I am running a Deye 8kW inverter with 3 Pylontech Batteries. 2 off US3000C of which the newest one is the master plus a US3000.

When discharging, the SOC has a rapid dip from 51% to 30% in 17 minutes without any change in load. Non of the other parameters as per Solar Assistant seems out of place.

This is due to the 3rd battery (US3000) dropping much more than the other two during the same time. It is running then behind for the entire day. It only catches up when the SOC gets above 90%.

Can it be a firmware problem that the BMS communication between the new and old batteries are not efficient?

Use battery view to pull the event and history logs for each of the batteries.

Assuming the batteries are correctly connected to each other so you get an even draw thru them, either via each battery feeding a common busbar or by connecting the positive from the inverter to the first battery and negative to the last one?

They may need balancing for a prolonged period (24 hours?), maybe connect to mains to fully charge overnight? but if this fails to help it could be a faulty cell in the third pack? Battery view history and event logs will give clues about this, and your supplier will probably ask for these or for the battery to be taken in?

Having the same type of issue with 2 x up5000's, (around 500 cycles each) of 11, that will need replacement all the others are working well. Using Multisib control to log full details of charge and discharge and seeing the same type of behavior with 2 of the 11 battery's.