Thread: External Battery Packs
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xiojason is offline xiojason
2009-12-17 , 20:13
Posts: 36 | Thanked: 74 times | Joined on Jun 2009 @ Seattle, WA, USA
#49
You need an adapter that ties the D+ and D- (TX/RX) lines together on the device side (but doesn't just ground them).

From Wikipedia:
The Dedicated Charging Port shorts the D+ and D- pins with a resistance of at most 200Ω. The short disables data transfer, but allows devices to detect the Dedicated Charging Port and allows very simple, high current chargers to be manufactured.
You can make an adapter by cutting and soldering two wires from a USB extension cable:
Code:
             Red <------------------------> Red
     Male  White X cut and         solder / White  Female
Connector  Green X insulate      together \ Green  Socket
      End  Black <------------------------> Black  End
Leave the red (+5V) and black (ground) lines connected straight-through. Cut the green and white (data) lines. On the female end of the cable (the end where you will plug in your USB<->microUSB cable), tie the two lines together. On the male end (the end that plugs into your power source), leave them cut and insulate them so they don't accidentally ground or short.

The N900 senses the tied-together data lines and will begin charging without expecting an actual smart USB host on the other end to negotiate voltages and other bus details. This should also work for other devices that don't like "dumb" power-only charging sources, like iPods and iPhones.

Disclaimer: While this adapter works for me and has not caused any damage to my N900, I do not take any responsibility for any of your equipment.
 

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