Some of the early audio jack magnetic stripe readers had no battery. (Think Square, although I don't know if theirs ever worked this way.) They sourced all their power by rectifying the weak AC signal of a tone played out of the audio jack.
About a year ago, I either read somewhere or was told by a co-worker that unpowered MSRs will not work on a majority of newer (ICS+) devices.
Edit: Since it's apparently not obvious, every MSR that lacks a battery is going to have a certain minimum power requirement. How much power you can source from the audio port is going to vary by device. I would expect that Google has established a standard for the minimum amount of power you can source from an Android device's audio jack, much like there's a standard minimum amount of power you can source from USB. I found the audio headset spec, but I don't see anything in there about minimum power.
I'm finding multiple sources that say the iPhone 5 can source about 30mW from its audio jack, or 15mW per channel. According to at least one source, some Android devices can only source a small fraction of that. So I'm trying to find out:
- if and when the minimum standard was lowered (or if there isn't a standard, if and when manufacturers started producing devices that could not source adequate power)
- whether this actually means that non-powered audio jack MSRs will not work on newer devices, or if that's just a rumor