Can You Overcharge Your Lithium-ion Cell Phone Battery?
As has been seen, the regular recharging of a Li-ion or cell phone battery is a basic redox reaction that causes lithium ions to travel from the negative (cathode) to the positive (anode) electrode. Li-ion batteries can be recharged a number of times, but eventually, after a number of charge discharge cycles, this battery will need replacement, too.
It is important not to overcharge the cell phone battery, or to let it drain out completely. If it runs out of charge completely, it will spoil the battery cells, so it is required to plug it in the charger while it still contains some charge. Furthermore, if you overcharge the battery, it creates heat which could be a fire hazard, or even cause a fire itself.
Since lithium-ion (and lithium-polymer) are sensitive to overcharging and over-discharging, the cells that make up the battery pack are charged independently. With NiMH and NiCad packs, the standard charging phenomena is to provide a current and voltage to cells connected in series so all the cells receive the same amount of current. The cell voltages in that case are not necessarily equal, in fact it is almost certain that they are not, but the voltage across the battery pack will be the same. Since NiMH and NiCad relatively show resistance to overcharging, there is usually no danger. It is common for one cell in a NiMH or NiCad battery pack to be dead without an effect on the charging of other cells. What does that mean? Let me explain it with an example.
Let’s suppose that we have a 6-cell battery pack, with the nominal voltage of each cell being 1.2 Volts (7.2 Volts total). If a charging voltage of 5 Volts is present, each cell would ideally receive 0.83 Volts approx. If one cell is dead, the voltage across each cell would be 1 Volt. That means a 20.5% increase after the death of one cell. This percentage increase is catered to by most Nickel-based cells, but a 20.5% overvoltage to lithium cells would ultimately result in a failure, and likely a fire. Li-Ion/Li-Poly batteries cannot tolerate over discharging as well, and tend to die if discharged below around 2.5 V.