Thanks folks for your help. So I have 1 epever Trace 60A max (I forget which one, I had that info when I joined the forum, but I guess it doesn't show publicly?) SCC that has the maximum charge parameter set to 14.6V, the boost voltage set to 14.5. I have to manually change the equalization voltage once a month for a day to 15.5, otherwise it is set to 14.5. The inverter is a Kinverch pure sine wave 2000 watts. My panels are ecoworthy 200W panels connected in series. The wires connecting the solar panel to the SCC are 12 gauge cables that have the 3 wires all twisted together at both ends, about 14 feet long (yes, I like to save money). The other wires connecting the inverter and SCC to batteries are thick gauge, short (<4 feet) single wire cables that came with the inverter and SCC. The batteries are heavy duty "street cleaner" batteries, equivalent to L16s (but cheaper), 2 6V ones connected in series. I can't upload a pic right now, since I'm not there right this moment. Hopefully the words are good enough.
Sunsurfer, why would you connect several SCCs together, of course they would conflict with each other?
740GLE, no the batteries are not smart, they don't have overvoltage protection like some lithium ion banks. It's the inverter that has its overvoltage protection circuitry triggered (at 15V). I can lower the "max charge" parameter of the SCC further, and the problem goes away, but then the boost voltage has to be lowered too, which is not good for the batteries. I am a bit confused how the MPPT algorithm interfaces with the whole idea of a boost voltage. The other thing I did is reduce the boost time, so the problem doesn't keep happening for more than 20 minutes a day (it was 120 minutes of boost time before). But my batteries probably don't like it and will sulfate faster.
As far as solar panel output, it is possible that it spikes the voltage when the clouds part, exceeding the range of the SCC. I might check if I am willing to sit under the house for a bit till the problem happens, monitoring the solar panels' output. I think I would have noticed if the current or voltage was going higher than the SCC bounds. I actually took a video of the spike so I can convince epever that the problem is real, so I might look at that (I had only focused on SCC output, not input before). Epever, btw, just sent me a new SCC, and it didn't solve the problem, just wasted my time.