I assume you're using it to record shows. For playback only, those NiMH rechargables work well as long as you use them properly by completely draining them and completely charging them, making sure to rotate between a couple at least.
For recording shows, Lithiums work well for a couple shows solid if you're going to be on long stretches of two and three night runs. Expensive though, and not very eco-friendly.
Check out the Energizer rechargable NiMH's. I've had a good run with those for the last couple years. I haven't used my MZ-N505 to record shows, but from what I've seen it seems to draw less power than my Sharp MD-SR60, so it should do even better for you.