Re: AW\'s Rechargeable solution vs. Batterystation\'
Hello AFAustin,
In general, Lithium cells need to be protected from over charging, over discharging, and high current draws (including dead shorts).
Over charging results in reduced cycle life as long as the voltage is kept below 4.4 volts. At around 4.5 volts they go into thermal run away resulting in "rapid venting with flame." At 4.4 volts, they are good for about 5 cycles, then die. At 4.2 volts, they are good for about 200-500 cycles. At 4.1 volts, the cycles go up to around 1500. It should also be pointed out that charging at too high a current can also damage the longevity of the cell.
Over discharging (and storing cells at a low state of charge) has the effect of forming copper shunts inside the cell. The next time the cell is charged, these shunts form micro shorts that reduce the cells capacity and raise the cells internal resistance. When enough of them form, the cell shorts out with "rapid venting with flame" during the charging cycle (or sometimes while resting after a charge).
Too high a current draw will cause the chemistry to heat up. When the temperature gets high enough, thermal runaway occurs which results in "rapidly venting with flame."
Using two (or more) cells in series can allow some imbalance between the cells. A special case occurs when one cell drops to a low voltage before the others. The other cells try to "charge" the dead cell up rapidly, resulting in high currents and cell voltage reversal. This can result in "rapid venting with flame."
Protected cells have a circuit that can address these issues. The protection circuit takes space, so the downside is that protected cells have reduced capacity.
Tom
