I did some testing and couldn't reproduce this. Just to make sure I'm on the same page, you had a solar bank, and connected it to the junction box, then connected it to a battery bank? And power wouldn't flow for you?
I first connected the solar bank to the junction box (I tried with the concrete block, the fuse box and the junction box), then connected the junction box to the battery bank, then connected the battery bank to a spot light.
I had also tried it connecting the solar bank to the junction box, then connecting the junction box to the spot light. I was able to get the spot light to turn on in both instances, even when I had turned the battery bank off in the first setup.
Please let me know if this is how you had it set up (or how you had wanted it setup) and you were still experiencing issues.
That, unfortunately, is impossible, at least as far as my knowledge goes and being able to do something like that with a modlet. But there is a way where you can setup two battery banks to have a 'day grid' and a 'night grid' and not have to worry about charging the batteries. I know this worked in 16.4, but I haven't had a chance to test it in 17.1.
I will say that this is not my idea, I found it probably on the forums here, but failed to write down who posted it, so I'm sorry whoever posted it, I'm unable give you credit.
So the build:
Solar bank, with as much cells as you can buy.
It connects to the 1st timer relay, that is set to 4:00 - 19:30, connected to a 2nd timer relay that is set to 4:00 - 20:00. This little trick is required to skip the game's logic, should be easily done by one relay, but if I leave the 1st timer out, than the system doesn't powers up at nighttime, I need to start/stop the solar bank to make it work. So it's kind of workaround because of the game's logic, it just completely separates the networks.
The 2nd timer connects to the 1st battery bank which charges during daytime and connects to the 3rd timer that was set to 20:00 to 4:00. This is the "night grid", that powers up the sensors that switches the lights up in the house. This night network cuts of at 4 when the sunlight is enough to see easily and save power. Also this was connected to the 2nd battery bank that's output is the "24/7" network. During daytime it powers up the traps around the house (while the 1st is being charged by the sun), and at night the 1st battery pack charges up the 2nd.
This system fully works without any touch, of course you have to check your watt usage and get the best batteries you can. Of course you can play with the times but this is what works best for me.
So in schematic:
Solar bank --> timer relay 1 (4:00-19:30) --> timer relay 2 (4:00-20:00) --> 1st battery bank --> timer relay 3(20:00-4:00, nightpower grid) --> 2nd battery bank --> 24/7 grid.