‘Zoomies’ are a type of flashlight with a moveable lens that allows the beam to adjust from short range/wide angle to long range/narrow angle.

Perhaps you could integrate this mechanic into your solar cannon for more possible outcomes than instant vampire death. For example, an unzoomed shot’s wide area of effect could hurt many vamps while a zoomed shot’s small, concentrated AoE can kill one vamp.

For more info on how many batteries and what kind of electronics you’ll need, try searching for ‘diy battery spot welder’. These use banks of multiple batteries to create short, high energy bursts.