Hopefully this fits in the Forums Home < General < Renewable Energy and Storage (Non-EV) category.
I have a DC-DC charger with MPPT in our caravan. The DC-DC function works well.
I recently installed a couple of solar panels.
Problem is that it won't charge the Aux. batteries. Unless there is a 12V source at the input cable, solar charging won't work.
Here's how it's wired:
I have a spare AGM and connected it to the caravan Anderson plug, with an ammeter inline, and there was no current flow when it was connected. The AGM was at about 12V, and the caravan batteries were probably 13(ish)V. Solar charging worked.
Disconnected the battery.
I then connected a wire, with an inline spade fuse (30A resettable), from the DC-DC 12V output to the DC-DC 12V input, and that worked, but it (the fuse) got very hot after a short time, and tripped open. I had thought, if it worked, that I could use a switch or relay when disconnected from the wagon.
I tried a wire with a diode and a glass fuse between the main in and aux. out cables to see if that would work, it started the solar charging, but melted the solder on the cable/diode join eventually. I measured the current when I connected it, and IIRC there was only 0.1A. Reversed the cable and there was 15A. I don't know any specs of the diode it was one of several bits and pieces recovered from old (PC PS?) circuit board/s.
I then tried 3 diodes in parallel. Used a terminal strip at each end, so no melting solder. It didn't get hot though, and solar charging worked. There was a 0.03A current.
I read that I should put a resistor in series with each diode, so I'll do that if I can use it OK.
I was going to put a 24V NO relay controlled by solar input to switch my "fix" wire so it's only connected when there's solar. A 12V NO relay in the input cable to the DC-DC, so it only closes the "fix" wire circuit when the wagon is not connected, but I think that relay may stay closed with the 12v from the Aux batteries.
- Is this safe to leave connected?
- Am I possibly going to damage to the DC-DC? I can't see how, but other eyes/brains might.
- Is there a better (& cheap ) solution?
I have 3 other options:
- I can keep the DC-DC, and use 2 MPPT controllers I have, they are only suitable for one panel each, and that adds more wiring.
- I can keep the DC-DC, and buy another MPPT that will support 2 panels. That adds more wiring too. That costs more $$.
- I can return the DC-DC, and buy one that works when there's no 12V supply. I.E. caravan is disconnected. That costs more $$.