voltage - Troubleshooting why Powerbank Doesn't Charge Cell Phone

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.

Answer

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.