Q: Can a battery charger damage a battery ?
A: Yes, a charger can shorten battery life or reduce its capacity.
The most common cause of premature battery failure is overcharging. The type of chargers most likely to cause overcharging are the so-called old generation "rapid chargers"(5 to 8 hours chargers). The problem is that they do not have a charge control mechanism and often charge at their full charge rate for a fixed period of time, typically 5 or 8 hours, and then shut off or switch to a lower "trickle" charge rate. If they are used properly, these chargers are fine but if they are used improperly they can shorten a battery's useful life in a couple of ways.
First, suppose fully charged or partially charged batteries are put into the charger. The charger has no way to sense this, so it will give the batteries the full charge it was designed to deliver. It is not unusual to put partially charged batteries into a charger since it is easy to mix batteries up and inadvertently put fully charged batteries into a charger. Do this enough times with one of these battery chargers and the capacity of the battery will start to drop.
Another common situation is for the charge cycle to be interrupted part way through the charge. The charger is unplugged to see how warm the batteries feel or to use the electrical outlet for something else. Then the charger is plugged back in. Unfortunately, this will cause a complete charge cycle to start again, even if the previous charge cycle was almost complete.
The easiest way to avoid these scenarios is to use a smart charger, a charger with microprocessor control. A smart charger can determine when a battery is fully charged and then depending on its design, either shut off entirely or switch to trickle charge.