I've just installed a 12v socket to my motorcycle. The socket has a sticker saying "12V - 1A", and the user manual says that "use is possible up to a power input rating of 12W". Taking that into account I bought a USB adapter rated for 1A (the model is this one) and it seemed to work.

Now, I've been trying to use my mobile phone (a Galaxy Nexus) as a GPS (the app I use is Sygic, although I think that doesn't add much). The problem I've found is that, while using the GPS app with the mobile phone connected to the usb adapter in the 12v power socket, the "charging" icon appears, but the battery is not actually charged, nor kept at the same level. It keeps discharging, although at a slower pace. Eventually the phone runs out of battery and shuts down.

Given these facts, the problem seems that when using the GPS (haven't checked in standby mode), the energy is consumed at a faster speed than the charging speed. The question here is... does that make sense? It makes sense to me at least. The Sygic app running, plus the GPS module updating the location constantly, plus the screen at full brightness, could that need more than 1A?

I'm asking this just to discard an electrical problem. Is there a way of measuring if the 12v_to_usb adapter is actually providing 1A of current?

EDIT: There seems to be a related thread in reddit: http://www.reddit.com/r/Android/comments/nystt/galaxy_nexus_power_analysis_why_chargers_cant/

  • \$\begingroup\$ One thing to check first: Are you sure that the phone is actually charging at 1 A? When using USB 2.0, the maximum current a device can draw from port is 500 mA. Therefore many devices won't charge at higher rate unless they somehow detect that the source can in fact provide more than 500 mA. \$\endgroup\$ – AndrejaKo Jan 12 '14 at 22:13
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    \$\begingroup\$ There is actually a USB charging specification specifically for chargers, which allows device to detect a charger by using a specific resistor arrangement on the USB data pins. Some devices don't use the standard way of detection and instead have their own. I see that the charger you have is said to support Apple's iDevices. They do not support standard charging detection mechanism! It could very well be that the Galaxy Nexus is expecting the standard charging detection mechanism, but the charger isn't providing it, so it only charges at 500 mA instead of full current of 1 A. \$\endgroup\$ – AndrejaKo Jan 12 '14 at 22:15
  • \$\begingroup\$ As for measuring, do you have a multimeter available? Do you have an USB extension cord that you could sacrifice for the measurement? \$\endgroup\$ – AndrejaKo Jan 12 '14 at 22:16
  • \$\begingroup\$ Ahá, nope I'm not sure about the phone actually charging at 1A. There are some apps that tell how much current the phone is actually charging at, but none of them seem to work with the GalaxyNexus. I thought the charger didn't actually matter, but I see I'm wrong! About the last one, yep I have a multimeter and could sacrifice a USB extension cord, for science! \$\endgroup\$ – José Tomás Tocino Jan 12 '14 at 22:24
  • \$\begingroup\$ Added a related link to the question. \$\endgroup\$ – José Tomás Tocino Jan 12 '14 at 22:26

The socket can handle 1 amp if hooked up correctly. That does not mean you can tap off any circuit and get 1 amp. Wire directly to the battery.The phone draws about 4.5W. (0.9A at 5V). The battery is probably a Li-Ion 1750 mAh battery. You need about 1.5A charge circuit, that will get you about 7.5W. But the consumption I think will always be greater than the charge rate with that small of a battery. Try a higher capacity battery, or if you really wanted to get crazy, shell out an old battery, wire it to the terminals, less the battery, and use it as a "battery eliminator".


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