I'll say this once and back it up with actual verifiable information from Cadex - a battery charger manufacturer. Whilst current is an important factor in charging Lithium batteries it is actually over voltage that destabilises Lithium cells, here are the relevant extracts.
Courtesy of Cadex:
The charge rate of a typical consumer Li-ion battery is between 0.5 and 1C in Stage 1, and the charge time is about three hours. Manufacturers recommend charging the 18650 cell at 0.8C or less. Charge efficiency is 97 to 99 percent and the cell remains cool during charge. Some Li-ion packs may experience a temperature rise of about 5ºC (9ºF) when reaching full charge. This could be due to the protection circuit and/or elevated internal resistance. Full charge occurs when the battery reaches the voltage threshold and the current drops to three percent of the rated current. A battery is also considered fully charged if the current levels off and cannot go down further. Elevated self-discharge might be the cause of this condition.
Increasing the charge current does not hasten the full-charge state by much. Although the battery reaches the voltage peak quicker with a fast charge, the saturation charge will take longer accordingly. The amount of charge current applied simply alters the time required for each stage; Stage 1 will be shorter but the saturation Stage 2 will take longer. A high current charge will, however, quickly fill the battery to about 70 percent.
Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Since the consumer market promotes maximum runtime, these chargers go for maximum capacity rather than extended service life.
Lithium-ion operates safely within the designated operating voltages;
however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V forms plating of metallic lithium on the anode, while the cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO
2). The cell pressure rises, and if charging is allowed to continue the current interrupt device (CID) responsible for cell safety disconnects the current at 1,380kPa (200psi).
Should the pressure rise further, a safety membrane bursts open at 3,450kPa (500psi) and the cell might eventually vent with flame. The thermal runaway moves lower when the battery is fully charged; for Li-cobalt this threshold is between 130–150C°C (266–302°F), nickel-manganese-cobalt (NMC) is 170–180°C (338–356°F), and manganese is 250°C (482°F). Li-phosphate enjoys similar and better temperature stabilities than manganese.
Lithium-ion is not the only battery that is a safety hazard if overcharged. Lead- and nickel-based batteries are also known to melt down and cause fire if improperly handled. Nickel-based batteries have also been recalled for safety concerns. Properly designed charging equipment is paramount for all battery systems.
It should be obvious from this that whilst current is limited to a maximum level this is to prevent heating of the battery during charging and high inrush currents on highly discharged batteries. It should also be noted that as the battery nears full charge the current drops - current is not being "Forced" into the battery it is being "drawn" this should have been obvious to everyone as we refer to coils "drawing" x number of milliamps.
It should also be obvious from the extracts above that the culprit in a charging venting event like this one would be the battery being charged over voltage and since USB is a standard voltage all the arguing over iPad chargers, Kindle chargers, car USB ports is more or less moot since A: they all supply the same voltage & B: The point of failure was most likely the dongle part allowing a too high voltage into the battery, with an actual faulty battery coming in a reasonable second option.
Hopefully this will put an end to the he said she said fight
and let everyone focus on something more important...
