Right, so this should be independent from whether you are using the battery or the USB PSU, right?
To find out if the piezos are clipping, simply look at the input signals on the Bela scope (there are examples both for C++ and PureData to use it). You may have to change the scope setting to better visualize the signals (e.g.: trigger mode Normal, triggering level 0.1, or similar).
Are the piezos going into the audio inputs?
If so, you can reduce the PGA (programmable gain amplifier) for the channels. If that is not sufficient, you can also reduce the ADC level. These settings are available from the project settings tab in the Bela IDE.
If, instead, you are connecting them to the analog inputs, we normally recommend a passive biasing circuit as explained here: use two 1M resistor, as follows:
Z 1M (or 690k)
piezo in ---------- analog in
This will at least center the signal, so that you will read a symmetric signal (but DC-shifted), and the negative half-wave is more unlikely to clip. Look at the signal, if there is still some clipping (positive or negative), then try adding a resistor in series with the piezo.