Electronic – USB power in Parallel, increasing the maximum current using a Y splitter idea[POWER ONLY]

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When you connect batteries or power sources of the same voltage, in series, the voltage adds together. When in parallel the capacity of the two add together.

Ive seen a few usb>sata cables in which there are two usb cables at one end. One for data+power and one for the extra power that larger HDD 3.5inch needs.

So my question is. If I treat the usb ports on my external battery as seperate sources of power, if i join the positive of port 1 to the positive of port 2 and then to a Micro usb cable. The same with the negative. This puts both sources of power/ports in parallel. This is confirmed by the face that if one port breaks the circuit is not broken.

By connecting them in parallel I should increase the capacity. In micro usb battery bank or whatever you want to call it. Capacity is measured in Ah or mAh. 1Ah is 1 solid amp for 1h. So by joining these two ports in parallel, I cant increase the total capacity as they are both the same battery. But should this increase the current.

The above was just to get the idea going, what im planning to do is below

Another mod I have is to take a usb hub, with ONLY a Usb cable with a usb male connector on it. Slit the cable open half way, find the red and black wires and make a T junction out of both. connecting the extra cable I just added to a male usb port that will go to my battery (usb power bank). Im joining 5v to 5v, whats the problem, the power system is working in parallel (because the hub can take it from a pc or the battery) and the data pins are unaffected. Allowing me to take a LOT more current when nessesery. Meaning I could hook up like 12 Hard drives or charge a few phones quickly. This being current each device will only take what it needs?

Basically my plan

Any thoughts or issues spotted. What im aiming to do in total is that I have a windows tablet, but it only has a micro usb 3 port, im gunna use an adapter to make it into one female usb3 port. then give it some power by making a t junction to my power bank (it will be fixed to the back of the tablet) and then will go to a usb3 hub also on the back of the tablet:: http://www.amazon.co.uk/gp/product/B00Y25XFGK/ref=ox_sc_imb_mini_detail?ie=UTF8&psc=1&smid=A2PGPJL0BBLHLX

Not important but the power bank is "Anker® 2nd Gen Astro Pro 15000mAh Triple Port "

Im also using its 9v port to extend my windows 8 tablets battery to about 16-20 hours by my estimate

just to check and reduce peoples time that I waste (apologies in advance)
Im only touching power pins, so I do not think I need the processing chips in smarthubs or whatever
Its current so it does not matter how much I supply, each device "Should" take what it needs.
The voltage is identical from both sources. Only the current is different as the pc provides 0.5A and the charger 1A and 2A ports.

Best Answer

The problem with this approach is, as with every other "connecting two power supplies", that the voltages are not the same. The nominal voltage is 5V - yes, but the actual voltage is different. One might be 5.05V and the other 4.8V. Both are in the spec for USB 5V.

But if you connect those two together, one will start to drive the other because it has a higher voltage.

Why the normal Y-Cables work? You plug them into the same device which has a single 5V source, so the voltage is really the same. Only each port is current limited.

And I see another possible question coming:

But there are USB hubs which use an external power supply, which is also a different voltage than the original port - how does that work?

Right, but those use either completely the external power supply or have some switching logic inside to switch from bus-powered to self-powered mode. For example the TPS2070 from TI.

So I would not connect two seperate power supplies to a single cable as long as I don't know the exact inner workings and can be sure that nothing will go wrong. It might work (if the things are reverse current protected for example) or it might not.

But I'm not an expert on USB power, so maybe I'm completely wrong.

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