On my last day of vacation, I sat on a pristine beach, sipping on a piña colada while staring at a turquoise Caribbean Sea. In four days, I’d charged my Apple Watch Ultra 2 three times, and I was down to about 30 percent. On the other wrist, I had the more modest $249.99 Garmin Lily 2 Sport. It was at about 15 percent, but I hadn’t charged it once. Actually, I’d left the cable hundreds of miles away at home. While pondering this, the Ultra 2 started buzzing. My phone may have been…