Yes, it is dangerous to attempt to charge a deeply discharged Lithium battery. Most Lithium charger ICs measure each cell's voltage when charging begins and if the voltage is below a minimum of 2.5V to 3.0V it attempts a charge at a very low current . If the voltage does not rise then the charger IC stops charging and alerts an alarm.
Full eruptions should be avoided because they put additional strain on the battery. Studies have shown that a lithium-ion battery regularly discharged to 50% before recharging will have a longer lifespan and may retain up to 1,500-2,500 cycles, compared to just 500-1,000 processes if regularly fully discharged.
It is well known that Li-Ion batteries should not be deep discharged. But sometimes they do discharge deeply. Is it OK for the device to remain in such state for a long time (and recharge again only when the device is needed again after a year) or it should be charged back as soon as possible? In other words, the battery was discharged deeply.
According to tests by BigClive (sorry, I cannot recall exactly in which video he mentions this) some batteries can be completely discharged (0 Volts), charged again and then appear to still have their full capacity. Note that the "fully discharged" state did not last long (hours or days perhaps). Lithium-ion batteries age
This means the battery will only charge if left on the charger, addressing concerns about leaving devices plugged in overnight. Storing lithium-ion batteries at full charge for an extended period can increase stress and decrease capacity. It’s recommended to store lithium-ion batteries at a 40-50% charge level.
The situation is simple: when a battery gets discharged below certain limit set by protection circuitry (say 2.9 or 2.5 V), the circuit disconnects the battery output. The terminal would show "zero voltage", which looks like "dead". However the INPUT path is there.