Cell to cell mismatch in either capacity or SoC may severely reduce the usable battery stack capacity unless the cells are balanced. Maximizing stack capacity requires that the cells are balanced both during stack charging as well as stack discharging.
In the example shown in Figure 2, a 10-cell series stack comprised of (nominal) 100A-hr cells with a +/- 10-percent capacity error from the minimum capacity cell to the maximum is charged and discharged until predetermined SoC limits are reached. If SoC levels are constrained to between 30 percent and 70 percent and no balancing is performed, the usable stack capacity is reduced by 25 percent after a complete charge/discharge cycle relative to the theoretical usable capacity of the cells.
Figure 2: Stack capacity loss example due to cell to cell mismatch
Passive balancing could theoretically equalize each cell's SoC during the stack charging phase, but could do nothing to prevent cell 10 from reaching its 30 percent SoC level before the others during discharge. Even with passive balancing during stack charging, significant capacity is “lost” (not usable) during stack discharge. Only an active balancing solution can achieve “capacity recovery” by redistributing charge from high SoC cells to low SoC cells during stack discharging.
Figure 3 illustrates how the use of “ideal” active balancing enables 100-percent recovery of the “lost” capacity due to cell-to-cell mismatch. During steady state use when the stack is discharging from its 70-percent SoC “fully” recharged state, stored charge must in effect be taken from cell 1 (the highest capacity cell) and transferred to cell 10 (the lowest capacity cell) – otherwise cell 10 reaches its 30 percent minimum SoC point before the rest of the cells, and the stack discharging must stop to prevent further lifetime degradation. Similarly, charge must be removed from cell 10 and redistributed to cell 1 during the charging phase – otherwise cell 10 reaches its 70-percent upper SoC limit first and the charging cycle must stop.
Figure 3: Capacity recovery due to ideal active balancing
At some point over the operating life of a battery stack, variations in cell aging will inevitably create cell to cell capacity mismatch. Only an active balancing solution can achieve “capacity recovery” by redistributing charge from high SoC cells to low SoC cells as needed. Achieving maximum battery stack capacity over the life of the battery stack requires an active balancing solution to efficiently charge and discharge individual cells to maintain SoC balance throughout the stack.
This is stupid you circulate currents and waste energy.
move the ground on the balance output side to the top of Cap N.
You only need to steer current around week cells.
No sense in passing balance current through un needed cells
Again move the ground to the top of N
This is a LifePO4 Kickstarter project that needs additional backing to sucsseed. I am a backer and would like to see this funded before it ends 23 April 2014. Thie creater of the project is an eletrical engineer who is trying to solve his own solar energy problem, and Lifepo4 is 1/2 the price, and more enviromentaly friendly then lead acid over it life time.
Join our online Radio Show on Friday 11th July starting at 2:00pm Eastern, when EETimes editor of all things fun and interesting, Max Maxfield, and embedded systems expert, Jack Ganssle, will debate as to just what is, and is not, and embedded system.