holy grail" of battery management is full time active balancing that is "up to the task" and can transfer charge fast enough to keep up with the needs! Most active balancing technologies are far from "up to the task" especially with high charge and discharge rates and very "flat" capacity curves. With LIFEPO4 most all the charge transfer has to take place in the nearly full and nearly empty ends of the cycle between 3.3 volts and 3.0 volts very little balancing can be done.