The purpose of this project is to allow the Motorola/Tracfone V176 cell phone to be charged through a USB port.
Recent Motorola cell handsets (RAZR, W370, etc.) already provide a mini-USB port for charging. Future cell phones will standardize on the micro-USB connector. Standardizing on USB for charging has certain advantages:
A USB port can supply 100mA at 4.75 to 5.25volts. A higher current, to a maximum of 500mA, can be drawn if the port follows a particular handshaking protocol.
Pin |
Color |
Name |
1 |
Red |
VBus(5V) |
2 |
White |
D- |
3 |
Green |
D+ |
4 |
Black |
Gnd |
The cell phone power adapter output is measured at 6.4V open circuit. The internal lithium-ion cell phone battery is nominally 3.6V open circuit, with a storage capacity of roughly 900mAH. The power socket takes a 3.0MM outer diameter 9MM long barrel-type power plug, similar to Digikey part CP-119-ND.
Lithium-ion batteries have an open circuit voltage of 3.6V, and are typically charged at 4.2V. The typical charging procedure is to
Small batteries such as this can be charged at 1C, so we expect a maximum charging current of 900mA, more than the current available in the USB spec. Connecting the cell phone to the bundled wall-wart charger, we measured an actual maximum DC current of 470mA, when charging a mobile phone with very low battery charge.
We expect a voltage regulator or charging circuit in the phone to require a higher voltage than the battery voltage. This difference between input and output DC voltages in a voltage regulator is termed dropout. Dropout voltages for typical voltage regulators are on the order of 1 to 3 volts, and low-dropout voltage regulators are availabe at less than a volt. Is 5V (4.75V worst case) high enough input voltage to charge this phone? We could use a voltage booster to create a higher voltage from the USB voltage, but we would prefer a simple design with few components if at all possible.
If the charger cable is connected from a charged phone to an unpowered USB, does this drain the battery? If so, we might need a blocking diode to prevent current going the wrong direction. This would reduce the voltage available for charging even more. Voltage drops at low current across a silicon diode are approximately 0.6V, or 0.3V for a more expensive Schottky diode. With a phone at full charge, we connect a 10kOhm resistor across the positive and negative charging socket terminal connections and read no voltage across the resistor, demonstrating that no current can flow out of the charging socket, so we do not need a blocking diode.
As an experiment, we connected the USB to cell phone power plug through a 50ohm resistor, to limit maximum current to 100mA. The cell phone powers up, and its display indicates charging, but the battery is not completely charged after several hours. Next, we tried connecting USB power directly to the cell phone power plug (USB red wire to power plug inner conductor, USB black wire to power plug outer conductor) and the battery charges completely after an hour or two. The computer does not complain about exceeding USB power limits, even though the maximum current measured, for a cell phone with very low battery charge, is 470mA.
We also tried charging from a USB port on a different laptop computer, one that often gives spurious "Power surge on port hub" messages when returning from sleep, so we expected more stringent USB current limit checking. This USB port also provided 470mA without any warnings about exceeding current.
Finally we tried charging from an external powered USB hub, and charging also measured maximum 470mA from this port.
Disclaimer: Use this information at your own risk. We are not responsible for damage to your cell phone or computer.
Start with a spare USB cable ending in the usual type "A" plug, and cut the cable approximately 15cm from the connector. Strip the outer insulator 2cm from the end, remove the shield, and snip off the green (D+) and white (D-) wires, which are not used here. Thread the power plug strain-relief shroud on the cable, remove 0.5cm insulation from end of the red (VBus) and black (Gnd) wires.
Solder the red wire to the center plug conductor and the black wire to the outer plug conductor. Use a bit of electrical tape to insulate the two connections if needed.
Screw the shroud back onto the plug, and use a multimeter to check for continuity. Also check for shorts between the conductors. Try flexing the cable near the power connector while checking for intermittent opens or shorts.
Plug the power connector into a v176 mobile phone, and the USB plug into a computer. You should observe a "charging" message on the phone display. We didn't get any computer warning about excessive USB current when charging on our laptop-- your results may vary. During the first charge attempt, check the phone temperature by hand occasionally-- just in case. After an hour, unplug the phone and see if the battery is completely charged.
It works! (Strictly speaking, this circuit exceeds the lower USB current source specification of 100mA available unless the device requests more current by defined protocol, but in practice several USB ports we tried were able to provide the necessary charging current without complaint.)
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