Every time I take off I'm getting Battery signal error. At first a few times during the flight, and now short after taking off, and I cannot see battery details untill I land, and that is big problem. I have two batteries and problem repeats on both of them. The drone is new, and batteries have approx. 15 charging cycluses. I suspect that is the firmware problem (latest firmware installed). I made firmware refresh with DJI assistant 2, but problem continues. Please help!!

I don't think it is a FW issue, if you still are under warranty you better take a chance and go for a rep or replace from seller.
My P4P was replaced to one with this issue among other, i don't get it in all flights but when i get it, it don't matter what battery is mounted, it come and go during the flight and while active as you say the battery status won't update.
Neither down- nor upgrade of FW has changed this error so it is something in the hardware/connections that fail.

Sorry about the issue. Please try to discharge and charge the battery fully to see whether the issue remains. If no helps, please kindly start a case and send it in from here: https://www.dji.com/support