During fast charging it is possible to pump electrical energy into the battery faster than the chemical process can react to it, with damaging results.
The chemical action can not take place instantaneously and there will be a reaction gradient in the bulk of the electrolyte between the electrodes with the electrolyte nearest to the electrodes being converted or "charged" before the electrolyte further away. This is particularly noticeable in high capacity cells which contain a large volume of electrolyte.
There are in fact at least two key processes involved in this chemical conversion. One is the "charge transfer", which is the actual chemical reaction taking place at the interface of the electrode with the electrolyte and this proceeds relatively quickly. The other is the "mass transport" or "diffusion" process in which the materials transformed in the charge transfer process are moved on from the electrode surface, making way for further materials to reach the electrode to take part in the transformation process. This is a relatively slow process which continues until all the materials have been transformed.
Both of these processes are also temperature dependent.
The charging process may be subject to other significant effects whose reaction time should also be taken into account such as the "intercallation process" by which Lithium cells are charged in which Lithium ions are inserted into the crystal lattice of the host electrode.
In addition there may be other parasitic or side effects such as passivation of the electrodes, crystal formation and gas build up, which all affect charging times and efficiencies, but these may be relatively minor or infrequent, or may occur only during conditions of abuse. They are therefore not considered here.
The battery charging process thus has at least two characteristic time constants associated with achieving complete conversion of the active chemicals which depend on both the chemicals employed and on the cell construction. The time constant associated with the charge transfer could be one minute or less, whereas the mass transport time constant can be as high as several hours or more in a large high capacity cell. This is one of the the reasons why cells can deliver or accept very high pulse currents, but much lower continuous currents.(Another major factor is the heat dissipation involved). These phenomena are non linear and apply to the discharging process as well as to charging. There is thus a limit to the charge acceptance rate of the cell. Continuing to pump energy into the cell faster than the chemicals can react to the charge can cause local overcharge conditions including polarisation, overheating as well as unwanted chemical reactions, near to the electrodes thus damaging the cell. Fast charging forces up the rate of chemical reaction in the cell (as does fast discharging) and it may be necessary to allow "rest periods" during the charging process for the chemical actions to propagate throughout the bulk of the chemical mass in the cell and to stabilise at progressive levels of charge.
A memorable though not quite equivalent phenomenon is the pouring of beer into a glass. Pouring very quickly results in a lot of froth and a small amount of beer at the bottom of the glass. Pouring slowly down the side of the glass or alternatively letting the beer settle till the froth disperses and then topping up allows the glass to be filled completely.
Fast charging also causes increased Joule heating of the cell because of the higher currents involved and the higher temperature in turn causes an increase in the rate of the chemical conversion processes.
