Is it harmful to use an external battery with my iPhone?

Episode 1254 (1:04:52)

Kenny from Temple, TX
Apple iPhone Battery Case

Leo says it isn't harmful at all, and Apple even sells an external battery for the iPhone 6/6s. In general with these charging cases, when you charge the case it will also charge the phone. On the Mophie cases, there's a switch for when you want to switch to the battery. When it comes to Lithium Ion batteries, though, the best thing to do is keep it charging. The battery will last longer if it stays charged vs charging and discharging. It's definitely not good to let it fully drain.

Lithium Ion batteries have a limited amount of cycles. When it fully charges and fully discharges, that's one cycle. And batteries typically get about 500 cycles. If Kenny only partially discharges it and recharges it, that will only count as a part of a cycle, and will extend the life further. In general, its best to keep the device plugged in. If Kenny is going to store it, he should store it with about half of the charge.