SolanaJohn wrote:
Turn on lights sequentially until all lights fail.
Turn off all light switches. Switch on lights (none light).
Return all light switches to off position.
Across battery 12.8V
You are barking up the wrong tree, so to speak.
You NEED to do a similar set of measurements WHILE THE LIGHTS ARE FAILING.
I suggest that you disconnect the solar system completely and then make some more measurements.
It should be bridged in parallel with the batteries and converter/charger and should NOT be causing what you are seeing even if it is failing.......unless someone has made a huge wiring error.