Doing so will reduce the capacity of the battery by about 10% or less, which is within the normal variations we see in batteries. If you're worried about getting caught by someone who's measuring battery capacity, you can add an extra explosive sheet to the battery's interior, increasing the thickness of a 10-sheet battery by 10%, which is within the tolerance for normal swelling.
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Once the explosive is laminated inside its (carefully cleaned) aluminum pouch, there's no way to detect the chemical signature of the PETN. The pouch seals that all in. The PETN and other components of the battery are too similar to one another to be detected with X-ray fluorescence, and the multi-layer construction of a battery also foils attempts to peer inside it with Spatially Offset Raman Spectroscopy.
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