"Most of the problems with rechargeable batteries
can be traced to misuse"
HOW CELLS ARE DAMAGED Sustained high-current overcharge and cell polarity reversal (during discharge) are the main killers of Ni-Cd and Ni-MH batteries: If a high charge rate is used, it is essential to terminate charge when the cell is full. If this is not done, the temperature and pressure within the cell will rise quickly as the charging current is dissipated as heat. Both Ni-Cd and Ni-MH cells have internal vents which will open to allow gas to escape and prevent explosion of the cell. In the case of Ni-Cd, the gas released is oxygen, while a Ni-MH cell will vent hydrogen. The gas that is lost can never be replaced, which means that the lost cell capacity which results from a severe overcharge is not recoverable. It is never wise to rely on the cell's vent as a failsafe, because they often corrode overtime and can not be assumed to be absolutely reliable. Avoiding abusive high-current overcharge can only be ensured with a well-designed charging system that responds to the signal the battery gives when fully charged. Cell polarity reversal is a potential problem with any series-connected string of cells: as the battery is discharged, the cell that goes down to zero volts first will continue to have current forced through it by the other cells. When this occurs, the voltage across the fully-discharged cell is reversed. A cell that has current forced through it with a reverse voltage across it will heat up very quickly and vent gas in a similar mode to that described for the sustained overcharge, with the same resultant damage. LI-ION: HOW CELLS ARE DAMAGED The biggest problem with the Li-Ion battery is the ease with which it can be damaged during use: The Li-Ion battery carries a very large amount of energy in a small package. Combined with the fact that the internal resistance is fairly high, you have the potential for a very dangerous product: If the cell is accidentally shorted, it could get hot enough to burn a user (and possibly explode). The makers of Li-Ion cells handle the explosion threat by designing the case of the cell so that it will "die with honor", and not explode in someone's pants pocket if the battery hits their car keys. More important, the actual battery terminals are simply never allowed to reach the outside world. The only manufacturer presently shipping Li-Ion batteries for consumer products does not sell single cells, only battery packs. Contained within the pack is circuitry that isolates the battery power leads from the outside world if excessive current is sensed, pre-venting battery damage and protecting the user. Another easy way to destroy an Li-Ion battery is by discharging it too far. The Li-Ion cell should never be allowed to drop below about 2.4V, or an internal chemical reaction will occur where one of the battery electrodes can oxidize (corrode) through a process which can not be reversed by recharging. If this occurs, battery capacity will be lost (and the cell may be completely destroyed). A similar process will occur if an Li-Ion cell is charged to too high of a voltage. If current is continually forced into a fully charged cell, internal corrosion can take place which will reduce cell capacity (possibly completely). For this reason, Li-Ion cells can not be trickle charged for extended time periods without cutting off the current when full charge is reached. Sustained charge currents (even a few ľA) can damage the cell if allowed to run continuously. AGE-RELATED FAILURE MODES Ni-Cd: After a period of time, the insulator within a Ni-Cd battery often develops holes which allow the cell to grow crystalline "shorts" that provide a conduction path between the positive and negative electrodes of the cell (which basically shorts out the cell). If this happens, you may have to blow open this short with a high current pulse before the cell will again accept charge (a process sometimes referred to as "zapping"). A leaky Ni-Cd cell will always have a high self-discharge rate and will re-grow internal shorts if left on the shelf without some kind of trickle charge. The annoyance factor of batteries that go dead quickly often prompts users to throw away leaky Ni-Cd batteries, even though they may still be able to give nearly full A-hr capacity during discharge. Ni-MH: The Ni-MH cell (according to the manufacturers) is not supposed to be prone to developing internal shorts like a Ni-Cd battery. User feedback (so far) on Ni-MH has been good, with no major problems reported. Li-Ion: The Li-Ion battery got off to a weak start, as there were many failures in the first batteries shipped. However, the addition of the internal protection circuitry inside the battery (and increased knowledge about the failure modes) has improved this. At present, there are no known problems which appear significant enough to prevent Li-Ion from successfully penetrating the high-end consumer market. |
