i wanna try to mod a hollow body in order to play not only electric but sound acoustic problem is i can't find any piezo's that do not need batteries, do they even exist?

Yes they exist. In fact most run fine without batteries. Just need a bigger amp.

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It's not the pickups themselves that need batteries - it's the preamps that usually go with them. The preamps are important not only for boosting the signal but also for impedance matching. Without the impedance matching they tend to sound brittle and nasty. BUT - you don't need an onboard preamp. Fishman, LR Baggs and a few others make external preamps that do exactly teh same thing. My dulcimer has a piezo bridge pickup but there is really not much room inside to have a battery compartment so I use an external Fishman ProEQ preamp and it works well.

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Crustaceo ergo sum

Fixed that for you. The rest of the text wasn't needed.

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My Gear , please wack with the side of your hand for normal service.
Bringing you EVIL VOODOO WAX MAGIC since 2009

Some do on some instruments for sure. Not all do.

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Crustaceo ergo sum