Cell monitoring chip simplifies balancing of lithium-ion battery cells

Cell monitoring chip simplifies balancing of lithium-ion battery cells

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By eeNews Europe

The AS8506 performs distributed cell monitoring and balancing operations for stacked cell modules, including Safe Operating Area (SOA) checks and passive or active cell balancing. It suits for all lithium-based cell chemistries, such as those found in hybrid and fully electric vehicles, as well as for supercaps (EDLCs).

In conventional systems, a complicated algorithm running remotely on a high-end microcontroller decides which cells have to be balanced. The architecture implemented in the AS8506 can control balancing locally at the cells, enabling designers to create a more streamlined cell management system which eliminates the powerful host controller, complex software and vulnerable serial communication links normally used today.

The AS8506 can implement both passive and active cell balancing autonomously, or it can support a microcontroller-based system via its Serial Peripheral Interface. An advanced analog circuit in the AS8506 compares up to seven cell voltages against an internal or external reference with an accuracy of 1mV, to support cell-balancing and cell-monitoring functions. Cell voltage measurements can also be digitized with an accuracy of 5mV and reported to a host controller.

Active and passive cell balancing use a similar circuit design, but active balancing requires an additional flyback transformer. The control circuit is integrated in the AS8506.

The device also features internally or external adjustable upper and lower cell voltage limits. Temperature measurement is carried out through two external NTC sensors.

The fully autonomous cell management architecture enabled by the AS8506 is in stark contrast to the typical system implementations found today using existing cell monitoring ICs. These are typically limited to the sequential capturing of cell voltage measurements that must be processed by a host controller. These prior architectures have several drawbacks for designers of battery management systems:

  • The system must stream large amounts of data over a serial link – a communications channel that is vulnerable to interference in noisy environments.
  • A powerful host controller running complex software is required to turn individual cell voltage measurements into useful functions such cell balancing decisions. Writing this software is a difficult and time-consuming task. The software requires exhaustive testing in order to qualify for use in automotive ISO26262-compliant systems.
  • Sequential cell measurements require complex compensation algorithms in order to produce valid voltage and current readings across a stack of cells. By contrast, the simultaneous measurements captured by the AS8506 require no compensation.

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