One reason why we want to charge at a slow rate is to allow all the chemicals in the battery to migrate properly and slowly into their charge state. Today’s technology is good for about 1C as they say, or charger at the speed on one capacity of the battery, which puts us at the 1-hour mark. Anything faster than that, and we are damaging the cells more than the normal rate.
Second reason to charge slow is to keep the battery voltage at a lower rate. Today’s cells are considered full at the 4.200V mark, there is newer technology that bumps this up to 4.350V. As lithium batteries near completion in their charge cycle, all chargers, slow, fast, and anything in between has to slow down in the last 20%-30% charging rate. Fuller batteries are not able to convert all of the energy supplied and they shed the excess energy by heating up. Thus why all chargers have to slow down until a full state of charge is achieved, otherwise cells get over charged, damaging the cells.
During the charging process, a lithium battery has the same characteristics and a car lead acid battery. The
CC/CV charge cycle,
Constant Current – the fast charge at the beginning of the charge cycle when the battery is completely empty. The charger can apply maximum current it is designed for and monitors cells as they near the 4.200v maximum. /
Constant Voltage – when the battery is no longer able to handle all the energy supplied by the charger, voltage reaches 4.200V and now the charger has to scale back the current as to not over charge or raise the cell voltage over 4.200V. Over voltage, damages the cells by oxidizing the electrodes, further overcharging raises the cell temperature further damaging the cell, more further overcharging causes a thermal run-away reaction and combined with oxygen, cell catches on fire.
The concept of slow charging is that of a dimmer switch. The battery applies a load on the slow charger, is able to consume all current applied, cell voltage is kept far from the 4.200v threshold. As we turn down a dimmer switch, we restrict the flow of energy, (current) light bulb is able to consume all supplied energy, voltage drops, bulb starts to dim. Same concept applies to old school flash lights without electronic regulation when batteries were at the end of life.
A fast charger is a full open dimmer switch, during the
constant current phase, charger blasts the battery with the max current it is rated for, causing a higher overall voltage throughout the charge cycle. A fast charger will have to start scaling back much sooner than a slow charger as the battery will reach the 4.200V threshold much faster. The battery is not fully charged, it may only be at the 50% charger rate, the higher current causes the initial voltage spike, requiring a much sooner throttle back response than a slow charger. That is why a fast charger has active cooling, while a slow charger does not, as passive cooling is more than enough. A slow charger will also get to 4.200V but that happens much later in the 80%-90% thus saving the cell from heat and higher voltage.