I have 2 Pylontech UP5000 batteries. The SOC on my Sunsynk inverter shows 99% but the lights on the batteries only show 3 full lights with the 4th blinking. Why would there be such a big difference?

There's no comms between battery pack and inverter. So the inverter has to infer the SOC. You shouldn't have problems getting comms between Pylontech and pretty much any inverter.
 

The batteries aren't linked correctly...  

The 'link ports' should be chained together - 'linkport 1' to 'linkport 0' all the way down the stack, with 'linkport 0' empty on the 'master' battery and 'linkport 1' empty on the last.

Only the 'master battery' then has either the RS485 or CAN port (depending on your setup and with the correct cable, which is almost never the one that came in the box - other synsync users can advise) connected to the inverter.

Edited December 31, 20232 yr by HedgeSlammer
Typo

Thanks Hedgeslammer This solved the issue

Edited December 29, 20232 yr by Ddv86