expected normal voltage drop from load?

Agreed BFL13. The annoying thing is that I thought I had done a good job at preventing battery abuse by checking in on them periodically. And they still got abused, my first worry was when I checked on things to winterize in early October and noticed lots of corrosion on the battery terminals that had never been there before. Solar charging voltage looked fine, water levels down a bit but hardly low, but when I plugged my air compressor into the inverter the voltage dropped big time and it faulted out. I had to hook the inverter up to my truck battery to run the air compressor. That's when I first suspected I may have toasted batteries.