AMS releases automotive cell balancing chip
AMS has joined the firms offering cell balancing chips for electric vehicle Li-ion stacks. Its AS8506 can operate in both passive and active modes, and has a built-in balancing algorithm, cell monitoring and communication.
Cell balancing allows the maximum capacity to be squeezed from a battery whose cells inevitable differ in capacity, particularly as they age.
Passive balancing is simple – current which would over-charge a cell is dissipated through a resistor.
Active balancing is less wasteful, transferring unwanted power to uncharged cells. It is more complex to implement, and suitable power-transfer methods are still being invented.
“The architecture developed by AMS has been implemented to perform distributed cell monitoring and balancing operations for stacked cell modules, including safe operating area checks and passive or active cell balancing,” said the firm, claiming: “It is ideal for all lithium-based cell chemistries, such as those found in hybrid and electric vehicles, as well as for EDLCs, also known as supercaps or ultracaps.”
Including a balancing algorithm is unusual. All other cell balancing chips Electronics Weekly has heard of so far require a separate microcontroller to implement this.
“The new architecture can control balancing locally at the cells, enabling designers to implement a more streamlined cell management system that eliminates the host controller, complex software and vulnerable serial communication links normally used today. Both passive and active cell balancing is autonomous,” said AMS.
That said, the firm cannot be completely against remote control as it added: “Or it can support a microcontroller-based system via its serial peripheral interface.”
It argues that local balance control has access to all cell voltages simultaneously, whereas remote balancing requires the microcontroller to accumulate all cell data through a long serial link, and then re-time it to make decisions.
The firm’s architecture has far fewer components that Linear Tech’s recent LTC3300 – only one mosfet and one transformer per chip in active mode, compared with two mosfets and one transformer per cell.
However, the 8506 is unidirectional and less flexible. It can only take power from a fixed section of the stack and deliver it one cell at a time (see below). In the data sheet application notes it takes it from the whole stack and therefore needs a high-voltage mosfet. The 3300 can arbitrarily shift power to and from any cell or cells in a stack full time and needs only low-voltage mosfets.
The reason AMS’s chip needs so few external components is that internal 100mA power switches connect cells one at a time to a single external fly-back circuit for active balancing, or a single external resistor for passive balancing (see diagram below). Therefore, for any particular cell, balancing is pulsed rather than continuous.
In active mode, the fly-back PWM frequency (25-200kHz) and duty
-cycle are factory programmable and register
controllable. The external mosfet drive pin can also operate an opto-coupler for isolated drive.
“Cells which are below target will either cyclically receive charge packages from an isolated dc-dc converter or cells above target will cyclically be discharged by an external resistor through integrated switches in an autonomous way,” said AMS. “The device can be used for battery stacks of three up to seven cells [6-32V]. It can be chained to support battery packs of virtually any number of cells in a synchronised mode.”
It is worth noting that only passive balancing is possible with the over-the-counter chip. “Active balancing mode need to be enabled by factory setting. It is not available for the default ASSP,” said AMS.
On chip monitoring and balancing is through precision circuits, with pins reporting cell-voltage-within-limits and balance-ready, while cell voltages and temperatures (from two thermistors per chip) are sent to the outside world via an ADC and serial communication.
“An analogue circuit 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 digitised with an accuracy of 5mV and reported to a host controller,” said the firm.
For a seven cell ’24V’ stack, only one one chip is needed. Beyond this, multiple chip are chained along the battery with each chip passing communications to its immediate neighbours.
Target cell voltage and minimum and maximum cell voltage can be broadcast to all chained devices.
Application notes in the data sheet cover balancing for two-chip ’48V’ connection using 100V mosfets.
A stand-alone passive balancing evaluation kit for up to seven cells is available (pictured), as well as a more comprehensive active/passive balancer board.
Applications are also expected in electric motorcycles, scooters, bicycles, karts and power tools, as well as on or off-grid storage for solar or wind energy – solar street lights, for example.