Charging mobile phone using the battery of a rechargeable bike light? Most rechargeable bike light are charged via USB port. I wonder if is it possible to use them as a power bank for your mobile phone as well? Or only a subset of them can be use as a mobile phone charger?
Answer No, you need an item designed for this purpose.

Whether yours do or not, it totally depends on the product.  

Many modern lights are charged by USB, but they can't supply USB to other devices.

There are other products which combine a USB battery with a light, but the batteries inside these are much larger than a plain light.  Notice 80%+ of this item is batteries, and it probably weighs quite a bit.



That claims to be a Rugged 3Watt speaker and hands free kit with 8Ah Power Bank, in-built Charging Cable, LED Flash Torch and Bike Mount.  The price is not small.