"N"s are about the same physical dimention as a 2/3A. Unless the cells are specifically designed for high charge rates (for example sanyo SCR cells) I'd stick to lower charge rates, like the standard 1/10C charge. However, you can probably push it to 1/5C (220mAh in your case) without too much issue. It may reduce the life very slightly, but it won't fry your battery. Just check to see how hot the battery is getting after a C/10 charge. If it is noticably differnet with the 1/5C rate, drop your charge rate back to 1/10C. You can probably charge at much higher rates, but it will greatly reduce the life of the battery.