Power e-bikes with lithium ion battery reference design

March 18, 2014 // By Graham Prophet
ams has produced a cost-saving system management reference design for lithium pedelec/e-bike batteries; implementing controller-less balancing and voltage monitoring, it shows how to reduce component count and BoM cost while providing essential safety functions.

The example design for lithium pedelec/e-bike batteries implements accurate cell monitoring and balancing without the need for a microcontroller in the Battery Management System (BMS). Battery pack and pedelec manufacturers which use the design will 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 48V pedelec battery consisting of up to 14 lithium-ion cells. It uses two AS8506 smart cell monitoring ICs, with some 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 simple 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 by contrast, includes built-in logic functions for controlling cell safety, protection and balancing. These functions can 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.

 

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