This is a picture of an exploded battery on a 45 KW backup generator:
850 CCA battery exploded during generator startup
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 electrolite, so there was an internal arc and the thing exploded in short order.
These events happend in rapid sucession, giving the illusion that lightning struck the generator.
I removed the old battery and hosed the inside of the generator out with copious amounts of water. There is nothing that can be done about the spilled electrolite, since it was likely washed into the storm sewer during the storm. After replacement of the battery, I tested the generator and all is well.
It looks like the battery was over-charged which “gassed” all of the electrolyte from the cells. The generator manufacturers almost always supply chargers that will overcharge lead-acid cells. I order our generators WITHOUT chargers as there is usually a “delete” option if you ask for it. We use Schumacher Battery Maintainers on all of our generators which WILL NOT overcharge lead-acid batteries. We have had batteries in service 10 years without a problem. These Schumacher units carefully float charge and keep the specific-gravity up such that sulphation doesn’t occur and render the cells useless. They typically cost about $30 and will save money initially on the generator purchase and down the road on batteries.