How severe exactly are the hazards of over-charging and over-discharging?
When a battery gets over-charged, it is like a balloon being continuously inflated, with its internal pressure rising non-stop.
Take the commonly-used ternary lithium-ion battery as an example.
Its normal charging cut-off voltage stands at around 4.2 V. If charging carries on beyond this threshold, a string of complicated and hazardous chemical reactions will take place inside the battery.
The electrolyte decomposes and generates gas, building up internal pressure which may deform or even rupture the battery casing. Worse still, lithium dendrites will form on the surface of the negative electrode. Resembling sharp tiny needles, these growing dendrites are highly likely to pierce the internal battery separator.
If the separator is punctured, the positive and negative electrodes will come into direct contact, resulting in an internal short circuit. This releases enormous energy instantaneously, triggering thermal runaway and ultimately leading to battery ignition or even explosion.
The discharge cut-off voltage of lithium-ion batteries ranges from approximately 2.5 V to 3.0 V, with the exact value determined by the battery's chemical composition and design.
When a battery is over-discharged, irreversible alterations occur in electrode materials, which directly degrade battery capacity and drastically reduce the amount of electric charge the battery can store and discharge.
Meanwhile, the electrolyte undergoes decomposition, and reactions take place on the SEI film covering the negative electrode to generate gas, causing slight bulging of the battery cell and a rise in internal resistance.
Severely over-discharged batteries may suffer permanent failure and become unchargeable. Even if recharging is possible, the charging efficiency will drop to an extremely low level.
