It is normal for the lights on the charger to flash red/green for up to 20 minutes. The charger is slowly building the charge in the battery until the battery is able to accept the full output of the charger. This protects the battery and increases the life of the battery. If the lights continue to flash red/green after 20 minutes but go to a steady green when the battery is removed, it is likely there is a problem with the battery.