Now lets move on to pushing a Li-Ion cell beyond it's design:
Operating Current Limits
Manufacturer's do not just randomly pick out ratings for their cells. Many Li-Ion cells datasheets will specify a *maximum* 2C discharge rate. If a cell is rated for 650mAH, then it's 2C rating would be 1300mAH. This is the *absolute maximum* rate you are supposed to discharge the cell at. This does not hold true for all Li-Ion cells. In some cases it is less. In all cases, it is inherently sane (and imho very important) to go and actually look at the manufacturer's datasheet for the cells maximum discharge rate.
So, just what happens if you push a cell too hard?
In a Li-Ion cell, if you push the cell beyond the maximum discharge rate, you in essence are pulling excessive current from the cell. This often results in permanent damage to the Li-Ion cell. It may be gradual, or it may be more immediate.
While discharging at high charging currents, the Li+ ion starts to diffuse improperly into the anode's graphite particles which starts a plating of lithium metal on the surface. This plating caused by high discharge current forms in a manner which causes dendritic growth that can puncture the separator and form a conductive short circuit to the cathode. In some cases these will be "soft shorts", where the cell has a momentary drop out of voltage. In other cases it can result in a chain reaction which results in very violent venting, or possible explosion. Even a venting stituation can be dangerous, and Li-Ion cells can vent with flame and hot molten metals, and very reactive hot Lithium.
If you are pushing your cells beyond their ratings, sometimes you will just see a rapid reduction in cell capacity. If the cell doesn not explode or vent, your cell suffers an early death. This plated lithium metal mentioned above is quite reactive toward the cell's electrolyte and results in electrolyte decomposition at the anode, growing the thickness of the SEI layer. This will also increase internal resistance, and reduction of available electrolyte.
Another situation that happens when running Li-Ion cells beyond their ratings is called "polarization". Polarization, is the inability to move lithium ions through the electrolyte, and in and out of the active materials, will greatly reduce the cell's performance.
When building Li-Ion cell packs, where the cells are in series, another technique is used during the build process, above and beyond all the aforementioned protection devices and circuit. In this case, battery pack manufacturers will actually run the cells over a number of charge and discharge cycles. Then they will match cells with similar characteristics. This helps prevent the over discharge (dangerous) of one of the cells in the series pack. A weak cell in a pack may also get warmer than other cells in the pack. Above and beyond all this, the associated electronics in a series pack actually monitor each cells individual voltage, and have various mechanisms to protect from explosion and venting of the weak cell.
The weak cell scenario also applies to the charging of the series pack. In this case, the weak cell may become fully charged before the rest of the cells. Then the cell starts to overcharge, which can result in venting and/or explosion. The electronics also protect against this scenario.
Unlike NiCd, Lead-Acid, or NiMH, there is no natural mechanism for Li-Ion cell
equalization during charging. This is another reason why the electronic protection circuits, and proper chargers are necessary. Otherwise you risk venting and/or explosion.
In situations where packs may be exposed to an unbalanced external thermal gradient, the cells will not discharge at the same rate. In this case, it is even more important to have the electronic protection circuitry. Otherwise, once again, you risk the vent/explosion senario. Though, the proper way to deal with this is not to put the cells in a situation where there is a thermal gradient.
I sure hope folks that are starting to buy some of the larger, and even more dangerous Li-Ion cells read and heed all the information presented so far.
Edited for spelling.
, but I was wondering
