New Magsafe iphone battery pack

When I charge my battery pack, the yellow light goes out, but it still charges. When I unplug it the yellow light shows up and then goes out. When it is fully charged, the green light doesn't come on, but when I unplug it, the green light shows up and then goes out.

When I attach it to my iphone it does charge. Why isn't the yellow or green light showing up? I am using a 20W charger.

Thanks,

Karin