I'm pretty sure polarity is correct.
After experimenting today, I found a workaround to the issue, and I'm able to run both the inverter/charger and the DC loads simultaneously. But I still have no idea why the issue (drop to 0 voltage) was happening.
There is a master battery disconnect switch connected to the battery positive terminal. Both the inverter/charger and the DC loads were connected to the output of this switch. So when the switch was turned off, there were no loads, no DC current flowed, and the battery terminal voltage was 13V. When the switch was turned on, power should have been supplied to both the inverter/charger and DC loads, but instead the terminal voltage would drop to almost 0 and neither the inverter/charger nor the DC loads would receive power.
What I tried today was to connect the DC loads direct to the positive terminal of the battery instead of the switch, while leaving the inverter/charger connected to the switch -- and the problem went away. Now, when I turn on the switch, the inverter/charger powers on fine, and the DC loads are also running since they are no longer controlled by the switch.
In summary, if the inverter/charger and DC loads are switched on together, neither received current and the terminal voltage drops to 0. But if instead the inverter/charger is switched on while the DC loads are already receiving power, then everything works fine.
What could be causing this? Is it just a fluke? Perhaps some sort of advanced "feature" inside the Victron inverter/charger?
Note the DC loads are only about 0.9 amps and the inverter/charger only draws another 0.9 amps, so the total draw of 1.8 amps is very small.
