300mohm!
They can do much better. I wonder what all the other components potted in the SSR are for, stability and LED?
Here is a FET with VDSS=100V, 50A Rdson=5mohm.
https://www.infineon.com/dgdl/irfh7185pbf.pdf?fileId=5546d462533600a40153561ee0b11ee1
Oh! Now that is a winner.
OK, Now do you have any BMS8T-300s left, 300A shunts and 250A (or 500A) SSRs? I suspect the FET is the same, only the heatsink size is different.
I only need the SSR to cut out the Charger at Low Temp. I have programmable over/under voltage control on the charger...