if you have a metal lid for your travel mug, the alignment of the lid can affect the mug’s ability to charge. If you do not properly screw the lid onto the mug can throw off the magnetic position of the lid which will throw off the receive coil for the mug.
No interference between mugs occurs. The magnetic fields are short range allowing the charging pad to only recognize the mug. Placing two mugs on the pad at once is fine.
The green light on mine has never dropped to red while on the charging pad. I have had mine for 4 years now. It remains green while on the pad and will go through another charge cycle when I place the mug back down on the pad. So the answer to the original question may also depend on which generation of mug you own for I have seen others comment that their mugs have begun to cycle back to red when on the pad.
