What voltage are you running your solar strings at? I was wondering what would happen if the loose cable was a cable from the solar panels instead of the low voltage battery.
If you want a DIY-friendly option, it's best to look towards DC optimizers. They are installed on each panel and they provide a steady 48V output. They also do MPPT tracking on the panel level, so you get the best possible performance if you have some shading.
Unlike microinverters that are notoriously unreliable, DC optimizers so far have excellent long-term reliability.
Are you suggesting using a DC Optimizer from your solar panels to charge 48V batteries directly? This would be interesting. It's the first I've heard of optimizers being used for anything other than prior to an inverter.
The panel strings are at ~500v so there are safety, fusing and grounding considerations. Pretty common electrical equipment and cable is rated for use at 600v. There are special locking water resistant connectors for solar panels called MC4.
Are the hybrid inverters you are using be able to detect arc faults (in series) from the strings? Running 500V DC is probably by far the most dangerous thing in your setup, fortunately your solar panels are mounted on the groud.
