The charger flashes red and green when there is nothing in it to charge. When the battery is inserted alone or attached to the radio, the light should go solid red when charging and turn solid green to indicated charging is complete.
The charger LED may also flash red and green if the battery and radio are not inserted properly and one or more metal contacts in the charger are not touching the contact points on the battery pack. It can also indicate the charging tray or battery pack may be defective.
If you have another battery pack for the UV-5R, try it in the tray. if the LED turns red, the other battery pack may be defective. If the LED continues to flash red and green, they charger tray may be defective. If you don’t have a spare battery pack, you may want to purchase one. They are very inexpensive and are always handy to have when carrying the radio around for extended periods.