BQ76PL455A-Q1: Series Pack Balancing

Hi Andrew,

I think the easiest way to explain this  one is through in example:

Say you have a 32S pack, which is comprised of two bq76pl455A modules with 16 cells each. The bottom  module will passively balance (burn off energy) through bleed resistors/FETs, and the top module will balance the upper sixteen cells. 

The pl455 device will not begin balancing on its own - you must write to registers in the device to enable balancing, and to set how long the cell will be balanced for. So as far as an algorithm goes, the end user is responsible for deciding which cells balance, and when. 

Thanks David,
So my question really is if in your example you had a 500uA variance in the supply current that the two pl455 devices (and peripheral components) were placing on the cells, over 1 year, this would be over 4Ah imbalance form module 1 to module 2 - how is this managed.

So i had hoped that we could configure the balancing at the PCB commissioning stage, and then monitor/gauge with the MCU throughout the life of the battery - what I think you are saying is that from the off, the MCU must actively control the two PL445 devices to avoid cell imbalance within modules, and module imbalance within the module stack. Is that correct?

Thanks

Andrew

Andrew,

You are correct, the variance in supply current would cause some mismatch. Ideally, the balancing algorithm would take care of that.

Another thing to consider is that each comm interface consumes about 2mA of current. So, the top module will consume less current with only the low side comms configured than the bottom module with high side and UART comms. One thing that some customers do is to use a GPIO enabled FET/Bleed resistor to equalize the module mismatch.

You are correct. In order to begin balancing you have to write to the CBENBL register of the 455, and set the bits that you wish to balance.