I had a similar problem when I bought my M18 grease gun. It came with a battery and charger but since I already had a charger from a previous Milwaukee purchase I didn't bother to unpack the new charger. Anyway, when I placed the 1.5AH battery into my existing charger it just flashed Red/Green. I unpacked the new charger and inserted the battery and it started charging just fine. Now that it is charged it works fine on both my Milwaukee chargers. Weird, eh?