I have a 300Ah 12V LiFePO4 battery setup with three 100Ah batteries (internal built-in BMS in each battery) in parallel. I have it fused at the parallel junction with a 400A fuse.

I want to put a 125A MRBF fuse on each battery instead. If I do install per-battery fuses, is there a reason to keep the combined big fuse as well? I see wiring guides recommending both but I'm not seeing any explanation for why.

The reason for moving is that the current fuse gets way too hot when pulling 300A sustained. I don't have room in my setup to easily install the really big fuse holders with heat sinks that are made to handle 300+A sustained current. (Yes, I checked the voltage drops across all my connections. They are very low. It really is the fuse getting hot and not a weak crimp etc.)