Don't have experience with the Cobra, but have spent some time experimenting with one of the G4 vibes which share the same charger and junk. With mine, the instructions on the charger said that it should glow solid red when connected, and go dark once the toy had finished charging. The toy itself had a pulsing light on the control panel while charging, also going dark once fully charged. Now, I found that when they were properly hooked up, all was as it should be - solid red on the charger, flashy on the toy - and all was well. However, once it was fully charged, the toy went dark, as expected, while the red light stayed on. As an experiment, I ran the battery empty, then left it connected, checking it periodically to make sure it wasn't falling out, for over 24 hours, and that light stayed solid. After that, ran the toy on full power to time how charged it had been, and it had indeed taken a 100% charge. So looks like that red light on the charger stays on as long as it's connected to the toy, and you have to go by the toy's display to see when it has been fully charged.
Maybe it interacts differently with different toys, but that's what I've found with mine, and both charger and toy are working perfectly, even if they are different to as instructed.