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It's your afternoon break and you know you have a long night ahead. You plug your phone into your power bank— many of the best ones are made by Anker —and find after 15 minutes that you're not getting as much juice as you should be. What's going on? Time to deploy a diagnostic checklist.

First, make sure neither your phone or power bank are running hot. If they are, your devices will negotiate lower charging rates down to a certain degree to minimize additional thermal stress. High heat can lead to adverse behaviors in your phone (including failure to charge and rebooting) and, in extreme cases, cause batteries in either device to expand and explode. A general rule to follow is to disconnect them when they're getting too hot to comfortably hold.…