I had to repair my solar power system recently. My battery wasn't charging.
It took me three months to find the time to spend an hour getting to the wiring, diagnosing the problem and repairing it.
It was just a connection that had vibrated loose between the charge controller and the battery. When I did the initial installation, must not have been paying enough attention, because I had only stripped the wire back a quarter inch, not enough for a solid connection on the inline splice block. I stripped the wire back a bit more and reattached it.
Everything is working fine now.