The Li-Ion battery contains an anode made out of LiCoO2 (Lithium cobalt oxide) and a cathode made of highly crystallised specialty carbon. As an electrolyte fluid, it uses an organic solvent optimised for use with the negative electrode material.

During use, the lithium of the anode is ionised (Li+). During charge, the lithium ions move from layer to layer towards the cathode. During discharge the reverse happens. The ions move towards the anode and re-unite to form the original compound.

The result is a high capacity battery with a terminal voltage of approximately 3.7V at full charge. Manufacturers manage to retain a compact and lightweight package inside a stainless steel or aluminum external case. This package may be prismatic or cylindrical.

For a given volume, a Li-Ion battery has a higher capacity than a Ni-Cd or Ni-MH cell. The terminal voltage is 3 times that of a Ni-MH, thus consenting use in applications of miniature dimensions where space utilisation is at a premium (ex. Notebooks).

Other characteristics of Li-Ion batteries are their clean discharge characteristics, maintaining a practically stable voltage throughout their entire capacity. Research over the past few years on specialty carbon has continued to optimise these characteristics. Similar to Ni-MH, the memory effect is minimal. This permits charge and discharge at will, without waiting for the battery to become fully discharged before re-charging it.

The normal charging voltage is 4.1V or 4.2V depending on the maker of the cells. For cells in series, this voltage will be a multiple according to the number of cells. The charging current is typically 0.5CmA ~0.7CmA for cells which have a terminal voltage over 2.9V.

It is not viable to increase the charging current to speed up the charging process. This is because in a typical 3 hour charge, the last 2 hours are spent topping up the battery at a reduced charging current. By then the battery will already be over 75% charged.

The charging temperature range is similar to that for Ni-MH cells, 0oC ~ 45oC. It is imperative that the polarity of the charger is never reversed. However, the Li-Ion battery charger is not as straightforward as it seems from the above. Moreover, the Li-Ion battery usually has a protective PCB which performs several functions. The charger is normally connected via the PCB.

The upper limit of 4.2V has to be accurately respected with not more than 0.05V tolerance. Above this voltage, the charger stops the charging current, unlike the Ni-MH charger which continues providing a trickle charge.

The temperature rise on correct charging is only around 2oC ( after a 3 hour charge). The protective PCB stops the battery from discharging below 2.9V typically. Some more energy could be obtained if this design voltage is reduced by the manufacturers. However, since Li-Ion batteries exhibit very flat voltage discharge characteristics, this does not handicap the battery by a large amount. The protective PCB also protects the batteries from high temperatures in case of misuse and act like a fuse in case the charging voltage exceeds the 4.3V limit.

Another asset of Li-Ion batteries is their shelf life, which is superior to other technologies. The only real disadvantage is their cost. The materials used, being rather rare, contribute to their high cost. Cobalt will probably be completely replaced by a cheaper alternative in the coming years. Moreover, research into different forms of specialty carbons and alternative cathode materials will help to at least double the battery capacity/volume in a few years time.