Turns out one of the 4 batteries was bad.
Many thanks to @SamGibson for his insight in solving this problem.
The well-deserved Accept is on his answer.
Here is what the charge plot looks like now:
The system now provides near-real-time montage plots here: https://SDsolarBlog.com/montage
This is a 24 Volt system comprised of 6 panels and 4 batteries connected by about 25 feet of 8 Gauge cable.
3 pairs of 100 Watt panelson single-axis hinges, with the mounts tilted south to catch the most sun. Each pair has positioning actuators - the three sets are run by a single Nano-based sun tracker which works well.
It sets the panels flat when it loses track of the sun to protect them from wind during night.
The PV panels connected by about
25 feet of 8 Gauge cable. All the PV side is connected with standard MC4-style connectors. 8 Gauge is the largest that will fit.
Renogy Commander MPPT Charge Controller- installed and working. According to its remote monitor screen it is doing its job just fine.
[Just did a daytime spot-check and with 95.2V x 1.9A = 180.88 Watts in, it is putting out 29.2V at 6.0A = 175 Watts for an efficiency of 96.7% to go to the batteries and inverter]
- Battery bank is comprised of
4x 29DC 122Ah deep-cycle marine batteries in series/parallelto present as a nominal 25.2 Volts. They are all about a year old.
They are wired together like this:
Inverter is a new 1000W Pure Sine Wave unitconnected directly to the batteries with
10 Gaugecable. [During the same spot check it is producing 165W AC with the input of 175W DC, for an efficiency of 94%)
I set up a voltage monitoring and inverter control system using a Raspberry Pi and Arduino Nano. The Nano takes the readings and can send a shutoff signal to the inverter when the voltage is at 21 or lower for 5 readings in a row (1 minute intervals).
It also shuts it off at 4pm to give it time to top off the batteries each evening.
The Pi part is the data logger & RTC, and is used to plot the voltages which it displays continuously in real time by virtue of the gnuplot
reread. I keep it up on the screen all the time via RDP.
I also set up an
automatic power-transfer relay that is energized by the inverter, switching the entire load from house power to Inverter power. The automatic transfer relay feeds power out to two UPSs, so there is no interruption of AC power during the switchovers. Turns out you can't buy this little gem. You have to build it.
Total load is typically about 300 Watts to run the computers and the 2-tube 37 Watt LED room light. it is quite sufficient while the sun is up to run nearly everything in my little place while the sun is up. (No A/C or kitchen appliances, obviously).
I have a 24-to-12 converter to run the ham radios, and thank goodness for getting rid of the PWM charge controller. Much less radio noise (QRN).
The battery monitor shuts down the inverter at 4pm - time for the batteries to get topped off before the sun goes down.
The charge controller flashes the battery light at me on the panel to indicate that the batteries are fully charged. Almost any time the inverter is off while there is sun it indicates that it is full. The batteries can't hold all the power this system produces.
The controller indicates that the battery bank is full. The voltages when they settle do not, though.
The batteries are new, made by Johnson Marine. The water is fine in them, and they are barely 1 year old.
After the inverter is commanded off at 4pm, with 2 hours of sun yet to go, it is clearly shown on the MPPT's remote screen that the amount of current flow into the batteries decreases down to about 0.2A
When the inverter is turned on after the sun is down, it really drags the voltage down quickly. It is providing about less than 300 Watts of AC power according to the Kill-a-Watt meter before the inverter quits due to input DC low voltage.
The DC side is still good at that point for a lot longer. I have full confidence that if all I was running was a 12-V lamp and the ham radios I could run all night on DC.
The problem is when I try to use a reasonable amount of AC power. It is the inverter that decides when the voltage is too low.
With the MPPT charge controller screen showing a happy face and the icon of a full battery bank, it seems there should be more usable AC power in there.
With four 12V 122Ah batteries in series/parallel the system stores a total of 244Ah @ 25.2V when fully charged. This is 6,149 Watt-hours.
================== LOAD TEST =================
In these plots you will see that the voltage started to rise as the sun started to come up, and then I started the Inverter. But then thought better of it and used the breaker to disconnect the PV panels. Gave it time to settle down then at 7:30 began a 260W AC load test with the Inverter, sampling every 30 seconds.
It lasted less than a half hour before the inverter cut it off.
Then let the battery bank settle for a second run, again at 260 Watts - with sampling every 15 seconds. This time it only lasted a few minutes.
You can see it go off the chart when the PV Panels were turned back on.
After the test the PV panels went to work, feeding 10 Amps of 27-28V into the batteries for a while.
Then the batteries filled up and the amperage dropped to about 2 Amps which allowed the MPPT charge controller to go into
boost at 29.8V
The charge controller is showing a happy face and a 'full' battery icon as it thinks the batteries are getting back up to full charge. It then drops down to the float voltage for the battery bank.
The green light on the charge controller is now blinking green to indicate a full battery bank. The amperage has now dropped to 200 mA.
Here is the overall voltage plot of both the input and output of the charge controller.
Obviously there was no input during the load tests.
There were some clouds interfering with the PV in the morning:
The charge controller handles partly cloudy pretty well, generally.
SO.. I can't figure out what I am missing here.
It all looks like it is working fine except for the drawing out of power from the battery bank.
During the day it is no problem to run 300 Watts AC while the batteries charge at the same time..
All indications are that the battery bank is full.
Then this. It simply does not add up.
Need someone with a practiced eye to look at all this and help, please.
So the question is: Why is it providing such short runtime after dark?
--EDIT: Thanks to @SamGibson's answer, it was determined that one of the batteries was defective. When fully charged then separated and left to settle for two days, three of them measured roughly 12.7 (100%) but one of them measured 10.02V. Also it was determined that by using 8 Gauge cable in yellow crimp connectors designed for 10 Gauge that extra resistance was presented there. So the solution so far has been to obtain a replacement battery and set up a new screw-terminal barrier strip where the inverter, battery bank and charge controller can all come together, and to replace the 8 Gauge cable with 10 Gauge.
After the new battery and harness were installed, here is the new charge plot:
Amazing the difference it makes to replace one bad battery.
Thank you, @SamGibson. You have the Accepted answer on this one. I really appreciate your insight.