The fire hazard danger of lithium ion batteries is real. While instances of solar battery arrays catching fire are not common, they do occur.
When a lithium ion battery of any size catches fire, it cannot be extinguished with water. Additionally, there is a very high risk of explosion once the fire has begun.
It is for these reasons that firefighters in Maryland were recently warned by county commissioners that they are not equipped to respond to these types of fires. The Star Democrat reports:
Volunteer firefighters from several departments met with Caroline County Commissioners Wilber Levengood and Larry Porter Oct. 17 to discuss the dangers of lithium ion batteries that are used to store power from solar arrays. The commissioners warned firefighters to not even respond to fire calls for fires from these explosive chemical batteries that can not be extinguished with water.
The commissioners reference a solar battery array in Arizona that caught fire. As the firefighters were attempting to extinguish the fire, an explosion occurred, resulting in serious injury:
There is a horrific account about seriously injured firefighters from Peoria, Arizona, who responded to a lithium ion solar battery fire that exploded. Broken legs, lost teeth, chemical burns — the injuries were extensive.
The low, but non-zero risk of a lithium battery fire should be a factor in choosing between lead acid and lithium ion batteries for your solar system.
You can read the entire story about the Maryland firefighters here.