Battery cell balancing and monitoring

In order to keep Li-Ion cells within their allowed operational ranges regarding voltages, currents, and temperatures dedicated cell monitoring controllers are being used. They monitor the state of each individual cell according to pre-adjusted voltages and temperatures using high-precision ADCs. Synchronization of the distributed measurements across all cells inside a battery is key to achieve the best possible insights into the battery's dis-/charging state (SOD / SOC) and the overall battery health (SOH). The required battery state assessment becomes less precise if these measurements are being spread across wider time windows and if the measurement accuracy for each parameter is not high enough.

A robust high-speed communication link across multiple daisy-chained monitoring devices supports complex cell topologies for a battery. For additional robustness, the daisy-chaining realizes a redundant ring, which keeps communication up between all connected IC's in case of a broken link. CRC protected data frames complement communication robustness. To achieve extreme low-power dedicated housekeeping functions such as periodically scheduled cell measurements and state analysis required for functional safety can be performed by the cell controller independently from the master controller of the BMS. Safety features for signaling over-/under-voltage, thermal stress, etc. including emergency alarms are triggered autonomously.

To allow for maximum cell usage, cell charging/discharging is being passively balanced via cell controller internal (for lower balancing currents) or external (for higher balancing currents) resistor networks. If the cell management IC detects a charging unbalance between cells it will connect the cells with the highest SOC-level to a resistor (or grid of resistors for better heat distribution) as load and reduce the peaking SOC-level by controlled partial discharging of that single cell. The battery charging is interrupted during the balancing action, which is frequently repeated until all cells reach the same SOC-level. Cell manufacturers grant almost identical electrical characteristics for each cell which do no longer require active balancing to compensate for bigger inequalities of charging state.

Optionally the communication link between the central battery management controller and the individual cell balancing and monitoring controllers can be implemented using wireless communication links. This saves wiring efforts and avoids additional overhead for communication across voltage domains. It gives more flexibility to assemble e.g. battery packs for different car options: low module count option for cost-optimized vs. high module count option for range / performance-optimized implementations. The reduction of wiring costs is an important aspect to address the increasing cost pressure to be profitable especially with more complex battery packs.