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?
To charge a phone you need exactly 5V, at least 0.5A but ideally 1A or more. Forget about USB lights. Their internal batteries are 3.7V which is hard to convert; they're also inaccessible. But many of the high-brightness rechargeable lights use detachable battery packs at 7.4V.
This can be converted to 5V with some electronics: either a linear regulator like the old 7805 or better a switching regulator. You'd need a little electronics skill to do this - I won't go into detail here.
A better solution is to get a USB battery pack. These are cheap and can charge a USB light or your phone, without running your light's battery down.
Update 2018: Direct USB-powered lights are becoming quite common. These run off any source of 5V with a USB socket, typically a battery pack. So you could have one light and one battery pack, and be able to light your way and charge your phone.
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.
Most lights are not designed for this, and it will not work.
Anyhow, even if you had a light that this could work for, its not a great idea to get into the habit of. Why? Phones take a decent amount of energy to charge. With a battery the size of a bike light, you can easily end up draining the battery by trying to charge your phone (especially as the battery capacity degrades). And then, you're stuck in the dark, without a light. And that ain't good.
Its well worth carrying some usb charger instead. Small ones which are good for say one charge of your phone can easily fit in a saddle bag or your pocket.
Yes, some models have the second USB A type port to provide power, using the charged lamp as battery pack. You can charge your phone or the red rear light, for instance. Google 'bike light USB power bank' to find them. The photo shows Dosun light with 2500mAh capacity accumulator, enough to charge a typical phone battery from empty to almost full.
You obviously need to plan the battery usage and have enough also for the light. The light I use has few tiny LEDs at the top that work as a battery charge level indicator.
Some lights with internal battery can also be used as a power bank, but only if they are designed for that.
There are some external batteries that can be used for that. For example Lupine batteries have USB adapters (Usb one for old usb and Usb two for usb-C). There is wide range of different battery sizes to choose from.
Typically external batteries don't use usb, because standard usb connector usually can't be made waterproof. Also some lights require higher current than usb (except usb-C) can provide.
Other option is hub dynamo and for example B&M Luxos U and Sinewave Beacon. They are dynamo lights that can also be used to charge devices. Modern dynamos are very good for both light and charging.