10-01-2009 07:38 PM
I finally managed to figure out the proper cabling to get the Verizon Mifi 2200 to charge using a generic USB charger. To recap the problem... The mifi device will only charge when using the supplied micro usb charger, or when connected to a computer USB port using the supplied micro USB cable. When connected to a computer, the device reverts to it's connected data mode, which turns off the wifi ssid broadcast and requires the Verizon access manager application to be launched. Most third party chargers will not work... including most 12V lighter to USB adapters, and most backup battery solutions.
Here is what is going on... Per the USB specification, Pins 1 and 4 are 5V power and ground respectively. Pins 2 and 3 are data+ and data-. In most chargers, pins two and three are tied via a pullup resistor to the 5V line. If you measure the voltage on 2 and 3 you will see around 3V, equivalent to a logic high state. This has something to do with allowing the device being charged to identify whether it should be in a fast or slow charging mode. More importantly, it is what the Apple iPod expects to see . So, anyone wanting to build generic USB chargers will probably implement this way in order to tap into the single largest device market around. Unfortunately, the mifi does not like the logic high on pins 2 and 3. I haven't yet sorted out why, but when it sees logic high on pins two and three, it refuses to go into charging mode.
The solution is to use a cable where pins 1 and 4 (5V and GND) are carried straight through. Pins 2 and 3 need to be disconnected from the charging source and shorted together at the Mifi end. If this was a miniUSB instead of a microUSB, it would be what is called a "charge only" cable. I have not found any vendors yet for a charge only microUSB cable so I went ahead and performed surgery on a standard microUSB cable. I took a standard USB cable, cut it and then connected it to a microUSB connector from an old Motorola bluetooth headset charger. Interestingly, the microUSB connector from the Moto charger already has pins 2 and 3 shorted together internally so all I had to do was connect 5V and ground and heat shrink tube the whole thing. With this cable I can charge the mifi from any generic USB port, including those on 12V lighter adapters, and rechargeable backup batteries. In addition, I can also connect the Mifi to my laptop to charge while continuing to access the Mifi via wifi.
Only word of caution... You really need to connect the Mifi to your computer using the standard data cable on occasion, say once per month. The reason is that it is only in this mode that you will receive notifications of S/W updates and/or the need to update your PRL. (preferred roaming list).