Reference design cuts costs of lithium e-bike batteries

March 17, 2014 // By Paul Buckley
An example design for lithium pedelec/e-bike batteries which implements accurate cell monitoring and balancing without the need for a microcontroller in the Battery Management System (BMS) has been introduced by ams AG.

The design aims to enable battery pack and pedelec manufacturers to benefit from bill-of-materials (BoM) cost savings and a simpler circuit design, compared to batteries in production today.

The ams design is for a 48 V pedelec battery consisting of up to 14 lithium-ion cells.The design uses two AS8506 smart cell monitoring ICs, with few supporting components, to monitor the temperature and voltage of up to seven cells each and to implement passive balancing of the cells when charging.

By contrast, conventional BMS designs in pedelecs use dumb voltage monitoring ICs to measure the voltage and temperature of cells, reporting the values to a dedicated battery management microcontroller via a serial communications link. The MCU is required to control safety and protection functions (over- and under-voltage and over-temperature shut-down) and cell balancing.

The AS8506 from ams, however, includes built-in logic functions for controlling cell safety, protection and balancing. These functions can easily be configured by the user, with the settings saved in an on-board OTP memory. The device also features integrated MOSFETs for use in passive cell balancing operations. During charging, each cell’s voltage is compared to a user-programmable reference voltage threshold. Up to 100 mA may be discharged through the MOSFET from any cell exceeding the threshold, until all cells have reached the threshold and the battery module is fully charged.

The architecture, in which battery monitoring and cell balancing operations are implemented inside the AS8506 voltage-monitoring device, dispenses with the dedicated MCU required in conventional pedelec battery designs. When an AS8506 detects an over- or under-voltage or over-temperature condition, an interrupt signal is transmitted to the pedelec’s motor controller IC to complete the required safety shut-down operations.

"Battery lifetime, safety and run-time strongly affect consumers’ choice of pedelec, but so does cost," explained Bernd Gessner, Vice President and General Manager of the automotive business unit of ams. "The controllerless reference design from ams helps pedelec manufacturers meet consumer