Solar-powered power banks seem like a fantastic idea; attach some solar panels to a power bank, and you have a free power source whenever the sun is shining.

However, the reality is different from this idyllic vision of limitless energy.

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I've tested nearly two dozen solar-powered power banks, ranging from devices purchased online to ones bought from camping and outdoor supply stores. Unfortunately, every single device has been disappointingly ineffective -- and some have even been potentially dangerous.

Using such a device means placing a power bank, a box filled with lithium-ion batteries, in direct sunlight. And that's a big problem. I've found that a power bank left in the sun on a modestly hot day can hit temperatures as high as 80