charging a li po battery while using it through piezoelectric crytals
Based on your comments, my advice is: Stop. Just stop. You cannot do what you want, and you are so comprehensively ignorant that you cannot understand what you need.

To begin with, the PZTs you linked are clearly intended to be used as power users, rather than power generators. The links you provided give no data to establish 0.4 volts, and no information about current. To use them as generators would require some means of providing a very large force at high (kHz) frequencies.

Even worse, you claim that they must be used in parallel because " apparently, It doesn't create enough current" Let's pretend that you can, in fact, drive them to 0.4 volts. If you connect them in parallel you will get 0.4 volts, rather than the 4 volts or so that you need. The fact that you don't understand this says that you really, really have no idea what you are doing.

So, give up on piezoelectrics. In fact, you have not justified this requirement. Why, exactly, do you think PZTs will do what you want? When you say, "apparently", exactly where did you get this from? Mind you, it's perfectly true that they don't produce enough current to be useful, but that does not mean that the rest of what you right makes any sense.