TL;DR: You’ll always have a charger for your Apple Watch when you attach it to the PowerBand. It’s now on sale for less than $30 through August 25. Watching your Apple Watch hit the dreaded 20% battery level — or worse, 0% — is frustrating when you’re mid-run or while running errands. How else is it supposed to track your heart rate or step count? But what if you could bring a charger for your Apple Watch anywhere you go? The PowerBand is an Apple Watch strap with a built-in MagSafe charger for handy juice-ups, and it’s now available for only $29.97 (reg. $49). You might think the PowerBand is…