SparkFun Forums

[gandi67]

Hi, My name is Martyn for England & am a complete novice with electronics, i,m making a 7amp hr 12 volt battery powered charger for the familys mobile phones while camping, i have a 4 watt solar panel to keep it topped up, the battery, a 4 port self powered usb hub, my question is, can i use a 12volt to 5volt dc converter if i modify a usb cable for positive & negative from the output side of the converter, if so what wattage dc converter should i get (seen 5 watt & 25 watt ones on ebay) & also, will i need a reverse charging diode in line aswell
I have tried to get what info i can from the net, but the last thing i want to do is fry our mobile phones if i get it wrong

Thanks in advance for your help, most looking forward to your suggestions

Martyn

[Dave Mueller]

A safer, but not so much DIY, solution, is to buy an "octopus" cable that has 4 or more car lighter sockets, and use the phone's normal car charger.
The phones will charger faster since the USB hub will limit the current to the phones (100 or 500mA, iirc).

[MichaelN]

You could connect a 5V supply to the positive and negative of the input USB cable, but there are a couple of problems:
- Since the hub cannot talk to the host (normally a PC), it might not allow the phones to draw any current
- Even if it does allow the phones to draw current, as a self-powered USB hub it will current-limit the total output to (theoretically) 500mA.

If you instead use a POWERED hub (i.e., with a socket for external power), you should be able to draw the full 500mA from each of the 4 ports, but I’m still not 100% sure it will let you do this without the hub plugged into a PC. If you do go down this route, you’d want a power converter rated to at least 10W, so the 25W unit would be suitable.

Of course, if all you want is a power splitter (i.e., not data) then a USB hub is “overkill”, and you could just make up a power splitter cable to use with a power converter. Since hubs only cost a few dollars the hub might be the easiest option though, and it would give the phones some protection against overcurrent etc.