Acer Aspire One charges via USB in Sleep Mode

I’ve discovered an interesting trait with the AAO. I left a few USB devices plugged in and charging off it, and then put it in sleep mode.
Strangely enough, they continued to charge.

This is handy for me, as I have a 6600mAH battery for it. Hence, it can charge my iPod, TYTN II, and two backup batteries, and still have enough for 4 hours of use. On my recent backpacking trip, it meant I could go for a few days without a power socket.

The laptop is completely cold in this state, so I assume it’s consuming little power itself.

However, although this works for my mini-USB TYTN and batteries, it doesn’t work on the iPod unless the laptop is actually switched on. Perhaps the notoriously tricky iPod requires a signal rather than just a voltage?