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.

  • \$\begingroup\$ As 3.5" disks need 12V and typically at least 2W in idle, I can't imagine them being used directly on USB. Some 2.5" disks need more power than a single port is able to supply. (just a minor detail I completely locked on while reading) \$\endgroup\$
    – Arsenal
    Aug 26, 2015 at 17:23
  • \$\begingroup\$ there are USB ports with common driver,Hence somehow the current to the USB ports is limited to a point,hence there is a limit to the amount of power you can extract from all combined USB ports,I dont think there could be any problem connecting sources in parallel,And I would like to say best of luck to your laptop battery. \$\endgroup\$
    – MaMba
    Aug 26, 2015 at 17:27

3 Answers 3


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.

  • \$\begingroup\$ Hm, only 8 seconds out :-) \$\endgroup\$
    – Asmyldof
    Aug 26, 2015 at 17:45
  • \$\begingroup\$ Like I commented on the other answer. What about a Powered hub that takes power from an external battery (assuming I get a battery with the correct dc output voltage and connector) \$\endgroup\$
    – kris2340k
    Aug 26, 2015 at 18:32
  • \$\begingroup\$ @kris2340k as long as the battery voltage stays in the specification and can supply all the current, I don't see a problem replacing just the power supply of a self powered hub with a battery. \$\endgroup\$
    – Arsenal
    Aug 26, 2015 at 18:36
  • \$\begingroup\$ schottky diode between each source, problem solved? 1020-Y 2A / 50V TO-92L for example, good one. \$\endgroup\$
    – Codebeat
    Oct 24, 2016 at 16:42
  • \$\begingroup\$ @Erwinus that won't help you much, you'd stop driving the other supply, but the load will be powered by the port providing the higher voltage. Shottky diodes don't provide load balancing. \$\endgroup\$
    – Arsenal
    Oct 24, 2016 at 18:10

Not using a Y-cable. No.

If your laptop is 5.1V and the power bank is 5V, what happens then? What happens if it's the other way round? Or 5.0V versus 4.9V? Are you going to be tweaking all your powerbanks and laptops to be millivolt compatible to each other?

Not to mention that opening up a USB cable will already demote it from USB2.0 to 1.1, as the shielding and exact twist vs power pair twist and distance and what-not are very important for the 480Mbit/s (burst) speed signal integrity. If the signal gives too many errors your hub and/or PC will auto switch back to high-speed 12Mbit/s. If you are fine with that, then that's not stopping you, but you're talking about hard-disks, so I'm guessing you're not fine with it. Just to be clear: That would be 12Mbit/s for everything on the hub combined. In the case of 12 ports with a HD each, which are all in use that's 1Mbit/s per HD = not even enough for HD video stream.

Then we can also throw in the limitation that if a USB hub manufacturer knows that a USB port is only allowed to supply 500mA they are NOT going to be designing it such that it can handle draining many amperes. If they put protections in they will be at about 750mA at best. If they didn't anything over 1A or so can cause serious unexpected problems. If I'm told "This connector is protected 100% of the cases to never give more than 500mA", I'm not going to waste my own time and money designing a PCB that can handle 5A, because that costs significantly more real-estate. Though I will definitely add my own protection, but that's more a condemnation of the reliability of the info managers usually give.

For this purpose there are hubs you can buy that include a power-path manager and a 2.5mm (or other size) DC jack 5V in, which allows you to source an alternative 5V at 2A or 4A or whatever to the hub separately and safely.

This will then stop the hub draining from the USB input and sourcing the total current to all USB ports equally. Which will still be limited to their specification internally, as functional USB ports are not allowed to just pump out 2A, that's in the official USB rules, so a hub that is commercially sold will stick to those rules and output at most 500mA normally, or if it's expensive enough to contain powered-negotiation up to 1A.

EDIT: As a side note:

Be aware that most dual-power hubs do hard-connect the ground of the incoming voltage to the ground of the USB. Fine, you may think, but please do check that the USB ground then is compatible with the ground of the voltages, if you use the same power bank for everything. If your powerbank happens to make a +5V for the USB and then a -5V to get 10V that gets smoothed into 9V, your tablet will "live" at -5V ground compared to USB ground = bad. Same goes for whatever a tablet manufacturer put in there to generate the USB 5V output.

It's very, very unlikely anything will happen, but unlikely is not impossible, whereas a multimeter first on resistance measurement, then on voltage measurement to compare would give you exclusive proof.

  • \$\begingroup\$ +1 for the side note :D \$\endgroup\$
    – Arsenal
    Aug 26, 2015 at 17:54
  • \$\begingroup\$ What If I had a USB3 powered hub that needed a usb3 input in addition to a DC coaxial connector. And I had a portable battery bank that had a coaxial of the same size and voltage. No cutting up so no unexpected issues? The main inspiration for this was from here:sites.google.com/site/sonicboomworld/my-projects/otg-diagrams he managed to get his phone plugged into a hub, both the phone and dock were getting power from the portable charger. I wanted to do the same but with usb3? \$\endgroup\$
    – kris2340k
    Aug 26, 2015 at 18:27
  • 1
    \$\begingroup\$ @kris2340k if it has a plug for power, it's designed for extra power. Just take heed of my side note. Powering many different devices from one battery/source that supplies multiple voltages is not clear cut. Chances of a problem may be in the 1% range, but such a problem can destroy any one of the devices in your setup. Electrons don't care which device was most expensive. \$\endgroup\$
    – Asmyldof
    Aug 26, 2015 at 18:59
  • \$\begingroup\$ So your saying to be on the safe side I should only keep the power bank supplying 1 voltage at once? \$\endgroup\$
    – kris2340k
    Aug 26, 2015 at 21:41
  • \$\begingroup\$ This is taking the electronics a tad too far, promise I wont do it but what about using 5v voltage regulators ? If needed im fine with simply breaking the connection that connects the power pins of the usb to the laptop. Meaning the device only gets power from the battery? \$\endgroup\$
    – kris2340k
    Aug 27, 2015 at 8:49

Are you using the laptop as a power source? If so a couple of diodes in the cable: 1 in the red from laptop ( anode to laptop) and 1 in the red from power bank ( anode to bank) will stop the 2 of them trying to power each other. Should have said to use Schottky diodes, as they only drop about 0.2 V.

  • \$\begingroup\$ And it will drop the voltage from 5 V to 4.3 V. Do you think this will matter? \$\endgroup\$
    – Transistor
    Aug 24, 2019 at 11:06
  • 1
    \$\begingroup\$ Standard silicon types drop that much, but Schottky diodes only drop about 0.2 V, so if the outputs are bang on 5 V, the input to the hub will still be 4.8 V, which is above USB spec for minimum voltage. \$\endgroup\$ Aug 24, 2019 at 16:47
  • \$\begingroup\$ I think you need to add that into your answer. \$\endgroup\$
    – Transistor
    Aug 24, 2019 at 16:49
  • \$\begingroup\$ Yes, now added. Thanks for reminding me 😀 \$\endgroup\$ Aug 24, 2019 at 17:33

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