This is a picture of an exploded battery on a 45 KW backup generator:
This happened during a thunderstorm. The smoking crew was out on the back porch, during a thunderstorm (you have to have your priorities I suppose) and witnessed the entire incident. What was told to me was lightning stuck the generator. I find that improbable since there are many metal objects scattered around the area that are much taller. What likely happened was lightning stuck something close by, causing the power to drop out momentarily. This caused the generator to turn over. The battery was likely low in electrolyte, so there was an internal arc and the thing exploded in short order.
These events happened in rapid succession, giving the illusion that lightning struck the generator.
I removed the old battery and hosed the inside of the generator with copious amounts of water. There is nothing that can be done about the spilled electrolyte, since it was likely washed into the storm sewer during the storm. After the replacement of the battery, I tested the generator and all is well.