the wost thing you can do to your batteries is charge it aimlessly. do not at all cost overcharge your batteries since it may leak and explode. compute the charging power of your battery against your rechargeable batteries.
battery capacity = 2500mA
battery charger = 500mAh
total charging time = battery capacity / battery charger mAh
total charging time = 2500mA / 500mAh
total charging time = 5 hours
you can avoid all this nonesense computation if you buy a smart charger XD
or better yet check this thread to learn more about rechargeable batteries <click here for link>