I have powerbank that provides 5v (presumably regulated) power, as typical of most usb interface powerbanks. I have been trying out my powerbank with different phones. It works on all except one. I have tried it with various USB cables, making sure I didn't overlook anything foolish. Then, I looked at the AC adapter of the phone that it didn't seem to be compatible with. The adapter read: 5.4v DC output. The device's official website or manual doesn't tell me what the input voltage should be. So I have the following questions

  1. Is the fact that the device's AC adapter creates a DC 5.4v output enough to assume that, this is the correct voltage for my cell phone?

  2. If 5.4v is the input voltage, then when I attempt to power it with a 5v power bank, am I hurting it? Thus far, it just seems to be non-responsive when I try this.

  3. It seems another explanation is polarity mismatch. I'm aware that laptops have a polarity correction feature, which could explain why I can charge this phone via my laptop. Is there any other way to tell if polarity is an issue, given that I don't have access to detailed product information (unless the official website is updated, or something)

So that's the gist of it, I'm not sure why my powerbank isn't charging my phone. And I'm having difficulty troubleshooting the underlying problem by myself. Any tips big or small will be appreciated.

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    \$\begingroup\$ Are you sure the phone can be charged via USB? \$\endgroup\$ Nov 26, 2016 at 18:14
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    \$\begingroup\$ What is the device that seems to not be charging? Can it be charged when it is powered off? \$\endgroup\$
    – user57037
    Nov 26, 2016 at 18:32
  • \$\begingroup\$ It is an import phone by Sharp. I tried when it was powered off, but didn't work. I'm thinking it CAN charge via USB under the right circumstances \$\endgroup\$ Nov 26, 2016 at 18:38
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    \$\begingroup\$ @ArashHowaida - " I'm thinking it CAN charge via USB under the right circumstances " Why do you think that? And what is its model number? \$\endgroup\$ Nov 26, 2016 at 18:42
  • \$\begingroup\$ I was just thinking from a marketing standpoint, a phone without USB charging would be a bit backwards. But, I don't know for sure. It is the Sharp sh06d. A bit outdated, but I still use it. \$\endgroup\$ Nov 26, 2016 at 19:08

1 Answer 1


A device (phone) gets charged at optimal (high) rate only when it recognizes "charging port signature" on charger side. For Type-A port, there are several DIFFERENT port signatures that a charger port can provide:

  1. USB Battery Charging v1.2 Dedicated Charging Port, where D+ and D- are floating but tied together;

  2. USB Battery Charging v1.2 Charging Downstream Port, when a special low-voltage handshake over D+ and D- lines happens;

  3. Apple charger signature, when a combination of DC levels is present on D+ and D- lines with high-impedance resistor dividers (75k/50k);

  4. Sony charger signature, when D+ and D- have 5k/10k dividers to VBUS;

  5. Qualcomm "Quick Charger" signature, which uses an intelligent IC on charger side to understand a special sequence of DC levels driven by device on D+ and D-; can provide 9V and 12V on VBUS;

  6. Samsung charger signature (details unknown, to me);

  7. USB Power Delivery protocol V1.2 - uses complex packet-based protocol over VBUS wire using FSK modulation method at 23.2MHz carrier frequency. The port might have additional contacts to detect plug insertion.

  8. ...

For Type-C connector, there is a newer Power Delivery protocol v2.1, which solely uses the CC wire (Communication Channel) to negotiate voltages and available currents that can be delivered via VBUS.

A charging port usually provides only one signature, and most of mobile devices have an intelligence to recognize it, usually by sequentially probing different signatures. When a phone recognizes something it understands, it will take the charge. [However, there were attempts to make a charging port that tries several signatures sequentially, as some Microchip/SMSC hubs of the past.]

We don't know what kind of signature is used in this particular power bank, so all bets are off. Now, consider that your old phone can be designed for some other proprietary signature, so it should not come at any surprise that some phones will not charge well (or at all) from this particular power bank.

  • \$\begingroup\$ Thank you, that information helps me understand how different charging solutions really are. I didn't know there were differences between brands, or I would mention the powerbank brand earlier. My powerbank is a streamlight EPU 5200. The signature doesn't appear to be denoted anywhere in the manual. So, if I understand your post correctly, it could be voltage or polarity, or both, and there is no way to tell which? \$\endgroup\$ Nov 27, 2016 at 4:37
  • \$\begingroup\$ Even if the voltages match, if the charger signature is not right, the device will not charge? \$\endgroup\$ Nov 27, 2016 at 4:43
  • \$\begingroup\$ If a USB device with rechargeable battery does not recognize the charger signature, it will likely take 500mA from a charger, assuming that it is a regular USB port. The 500mA might be barely enough to keep the battery charging, since the limited budget must be shared with other internal phone activity, like screen backlight, keeping processor in suspend. Depending on consciousness of the phone, it might be slowly charging, or might not. However, even this behavior does not comply with USB specifications (unconfigured devices must take no more than 100mA), although it works most of the time. \$\endgroup\$ Nov 27, 2016 at 5:25
  • \$\begingroup\$ Actually, Samsung's Adaptive Fast Charging is interchangeable with Qualcomm Quick Charge 2.0. \$\endgroup\$
    – neverMind9
    Nov 28, 2018 at 18:57
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    \$\begingroup\$ @Kevin Reid, the method of short between D+ and D- is the China Federal standard, around 2006, formalized in 'YD/T 1591-2009, and only later incorporated into BC1.2. We need to know our actual history. \$\endgroup\$ Dec 23, 2018 at 0:36

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