I personally like the "cell phone emergency charger" devices. Energizer makes one, most retailers have them for $20 (ebay much, much less if you don't need it today!), or Wal-Mart has a generic case that takes 4AA batteries and has a USB port - just need to put a nokia power tip on a regular USB cable, I personally use this type of charger with 4 NiMH batteries. There are also 1 AA options, smaller of course, but less capacity. Either way, they're cheaper than a new battery, and I've clocked runtimes in excess of 18 hours playing music (no wifi on, if I remember right).