Part Number: BQ20Z655
We are using the 655 in preproduction and see packs where one cell in a 3S1P stack looks to be self-discharging rapidly once assembled into packs. Cells are mated into a 3S1P pack only if their OCV is within 30mV of each other, but once attached to the 655, one cell has it's OCV rapidly decay to diverge from the other two cells by 300mV or more. The other two cells remain within a few mV of each other. The cell number (1, 2, 3) is random and occurs with equal frequency, so it is not 'position dependent' in the stack. This is exactly what I expect would happen if a cell balancing FET remained switched on when not in CV charging mode. The only difference in our design from "datasheet-schematic" use is our cell balancing resistors are 150 ohms. This problem occurs in 1 to 5% of the packs we build and is impacting our ability to launch production.
Please help me understand if this is possible - and if not possible what could be causing this OCV divergence only once mated with the bq20z655. We are using high quality cells from a company known for high reliability cells used in space, ocean and on other planets, so it is very unlikely to be a problem caused by cells.
Are there any diagnostics or flags that can tell us when each individual balancing transistor turns on?
Thanks,
Doug
