Test is up and running. I disconnected one of the panels so I'm back to 280 watts total solar array capacity. Using the older of my 12V 300 watt elements. Things seem to be working as expected - battery voltage rose until reaching the absorption setpoint and then diversion started occurring while battery voltage remained constant. I took voltage readings on the diversion load and they are unexpectedly low. When battery voltage is 14.5V, the diversion load voltage is 9.2V with about 9 amps being dumped into heating element. It started the day at lower voltage than this and has settled around this value.
SCC is now in float mode (battery at 13.6V) and the diversion load voltage and current remains about the same. I measured voltage at the SCC terminals and at an Anderson quick-connect close to the SCC and both read 9.2V. I measured at the heating element and it was in the 7V range - voltage drop from 8 gauge wire running the entire length of 27-foot TT? Is this what anyone else expected? I thought I'd be seeing battery voltage or even panel voltage. Current at the element measured via clamp multimeter concurs with laptop SCC monitoring software. The diversion load voltage measurements at the SCC also agree with the array/load voltage value reported by laptop SCC software.
Temperature gun said that the wire leads leading into the heating element are 330F. These are the now-exposed wires that cooked off the heat shrink during initial day-long test run where I suspect I was simply putting too much wattage into the 300 watt element.
