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/