Sendyne, a developer of precision current and voltage measurement systems and modeling/simulation tools for battery systems and other applications, has been awarded a patent for a novel active cell balancing topology.
Cell balancing is achieved by transferring energy to and from individual cells in a battery pack, with the goal of having all cells operating at the same State of Charge (SOC). Because individual cells in a battery pack will have slightly different capacities, if energy is not redistributed from stronger cells to weaker cells, discharging must end when the cell with the lowest capacity is empty.
Therefore, without cell balancing, the amount of energy a pack can provide is limited. Further, as weaker cells are exercised more fully than stronger cells, total pack longevity is affected.
|Sendyne active balancing topology. Click to enlarge.|
Current approaches to cell balancing can be costly in terms of the number or physical bulk of switches, inductors, or capacitors employed (per cell) to bring about the balancing or charging. Some approaches have been disappointing in terms of the energy losses suffered during the balancing process.
Some approaches only achieve charging based upon an external energy input but cannot redistribute charge between cells in a string. Some approaches only serve to discharge particular cells, throwing away energy merely to ensure that no cell performs better than the weakest cell in the string.
Sendyne’s topology permits any cell in a pack to charge any other cell in a pack with maximum efficiency and minimal operational losses. The topology requires a single MOSFET switch for each cell; one low cost transformer for each set of four to eight cells in series; and one shared parallel energy bus utilized by the entire battery system.
The topology can be designed using ubiquitously available off-the-shelf components. Far fewer expensive, bulky components are needed than are usually required for active balancing. The Sendyne method does not require a transformer to store the energy before providing it to selected cells, which means that a smaller, less expensive transformer can be used.
Sendyne’s method can be deployed in two modes: autonomous and controlled. In the autonomous mode, no microcontroller is needed to make charge / discharge decisions for each cell. Instead, the circuit itself guarantees that all cells in the pack are at nearly the same SOC. Further, the circuit will automatically utilize the capacity of all cells, regardless of whether the capacity of the cells is different. For this reason, it is not necessary to use matched cells in a battery pack. The necessity for cell matching adds a significant cost to battery packs.
For critical battery systems, a microcontroller can be used to make dynamic charge / discharge decisions. In the control mode, the topology makes it possible to ensure that all cells in the battery pack age at the same rate; State of Health (SOH) is consistent for all cells.
Sendyne delivers key technologies for battery system management. These include the SFP family of ICs and modules for precise current, voltage and temperature measurements with built-in Coulomb counting; RTSim, a small footprint, high speed model solver for embedded predictive control; and CellMod—high accuracy, physics-based battery cell and pack models.