Traveling to/from Mexico with batteries

[mag3]

I've done it with no problem, years ago when I lived in Germany I had a video camera shipped to my German address so I could take it to Bosnia with me, I had import "issues" with that because it was a new camera and it looked like smuggling I guess, It was easily fixed though, but personal property I've never had an issue with. I have always wondered upon arrival back into the US how do you prove you left with the expensive watch, jewelry etc. and didn't buy them overseas?
The TSA people are idiots, customs people aren't as bad, actually they aren't customs anymore, but CBP now. I used to export several aircraft a year by flying them overseas, only problem I ever had was with the TSA idiots in Atlanta on my return.

US Customs (CBP) Registration Certificate for personal effects taken abroad - Form 4457

Before you leave the US, you go to the CBP station, bring all your items with the form filled out correctly and with serial #'s noted. You can etch or permanently mark your own serial # if needed. The agent will verify them all, and then sign and stamp the certificate form. Take the completed/signed forms with you when you clear US CBP again.

As you noted, it should only be an issue with things that "look new."