you can't replace the battery with anything but a P2 battery. There's a circuit inside is that prevents counterfeiting. Not to mention the battery is glued down inside and since it's li-ion it's soft shell. Doubt you will be able to get it out without puncturing it and and harming yourself.
DAPs: Samsung P2 8GB Black - KR 5.10 UMS, Sansa e250 2GB - Rockbox r21519, Samsung S2 2GB White - 1.00 UMS. RIP Samsung K3 2GB Black 3.09 MTP... Patiently awaiting Zune HD 32BG,
Headphones: Head-Direct RE-2, JVC Air Cushion