Batteries typically ignite with active current. Similar to Dave I do not leave chargers powered unless I am charging batteries. I have over 130 Lithium batteries on shop grounds. None are charged without staff present.
The chargers remain plugged in, but they are on manually powered AC outlet strips that are hooked into the motion lights. No movement, no power, so if staff forget to power down a section, it will do it on it's own.
There is an exception for solar, however all of those batteries are in a separate structure with a suppression system. Incidentally nothing about Lithium warrants a class D extinguishing system.
So as long as a battery's not being charged, it's safe? Or less likely to burst into flames? Yes, if it is not damaged. If there is damage these things are volatile, and I believe, and this is an opinion for anyone incapable of understanding what an opinion is, that damaged batteries are more of a culprit for fire than ones with just standard defects.
Cheap batteries are a quality issue and when it comes to Lithium a defect can be much more dangerous. I stay away from super-cheap batteries/chargers/panels etc. At the end of the day there are billions of lithium batteries out there, so from a percentage perspective they are safe. But from a brand perspective I am sure, and this is an opinion for anyone incapable of understanding what an opinion is, my opinion is certain brands or production runs will be more unsafe.
Main link:
https://www.nfpa.org/education-and-research/home-fire-safety/lithium-ion-batt... Sub-links:
https://www.nfpa.org/downloadable-resources/lesson-plans/lithium-ion-battery-...https://www.nfpa.org/downloadable-resources/safety-tip-sheets/lithium-ion-bat... Also if you don't have smoke alarms I'd get some.