Most I've seen have a resistor and a transistor across each cell and use that to discharge the cells with too high voltage.
During charging the charger will stop charging and measure the cell voltages, if any of the cells have a higher voltage than the others, they are discharged a little.
LiIon is usually charged at constant current until a max allowed voltage is reached and then is held at that voltage while current tails off under "control" of chemistry of battery until Ichg = k% of Imax where k% is chosen according to longevity or max energy concerns.50% or 25% of Imax gives longer life. 10% or 5% tail gives max capacity but lower life.
Lowering Vpedesatl by 0.1V greatly assists battery life.
Discharging to higher cutoff voltage aids cycle life.
LiIon also has calendar life and starts self destructing from day one so a lightly used battery still dies.
Best cycle life is achieved by stopping charge when Vpedestal is reached and systen changes from CC to Cv. By monitoring voltage this point can be observed. You could even do a "dumb" system that simply watched delta Vbat and declared constant V when delta fell to zero. Only slightly more than a comparator and an RC delay in one input would achieve that.(While Vin is ramping a delayed vin is lower. When Vin pedestals the delayed Vin almost catches up. An offset voltage is needed to allow comparator towork).
LiIon cells mechanically flex the cell as metallic Lithium is "plated" in and out of the cell*. Cycle life is in large part due to battery beating itself to death mechanically.(This is why LiFePO4 lasts much longer and has lower capacity - the material is held in an Olivine matrix that maintains constant shape as active material is moved in/out BUT it takes up some space. )
*Note: Bill Dubuque has suggested that this sentence would be better replaced with " 'LiIon cells mechanically flex the cell as Lithium ions are intercalated".
The distinction is a finer one than may be apparent. However, it is true to say that if you cut open a LiIon cell you would not usually find metallic Lithium in it. Bill notes that this makes primary Lithium cells, which do contain metallic Lithium, a greater fire hazard than LiIon cells.
If you charge a LiIon cell with excessive voltage metallic Lithium will be 'plated out' and "vent with flames" mode usually occurs at about the same time.
Charge to CV level as often and as soon as possible.
If charging all the way their "disconnect message" is a sign of bad ethos. They are probably trying to minimise the risk of fire without telling you.
For longest storage life (as opposed to long life in regular use) storing at a lower voltage than Vmax is in order. Probably at about 3.6 V and only about 30% state of charge. The various Mars Rovers use LiIon batteries and have a design life of about 8000 cycles - but charge to about 3.6 - 3.7 V maximum.
8000 / 365(~=) ~= 22 Terran years.
Best Answer
Constant current of x amps (x is usually a fraction or multiple of capacity, e.g. 0.5C, 1C, 2C. Usually never any higher than 5C or cell heating results) until 4.2V ±0.5% is reached. This is called the "CC" stage.
Then a constant voltage of 4.2V ±0.5% is applied until charge current drops below a specific current, usually 0.2C to 0.05C. This is called "CV".
If cell voltage is below 2.8V, a precharge is performed at about 1/10 to 1/20 normal charge current until cell voltage reaches 2.8V; from then on CC takes over. Discharging a li-ion below 2.8V can usually cause a loss of capacity or permanent damage, so avoid doing it. Not all chargers implement pre-charging; some will refuse to charge over-discharged batteries.
This is for newer li-ion cells with 3.7V nominal voltages; for older ones with 3.6V nominal voltage, use 4.1V ±0.5% instead of 4.2V as the CV point.
This is per cell. For multi-cell packs at low charge currents, cells are charged in series. With higher charge currents, they are still charged in series but if they become out of balance, a balancer draws a small current from the appropriate cell to re-balance the pack. Some lower-current charges charge each cell separately, but this increases the cost.