Update: I sent the SC54c N back to China the next day as they requested. By the time it arrived (I think it took about 5 days), they were out of stock so I had to wait about a week. I asked them to test the replacement before shipping and they said they would.
I received the replacement light a couple of days ago, no more blinks after 90 secs, plus I tested a brand new shrink wrapped (but expired) alkaline AA and it worked on all modes. Battery capacity indicator also worked perfectly on both an Eneloop (3 blinks on a partially used cell), 4 blinks when fully charged and still 4 for a while after using. 4 blinks on the new alkaline.
So all perfect, except 7 small chips in the anodizing right at the very end of the body threads, which I'm pretty sure the 1st light didn't have and also my H54c doesn't have. Anyway just a minor complaint, as it doesn't affect the functionality.
Awesome light, I'm very happy with it. It's replaced my HDS, CWF Micro Click and Baton 4 lights as my new favorite EDC. I'm not a clip guy, but this is the first clip that I've seen done right, it clips on the pocket very nicely and very smooth. It's also great to have both the SC and H in AA size as just one set of batteries to carry for EDC, hikes, travel etc. This combo would also be perfect for bug out bags and emergency kits, and for cars (with long lasting and cold resistant Eneloops and lithium primary cells as backups too).
These won't be my last ZLs. I wish I would have tried them years ago, but hey it's fun buying many lights to try and find the perfect one.