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I have two 20.000 mAh mobile power banks with two USB ports that can deliver 5V at 2.4A. I need a minimum of 9V at 2A in output.

I have 2 male USB connector cables and I want to connect the negative wire of one power bank port with positive wire of the other power bank port (not the same power bank port obviously.)

I have done a try, and it seems to work properly, I've powered a device with 10V by wiring in series two power banks.

I did another test, by connecting the two power banks in series, again, and switching off one of them.

Unexpectedly, the circuit was not interrupted, but I got 5V (instead of 10,) so in this way the current traverses the turned off power bank in reverse (+ flow in - wire of the second power bank, and exit by + wire?) Nothing was damaged, but can this be a problem?

I know that every current source can be wired in series to get a higher voltage, and the maximum current can be the same as the lowest current generator. Basically, every power bank has one or more 3,7V cells with a boost converter to convert 3,7V DC to 5V DC.

P.S. I know, I can simply use a QuickCharge power bank to trigger a higher voltage, but I can't in this case for some reasons.

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  • \$\begingroup\$ I wouldn't count on it not damaging your power bank if one is turned off. Also make sure not to exceed the max current for either power bank. \$\endgroup\$
    – user20574
    Commented Mar 6, 2020 at 14:32
  • \$\begingroup\$ There is probably a reverse biased diode across the powerbank to protect it against reverse connection (by shorting out any PSU you connect to it backwards). It protected your powerbank ... this time. Relying on it is like relying on your airbags ... it shouldn't be routine. \$\endgroup\$
    – user16324
    Commented Mar 6, 2020 at 15:13
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    \$\begingroup\$ Use 2 x <Enter> for paragraph break. Review before pressing "Post". Fixed. \$\endgroup\$
    – Transistor
    Commented Mar 6, 2020 at 17:08
  • \$\begingroup\$ Re, "I did another test, by connecting the two power banks in series, again, and switching off one of them." Did you attempt to draw significant current from the combined power banks when you performed that experiment? or did you only measure the open-circuit voltage? \$\endgroup\$
    – Solomon Slow
    Commented Apr 26, 2022 at 1:11
  • \$\begingroup\$ I've tried to draw significant current, but I've solved in another way. I have now 2 power bank Quick Charge and/or Power deivery wired in parallel. I trigger the quick charge protocol on both (with 2 simple qucikcharge trigger bought from aliexpress at 1 dollar ) and then wire in parallel. The output is about 12V @ 3A that is fine for me \$\endgroup\$
    – andrea1493
    Commented May 3, 2022 at 9:22

1 Answer 1

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Without looking at a schematic, it's probably safe if:

  • Both units are always on when load is connected
  • There are no other connections to the batteries
  • There are no other connections to your load

It's probably ok if only one battery or if just the load is connected to something else, but if more than one is connected you may risk a short.

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  • \$\begingroup\$ I've tried to place a diode between negative and positive wires of power bank 1 and power bank 2, and I've turned off one of the power bank, but the circuit was not interrupted (only the voltage dropped from 10 to 5V). Maybe because the current Flow is always the same? \$\endgroup\$
    – andrea1493
    Commented Mar 6, 2020 at 15:49
  • \$\begingroup\$ It's possible your power bank grounds its output when turned off to get rid of any residual voltage. It's also possible it's conducting through some parasitic phenomenon which may or may not be able to handle the current in the long run. Unless you know, I'd suggest using an external switch to turn the power on and off. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Mar 6, 2020 at 15:54
  • \$\begingroup\$ Ok thanks for the answer, but when I have two generators and one load (in this case the two power banks are the generators) the current flow is negative wire of PB1 --> positive wire PB2 --> Negative Wire PB2 --> LOAD --> Positive wire PB1 ? This is always the same, even when One of the power bank (or generator) is off? \$\endgroup\$
    – andrea1493
    Commented Mar 6, 2020 at 16:02
  • \$\begingroup\$ That's right, although most of us use positive current flow, and as mentioned before I'm not sure how much current one of those batteries can safely pass when it's off. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Mar 6, 2020 at 17:47
  • \$\begingroup\$ No no, USB ports are NOT directly connected to lithium cells. Usually there are one ore more cells connected in parallel between them. Cells are connected to a BMS (to cutoff the circuit and prevent over charge/discharge) and finally a Step UP DC-DC converter to boost the voltage from 3,7V to 5V. Connecting 2 power banks in series, maybe, is like wiring two dc boost converter in series... \$\endgroup\$
    – andrea1493
    Commented Mar 6, 2020 at 17:58

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