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Here's yet another way to build a battery-powered USB port. You can use it to charge your iPod or phone or lights or whatever. It should last about ten times as long as designs based on 9V batteries.

It's based around my favourite batteries of all time, AA NiMH cells. They're cheap, rechargeable, and perfectly suited to this project. I used this cable to keep my iPod charged on a bicycle tour of Victoria last year.

The cable, charging my iPod

You'll need:

• a 4xAA battery holder
• some wire, if your battery holder doesn't come with any
• a USB socket

I got my battery holder and USB socket from Jaycar. My cable has a DC plug and socket in between the batteries and the USB socket, because I use the same battery pack to run my bike lights.

Solder the red (positive) wire from the battery holder to pin 1 on the USB socket. There are tiny numbers printed on the inside of the socket, if you look carefully. You can also check my wiring diagram below. The black (negative) wire goes to pin 4 on the USB socket. Seal it up with some hot glue, and you're done!

The NiMH AA cells are exactly the right voltage for the USB port. You can use non-rechargeable cells in a pinch, but there's a slight risk that devices will blow up - the terminal voltage of non-rechargeable cells is higher than rechargeables (1.5V/cell instead of 1.2V/cell). Most devices that I can think of (phones, PDAs, iPods, lights, fans) will run fine off non-rechargeable AA cells.

Only use lithium AA cells if you're brave or desperate, because their terminal voltage (1.7V/cell) is too high. Some devices will work (probably iPods, phone and PDAs), but you increase the risk of blowing something up.

You could also use NiMH AAA cells if you want a really tiny, lightweight battery pack. You'll still get several times the battery life of a 9V design!

USB charge cable

The end of the USB socket: Don't get the pins the wrong way around!