usb hub - How many devices can be plugged into a USB hub, and how is power distributed amongst them?

1. 
   

   Is it possible to connect a USB hub to a hub that is connected to a hub and a hub and.. and.. and to a laptop?

   
   


2. 
   

   I was thinking there are bound to be some "overhead" like if without the hub we have 100%, with the hub we lose 1% to overhead and have 99% left to split, and if i add it to another hub I will lose another 1% overhead, but is that assumption right?

   
   


3. 
   

   Are the power divided evenly among the devices? so if I have 5 connected to the hub, each will receive 20%? And what if one has "finished using" (like say I'm charging a iPhone and it's already at 100% / or say i connect to an external mouse but the mouse is switched off), does the power of the other 4 stay capped at 20% as long as the 5th one is attached?

   
   

Answer

The Wikipedia article will tell you everything you need to know, but the basics are that you can have a total of up to 127 devices, this includes the hubs themselves when daisy chained. Each device has available a specified amount from the port it is plugged into, not a division based on how much is plugged in. Each port will provide no less than 4.75 volts and no more than 5.25 volts and up to 500 mA to the connected device. A hub can supply no more than 500 mA in total load. If a device needs more than 500 mA, it needs to draw external power. If the hub is supplying power to multiple devices, the draw is divided amongst them as they require it (all devices start out as low-power (100 mA), but may change mode to high-power after and pull current as needed if available). If the hub is powered from another USB port and does not have external power, it will supply no more than 100 mA to any port at any given time.

USB as a battery charging specification works differently. If you plug your phone into your computer it will charge differently than when plugged into a wall or your car. When connected to a computer, you are bound by those limits above as it is treated as a connected device, but to a wall or your car it can pull up to 1.8 A (though 1.5 is pretty standard for dedicated charging ports).

EDIT: Updated post after reading specifications a bit more in depth.