I have 2 different powerbanks, both have 10000 mAh and both are rated for 5V 2A power input.

I have noticed that they draw a different amount of current despite using the exact same charger (5V 3A), same port and the exact same USB-B-Micro cable.

One of them draws 1.6A while the other one draws only 0.7A. Same results on further retries.

How can there be such a difference?

Is the flawed latter charging IC (0.7A) too paranoid of overdrawing current?

Edit 1:

  • The slower powerbank that charges at 0.7A is newer and has been used much lesser than the faster powerbank so far.
  • Both powerbanks only charge faster when compromising cable length. The limit in each case is 2A as the specification said, which can only be reached on slower powerbank when using a cable shorter than 10cm/4 inches. However, one of the powerbanks charges significantly slower with the same cable length, in fact the exact same cable. 0.7A with the exact same cable as the faster powerbank at 1.6A.

Edit 2:

  • Both of the powerbanks were at around 25% of charging state during the test. This means that their internal terminal voltage of the Lithium-Ion cells is rather low but very similar.
  • According to imprint, both powerbanks use Lithium Ion, not Lithium Polymer.
  • Again: Both are tested using the exact same cable.
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    \$\begingroup\$ Why do you call the 700mA one flawed? Maybe it's working at the maximum current that its battery is rated to charge at. \$\endgroup\$ – Hearth Nov 26 '18 at 3:02
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    \$\begingroup\$ Chargers that use USB cables for charging do have a variety of "charging signatures". electronics.stackexchange.com/a/271681/117785 Two different powerbanks can have different interpretation of the same signature, so they take different inputs. \$\endgroup\$ – Ale..chenski Nov 26 '18 at 23:28
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    \$\begingroup\$ Are you saying that with very short cables, both banks consume 2A, but with longer cables, the current goes down? If so, then you are using crappy cables, and the power bank are noticing the voltage drop and "backing off" to avoid crashing USB VBUS. \$\endgroup\$ – mkeith Nov 27 '18 at 6:41
  • \$\begingroup\$ @mkeith The cable has a length of 2m. Yet one powerbank draws 0.7 (occasionally 0.6A or 0.8A actually) while the other powerbank draws 1.5A with the exact same cable. \$\endgroup\$ – neverMind9 Nov 27 '18 at 14:52
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    \$\begingroup\$ Unfortunately, the question is not completely clear. I am trying to understand all the information you have provided in the question. You said "Both powerbanks only charges faster towards 2A when compromising cable length". I would like to know what that means. Are you saying that when using very short cables, both powerbanks draw around 2A or close to 2A? \$\endgroup\$ – mkeith Nov 28 '18 at 3:32

The most likely reason is different charger requirements.

When you plug powerbank into charger, the powerbank needs to know max availiable current to prevent overloading it -- is it a regular PC port limited to 0.5A? Or 1A charger? Or 2A one?

Unfortunately, there is no clear consensus on how to do it. USB 1.0 and 2.0 allow up to 0.5A current. USB 3.0 allows high currents, but requires (relatively) expensive chip to announce it. but various manufacturers came up with different ideas (short data pins, put different voltages on data pins, draw increasingly more current until voltage starts to drop, and so on).

Most likely, one of your powerbanks does not recognize your charger, and thinks it is an 1A model, and thus limits the max current. Since there is no standards, it is entirely possible.

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I cannot be sure what is going on and some parts of your question are unclear. But here is a possible explanation. Let's say the 2m cable you are using is made with AWG28 wire. Since the cable is 2m, the current must flow through 4m of wire (round-trip distance). 4 meters of AWG 28 wire has a resistance of around 0.85 Ohms.

So, if 2A flows through 0.85 Ohms, that is a voltage drop of 1.7V. 5-1.7 = 3.3V. That is too low a voltage to charge a lithium ion battery. Now, I am not saying that your cable is made from AWG28 wire. It may be a bit larger than that. But resistance can add up over a 4 meter round-trip length with higher currents.

I can't tell for sure, but I think you are saying that different cables are producing different results. I believe what is happening is that the unit with the low current is designed to reduce charge current when VBUS is too low. So initially it tries to charge the battery quickly, but when it sees that USB VBUS voltage is dropping below 5V, it reduces its output current to allow VBUS to recover. This is actually a good thing. But you will have to use a decent cable to get the most out of the charger.

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  • \$\begingroup\$ Thank you for the answer. More information added to question under section “Edit 2:”. \$\endgroup\$ – neverMind9 Nov 28 '18 at 14:15
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    \$\begingroup\$ Thanks for the edit. However, I understand that they were tested under the same cable and had different results. You are focused on that. But I still am not clear on the meaning of one sentence: "Both powerbanks only charges faster towards 2A when compromising cable length." Would still appreciate if you could explain what that means. My current theory is that the difference between powerbanks is mostly due to the fact that one of them "backs off" when the voltage sags, and the other one does not back off, or not as much. \$\endgroup\$ – mkeith Nov 28 '18 at 15:03
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    \$\begingroup\$ Sure. I will edit it. \$\endgroup\$ – neverMind9 Nov 28 '18 at 18:53

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