Hi, we have the same issue and setup (additonal battery). Did you get a solution for this?
No, there is not a method to distinguish between the two batteries when receiving battery faults. The EBM connector has no logic, so the UPS is unable to distinguish between the two batteries. Troubleshooting the two batteries will require disconnecting the EBM to see if the fault condition is corrected with it removed. In the event of a complete battery failure you might be able to tell which battery is failed by the remaining battery life since the EBM has a much larger charge capacity than the primary battery.
Dell EMC, Enterprise Engineer