I use the Tekkeon MP3300 to power my HD-P2 and have been very pleased with it. It didn't come with the right adaptaplug for the HD-P2, but Radio Shack had it for about $5. I don't tape day-long festivals, so I haven't put its life to the test. But after 3 hours of use, it hasn't registered as having been depleted at all. In fact, even with fresh internal batteries in the HD-P2, after 3 hours it hasn't switched to the internal batteries (which it's supposed to do after the life of the external battery subsides to below that which can be provided by the internals).
What I have had POOR luck with is the combination of the Duracell 2650 mAh rechargeables and the LaCrosse BC-900 charger. I basically purchased all of the above-mentioned at once (last year) and tried them all out at a show together for the first time. Until I plugged in the Tekkeon, the unit would not even power on from the rechargeables. Luckily, I had some regular alkalines with me to replace them with (for backup to the Tekkeon). Once I got home, I tested the capacities of the rechargeables and discovered that several of them were registering only around 1500 mAh (though, upon receipt of them, I had put them all through the LENGTHY reconditioning process recommended by LaCrosse). I tried the same process again on all of them, and this time the results were the same, with different batteries than the first time failing to charge past about 1500 mAh. So I tried again, and again some of them didn't charge to full capacity, but it wasn't always the same battery nor the same "slot" in the 4-slot charging unit that was the culprit. I was left puzzled, and made a note to myself to call Duracell and LaCrosse to try to figure out whether it was the batteries or the charging unit's fault. And then I never did. So I'm probably screwed now that it's been 6 months or more since I purchased everything.