Thanks redhatter. Great explanation. I'm still unsure why routing the output from the dynamo to the USB then to a battery pack then to a device such as a tablet seems to be the preferred method for charging devices whilst cycling.

There are many cache batteries out there ranging from 1000 mAh to 20000 mAh. Since this is just a capacity does it matter if I just go with the lower end of the spectrum? The battery will be charging and discharging at the same time.

I've just found out my tablet battery has a capacity 5710mAh. It requires a 2A power supply.

Thanks