Found the reason for those strange brown spots on my setup

I went thru something similar last year when I was testing a deep cycle power station in my yard. It was one of those larger units, probaly around 5000 watt hours, and after maybe six weeks of sitting outside, I noticed these strange white and green patches forming near the connection ports on the casing. At first, I figured it was just dust buildup, but it seemed localized and persistent. I looked at the area and realized the unit had been positioned right near a clogged downspout that directed runoff straight onto the workbench where I kept the whole thing. The issue wasn't just plain water, but water that had picked up heavy particulate matter and maybe some chemicals from the surrounding soil. I moved the entire unit to a covered area and it has completly dried out and stabilized. It really hammered home the point that even if the electronics are rated for outdoor use, they aren't impervious to concentrated splash zones or chemically active runoff. I’ve since started using a simple rubber mat under everything I set out for a few days.

I had a different experience with my unit, which was a smaller model, maybe around the 1200 watt capacity. The brown spotting didn't come from runoff like the original poster described, or from stagnant water like the other person mentioned. Mine appeared after a long summer where the battery spent most of its time tucked into a narrow wooden shelf in the garage.

It wasn't direct moisture, but I think the issue was related to the heat soak. The unit was backed against a cinderblock wall, and even though it wasn't touching the wall, the heat radiating off the block combined with the summer humidity really started cooking the plastic housing. The darker patches seemed to concentrate around the power inlet and the temperature display area. I figured the internal materials were simply degrading from the continuous high ambient temperature, causing a type of surface oxidation on the exterior casing itself. Since then, I only store my systems in an open, shaded area and ensure there is ample space between the unit and any hot surfaces. It seems keeping it cool is key to preventing premature casing wear.

I noticed somthing similar but it wasn't water damage like you guys described. My unit sat in a place that got pretty hot in the summer, just near a vent. I thought maybe it was just a cosmetic issue with the plastic. But after a while, these faint, almost blurry spots started appearing on the shell. It felt less like external liquid and more like some internal component was reacting to the heat and leaving marks on the outside casing. I'm really not sure what the chemical process is there.