how do I measure the voltage at the "device terminals"?
Estragon - an observation on that question about falling below 50 V - last night I was running a load test on my new battery bank, letting it handle a modest load for the night by itself and see how it goes. Anyhow, I put the system on grid from 4-6 pm, to float the batteries with the last bit of sun and to not have any charging during the period I was measuring. Right before I put the load on the batteries, they were in 51.15 V, and the Mate3 had both the temperature compensated and uncompensated voltages at 51.2 V, even though the battery T was 33C. In other words, like you said, if the batteries are at rest the two are the same, and Mate3 recognized that. Then when I switched the load to the batteries (0.1 kW) they dropped to 50 V within 30 seconds, but the temperature compensated voltage went to 50.8 V. I am still trying to wrap my head around this, but it does seem like the green battery status light (50V and above) does work with uncompensated voltage and will turn yellow after less accumulated load if the batteries are warm, which is pretty well what Outback guy said. Back in December-February the ambient temperature was lower, and the temperature sensor was not properly located, so the green light stayed on longer for the same accumulated load. What I am not really sure of, is whether this means anything for the batteries further down the SOC curve - what is the best voltage to work with when figuring SOC from voltage. Probably T-compensated? If the batteries are at 33C and the uncompensated voltage is down around 49.2, they really might be around 80% SOC.
Interestingly, on my load test for the new battery string I used about 1.8 kWH accumulated load during the night, which is fairly approximate at such low levels, and the batteries were just dropping from 47.6 V to 47.2 V at the end, and when load was removed they recovered to 48.0 V, which being at rest is the same with and without T comp. The kWh and the log files inverter current amp column data both indicate that I went to about 60% SOC for a 100 amp string. Yet the Energy Cell manual says a rest voltage of 48.8 is 50% and the Outback guy said a rest voltage of 48.8 is 65%. And the Mate3 manual says that 60% SOC is 46 V with load. Granted I only let the batteries rest a few hours, but from my experience in full tests, they do not go up a whole lot more after the first hour or so. Is there any truth I can cling to here? I am thinking that if I set my HBX grid-switch to 47.6 V for 30 minutes when running the three banks, I would probably never get that low overnight, but if I did I would certainly be somewhere in the 65-70% SOC range. Sound reasonable?