I have a short micro USB cable that came with my bluetooth headset. It charges the headset just fine, but when I connect my phone to the same cable, using the same power source, it doesn't charge (no indicator, nothing). However, using the same power source with a different cable, the phone charges just fine.
What is it about the shorter cable that it charges certain devices but not others?
Best Answer
A portable device will accept charging source over USB connector when it sees certain "charger signature" at D+/D- pins. There are cost-reduced "charging cables" that have no pass-through D+/D- wires, but attempted to provide the "signature" insde the u-B mold, like shorting D+ with D-, which defines Chinese-style (or "dedicated charging port") signature.
If your phone doesn't understand this signature and expects something else (like motorola ID pin impedance, or QuickCharge), or your "short cable" doesn't provide what the phone expects, the phone will refuse to take charge from this cable. With a different cable and different source (which might have a different charger signature) your phone might be fine.
ADDENDUM: The BT device is a low-power device with a small battery. Charging the battery will require a very modest current, so the BT device doesn't care about higher power from USB port, and will likely rely on standard 100mA-500mA USB port capacity. So the dedicated BT charging cable likely doesn't have any "charging signature" at all, and that's why your phone refused to take the charge over the BT "short" cable.