I have connected the BMW battery to my grid. First alone, then in parallel with the one I have been using so far. The results were disappointing.
This is the initialstate.com’s graph about the uptime after I connected just the BMW battery on the solar grid, and with both batteries:
The pink line is uptime, yellow is temperature, the blue is a event triggered at every reboot.
As you can see from the last side, uptime was high (about 10 days, could have been longer, I rebooted it for some reason). After that, the machine started to restart constantly. Then, at the end, one can see that the uptime started to get smaller and smaller.
I am not sure why yet. My speculation is that, because I made in the middle of the night, both batteries were not exactly at top charge when I connected the Solar Panel controller.
This solar controller requires batteries to be fully charged when connected to it (that is, during its boot). This is to avoid overload, probably, so it will only keep charging the batteries up to this value.
So, as I connected the batteries to the controller in the middle of the night, took a low value as its top and didn’t charge them.
Living and learning, as the saying goes.
Next step? To fully charge both batteries (with a conventional electric charger) and only then connect them to the solar controller.