The short answers to your questions....
- Being able to charge a severely overdischarged cell is not a common charger feature but it's nothing special.
- You can use a cell that has been overdischarged and recharged but it won't work as well as before.
- I wouldn't throw away a cell that was overdischarged only once but I would not recommend continuing to use a cell that is overdischarged often. It will get hotter while being used, won't last as long when being used, and the charging needs to be done properly. It's a lot better to just determine why the cell keeps getting overdischarged and stop it from happening again.
What's really going on...
Many chargers will not charge a cell that is below a certain voltage, often in the 2.5V-2.8V range, because it can overheat the cell. This can either further damage the cell or cause it to vent because of high internal resistance leading to overheating.
It's not hard to create a charger that will charge an overdischarged cell but it makes the charger more expensive. This, along with the increased need for attention to safety means that a lot of companies will just design their chargers to ignore cells below a certain voltage.
A well designed charger will gently charge a cell at a reduced rate ("precharge"), often 1/10th the charging rate, until the cell voltage has risen above a certain threshold. Above this threshold the internal resistance has dropped significantly and the chance of overheating during charging is reduced. Once above this threshold, often in the 2.5V-2.8V range, the cell will be charged at normal rate until it's fully charged.
Well designed chargers will also have a couple of timers built in. They will not continue to charge a cell that hasn't risen above the precharge threshold within a certain amount of time (often in the 20-30 minute range). This is to prevent the charger from continuing to charge a truly dead cell for hours, potentially creating a safety hazard. The other timer turns off the charger if the cell doesn't complete charging within a certain amount of time (often in the 3-5 hour range), also minimizing potential safety problems.
Other well designed charger features include...
- turning off the charge voltage/current completely once the charging current has dropped to a certain level. This allows the cell to remain the charger indefinitely without damage.
- accurate voltage sensing to ensure proper charging.
- independent overvoltage protection, turning off the charger if cell voltage rises too high.
- overtemperature and undertemperature protection to make sure the cell isn't charged when it's too hot or cold.
The Xtar web site says that they invented the technology that allows for charging a cell that is at zero volts. That's ridiculous. Texas Instruments manufacturers a number of charger chips that will handle cells down to zero volts...safely. I've been using these chips in my charger designs for years.
The Xtar web site also says that other chargers can't charge these overdischarged batteries because they can't detect the low voltage. Untrue, they do detect the voltage. They are just designed to not start charging below a certain cell voltage.
The Xtar web site does list a large number of good features for their chargers. LOL, they might even use the Texas Instruments chips! The VC2 seems like a charger with a number of great features for ease of use, information about the cells being charged, and charging safety. If you can't find any negative feedback or
reviews (perhaps great design but low build quality, etc,), I say buy it!
