BQ76920 stacking

Normally the cells are connected in parallel to form a 5s3p cell assembly and monitored by a single battery management system.  The parallel cells increase the capacity and current capability.  As some forum members have indicated, if one cell fails the pack is not useful.  

If you want to parallel three 5s1p packs, you could do that with 3 separate battery management systems and a common controller to handle load sharing and charging of the packs when they are not at the same voltage.  Note that if 2 packs at different voltage are connected together one with higher voltage will discharge very quickly to charge one with a lower voltage and may exceed the capacity of both packs.  With this topology you could remove one pack for repair while the other 2 continue to run the system. Realize that this may work much better for increased run time rather than high current since the individual stacks may have different capacities particularly when they age or if one is replaced after some time.  To use a common controller, since there are not 3 different I2C addresses for the bq76920, you will need a MCU with multiple I2C buses or a bus multiplexer.  You will need to determine whether you want the MCU to handle switching of the packs or separate ORing controllers.