Electronic – Is it possible to make a usb charger to power multiple devices

Damage is unlikely. Most likely what will happen if you overload the USB charger is that it will either drop its voltage or cut out altogether. If it drops its voltage, then things won't charge right. If it cuts out, then it will be like it is off. It may cycle on for a short time every few seconds while the overload persists.

Actual overload is also unlikely,however. USB 2.0 only provides for 100 mA per device without enumeration, and 500 mA after requesting and being granted the extra current. Without special communication, your devices have to assume they are plugged into a normal USB port and not draw more than the maximum current for such a port. There is a reasonable chance that your devices are well enough behaved to not overload a ordinary USB port and only draw extra power form their special branded chargers after some private communication. However, this will mean that these devices will take longer to charge their batteries as fast. You can't have it both ways.

What is being requested is very well documented and widely available for DIY, and following Instructable links are examples of that.

In fact there are several questions on EE.stackexchange site, which are currently appearing on the right side panel, as I type this answer.

• Instructable#1
• Instructable#2
• Instructable#3
• Instructable#4

Check your favourite online/offline store and you'd find semi-flexible solar panels, in various dimensions. They claim to be flexible to 20-30 degrees, and could be quite fine for the messenger bag.

They key would be selection of a good solar panel -- since there can be some significant difference in energy efficiency between panels, i.e. the extent to which they can convert the sunlight into electrical energy.

Also note that, not all smartphones charge / charge-well with the standard USB charging, i.e. they make proprietary deviations from the standard, and can expect to be able to draw (much) more than 500mA @ 5V.

PS> I wanted to put this as a comment, but unfortunately, I still don't have enough reps to do anything like that. Surprisingly, I am allowed to post this as an answer.