ams AG has introduced a vastly simplified and more robust method of implementing cell monitoring and balancing in lithium battery systems. The new architecture has been implemented in a highly integrated chip, the AS8506, to perform distributed cell monitoring and balancing operations for stacked cell modules, including Safe Operating Area (SOA) checks and passive or active cell balancing.
It is suitable for all lithium-based cell chemistries, such as those found in hybrid and fully electric vehicles, as well as for EDLCs (also known as supercaps or ultracaps).
In conventional systems, a complicated algorithm running remotely on a high-end microcontroller decides which cells have to be balanced. The new architecture supported by the AS8506 can control balancing locally at the cells, enabling designers to implement a more streamlined cell management system that 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 analogue 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 contrast to the typical system implementations which use 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.
- Sequential cell measurements require complex compensation algorithms in order to pro-duce valid voltage and current readings across a stack of cells.
“We believe the AS8506 marks a breakthrough in cell monitoring – not an incremental improvement on previous cell monitoring ICs, but a completely new approach”, said Manfred Brandl, Product Manager for battery management in the automotive business unit at ams. “By offering local cell and temperature monitoring, the AS8506 gives system designers a simple and robust means to implement a battery management system, with just a simple microcontroller required for basic system functions.”