@skin: i had used my Nitecore i4 with 1pc, 2pc, 3pc, and 4pc AA/AAA batteries without any issue. but normally, i do it by pairs.
actually, i don't keep tally of the life of my rechargeable batteries. i bought my eneloop AA and AAA batteries in 2010 and buhay pa silang lahat up to now.
what is the model? is it a smart charger?
What makes a charger a "smart charger"?
Any charger that uses a computer chip to control various aspects of the charging process can be considered a smart charger. Technically even a charger that can detect and adjust the charge rate based on the battery inserted into the charge station can be considered a smart charger, but anything that is either manual (steady charge rate as long as it is plugged in) or uses a timer to manage the charging process, we do not consider a true smart charger. There are even various levels of smart chargers. Different features that work together, sometimes in mysterious ways because there are just so many variables with batteries and chargers. In order for us to consider a battery charger a smart charger it needs to have a common charging feature known as negative delta V. Negative delta V is basically a technical method for a charger to know when a battery has reached its charge capacity and then shut the charging off, or sometimes change to trickle charge mode. Other features that contribute to a battery chargers "smart" status are: battery rescue (implemented in various ways to attempt to "jump start" an overly discharged battery - i.e. less than 1.0 or 0.9 volts - so that it will take a charge), temperature sensors, discharge and conditioning features, battery test features and even timers to limit the total length of the charge so even if you leave it plugged in, it turns itself off after a preset time. Remember, all manufacturers consider their chargers "smart" with any or all of these features and they are not all the same!? Hey, neither are we for that matter...
Can a battery charger damage a battery (shorten its life or reduce its capacity)?
Yes. The most common cause of premature battery failure is overcharging. The type of chargers most likely to cause overcharging are the 5 or 8 hour so-called "rapid chargers". The problem with these chargers is that they really don't have a charge control mechanism. Most of them are simple designs which charge at their full charge rate for a fixed period of time, typically five or eight hours, and then shut off or switch to a lower "trickle" charge rate. If they are used properly, these chargers are fine. If they are used improperly they can shorten a battery's useful life in a couple of ways.